NBS Measurement Services:

Far Ultraviolet

Detector

Standards

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National

Sureau of Standards. The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support

to the National Measurement Support Systemfor ionizing radiation; conducts research in radiation related

fields to develop improved radiation measurement methodology; and generates, compiles, and critically

evaluates data to meet major national needs. The Center consists of five Divisions and one Group.

Atomic and Plasma Radiation Division

Carries out basic theoretical and experimental research into the • Atomic Spectroscopy

spectroscopic and radiative properties of atoms and highly ionized • Atomic Radiation Data

species; develops well-defined atomic radiation sources as radiometric • Plasma Radiation

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

Provides the central national basis for the measurement of far ultra- • Far UV Physics

violet, soft x-ray, and electron radiation; develops and disseminates • Electron Physics

radiation standards, measurement services, and data for for these radia- • Photon Physics

tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applications; determines fundamental mechanisms of electron and photon interactions with matter; and develops advanced electron- and photon-based

measurement tech iques. The Division consists of the following Groups:

Radiometric Physics Division

Provides national measurement standards and support services for ultra- • Spectral Radiometry

violet, visible, and infrared radiation; provides standards dissemination • Spectrophotometry

and measurement quality assurance services; conducts research in optical • Radiometric Measurement Services

radiation, pyrometry, photometry, and quantum radiometry; and develops

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities • Accelerator Research

including the electron Linac and race track microtron; develops, • Linac Operations

designs, and builds electronic and mechanical instrunentation for • Electronic Instrunentation

radiation programs and facilities; provides national leadership • Mechanical Instrunentation

in the standardization of nuclear instrtmentation; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

• Office of Radiation Measurement

• Radiation Theory

• Radiation Chemistry and Chemical

Dosimetry

• Neutron Measurements and Research

• Neutron Dosimetry

• Radioactivity

• X-Ray Physics

• Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

understanding of physical processes that underlie measurement science.
NBS MEASUREMENT SERVICES

FAR ULTRAVIOLET DETECTOR

STANDARDS

L. Randall Canfield

Nils Swanson

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1987
Library of Congress Catalog Card Number: 87-619832

National Bureau of Standards Special Publication 250-2

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-2, 86 pages (June 1987)

CODEN: XNBSAV

Commercial products— materials and instruments— are identified in tliis document for the sole pur- pose of adequately describing experimental or test procedures. In no event does sucli identification imply recommendation or endorsement by the National Bureau of Standards of a particular product; nor does it imply that a named material or instrument is necessarily the best available for the purpose

it serves.

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1987

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services allow customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document (SP 250-2), NBS Measurement Services: Far Ultraviolet

Detector Standards, by L. R. Canfield and N. Swanson, is the second to

be published in this new series of special publications. It covers the

calibration of the quantum efficiency, or responsivity, of photodiodes

over the wavelength range of 5 to 254 nm. Inquiries concerning the

technical content of this document or the specifications for these

services should be directed to the authors or one of the technical

contacts cited in SP 250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

.

George A. Uriano Chris E. Kuyatt

Director Director

Measurement Services Center for Radiation Research

i i i
Far Ultraviolet Detector Standards

ABSTRACT: A description is given of the NBS program in which

special photodiodes for the far ultraviolet spectral region

(5-254 nm) are made available as transfer standards. These

detectors are calibrated in terms of quantum efficiency

(photoelectrons per incident photon) as a function of

wavelength. Descriptions are also given of the calibration

principles, calibration systems, and photodiode types

involved in this program. Calibrations reference to the

photoionization of rare gases.

Key Words: Far ultraviolet; photodiodes; photoionization;

quantum efficiency; rare gases; standards.

1

V
TABLE OF CONTENTS

I Introduction 1

II Description of Service 2 a . Background

b. Goals

c. Present Services

d. Future Directions

III Description of Detector Types 4

a. Windowless Photodiode Transfer Standard (5-122nm)

b. Windowed Photodiode Transfer Standard (116-254nm)

c. Rare Gas Ionization Chamber (absolute standard)

d. Thermopile (working standard)

IV Calibration Techniques 20

a. Calibration Principles

b. Measurement Methods

V Calibration Systems 26

a. SURF-II System

b. Thermopile System

c. Photodiode Intercomparison System

VI Treatment of Errors 45

VII Safety 47

VIII Customer Interaction 48

IX Acknowledgement 49

References & Bibliography 50

Appendices

A. Sample Report of Test A-1

B. Detailed Operation of SURF-II System B-1

C. Detailed Operation of Thermopile System C-1

D. Detailed Operation of Intercomparison System D-1

V
LIST OF FIGURES

Page

Figure 1. Photocathode anodizing arrangement 5

Figure 2. NBS windowless far ultraviolet photodiode 7

Figure 2a. NBS windowless photodiode cathode support 8

Figure 2b. NBS windowless photodiode anode structure 9

Figure 2c. NBS windowless photodiode main body 10

Figure 2d. NBS windowless photodiode cathode support retainer 11

Figure 3. Efficiency of typical NBS windowless photodiode 12

Figure 4. EVSD photodiode 17

Figure 5. Schematic of thermopile and ionization chamber 19

Figure 6. Far ultraviolet detector calibration facility at SURF.. 27

Figure 7. Side view of SURF detector calibration facility 30

Figure 8. Photoelectric loss of gold black thermopile coating.... 32

Figure 9. Schematic of thermopile data acquisition 34

Figure 9a. Schematic of thermopile calibration system 35

Figure 10. Duoplasmatron light source 36

Figure 11. Schematic of thermopile 38

Figure 12. Power supply for duoplasmatron 42

Figure 13. Spatial uniformity of EVSD photocathode 44

vi
I INTRODUCTION

Calibrated transfer standard detectors for the far ultraviolet are available

from NBS to cover the spectral region 5-254 nm. These standards serve to

enable one working with relatively monochromatic radiation in this region to

determine the flux incident at an experiment without knowledge of the spectral

characteristics of the source. Two detector types are available to cover this

range: a windowless photoemissive photodiode for the region 5-122 nm, and a windowed photoemissive photodiode for the balance of the total region. The

detectors have been extensively studied as regards radiometrically important

parameters, such as spatial photocathode uniformity and temporal stability of

photoemission. The probable errors are 8-15% in the 5-122nm windowless

photodiode region and 6-10% in the 116-254nm windowed photodiode region.

All calibrations are based on the rare gas ionization chamber as an absolute

detector. In the windowless region, working standards are calibrated directly

against such a reference, while in the windowed region it is necessary to

transfer the calibration derived from the ionization chamber in wavelength

with the use of a special thermopile. The shortest portion of the windowless

region involves the use of a beam line at the NBS electron storage ring

facility, SURF-II, while the calibrations above 50 nm are conducted using

plasma light sources in a laboratory environment.

Outgoing detectors are calibrated by direct intercomparison with

pre-calibrated working standards, which are periodically recalibrated.

Detectors for the windowless region are fabricated in-house, others are procured commercially.

It is anticipated that in the future all detector calibrations in the far

ultraviolet at NBS may be based on the calculable flux at SURF-II, rather than

rare gas ionization. This will entail the funding and development of a completely new system designed for this purpose, and would likely depend on

the use of the present Beam Line 2 instrument calibration facility at SURF-II

as well.

1
II DESCRIPTION OF SERVICE

a . Background

During the period 1955-1965 there occurred a rapid increase in interest in

scientific activity in the far ultraviolet, primarily due to two developments.

First, advances in both vacuum and optical technology had led to the

development and manufacture of convenient instruments (spectrographs,

monochromators , etc.) with which many new experiments could be successfully

conducted in the far ultraviolet. Second, the accomplishment of operating

spacecraft had provided the opportunity to pursue solar and astronomical

studies beyond the absorption of the earth's atmosphere, hence into the far

ultraviolet

.

With the great increase in far ultraviolet activity came the parallel need for

radiometric capability in this region. The situation became rather obvious

when it developed that experiments orbiting on different spacecraft, but

observing the same phenomena, were recording vastly differing flux levels.

Clearly a common, accurate radiometric base was needed and NBS undertook the

establishment of a program which would lead to transfer standard detectors for

the far ultraviolet.

b. Goals

The far ultraviolet detector radiometry program attempts to furnish transfer

standards capable of determining absolute flux levels in the spectral range

5-254nm. These standards should be relatively stable, simple to use, and

within the typical laboratory budget. Calibrations furnished with these

standards should be state-of-the-art in accuracy. Improvements in accuracy and

stability should be constantly sought and incorporated when possible.

c. Present Calibration Services

Two detector types (described in III) are now available from NBS as calibrated

transfer standards covering the spectral regions 5-122nm (probable errors

8-15%) and 116-254nm (probable errors 6-10%). Users are furnished with the

quantum efficiency of their detector as a function of wavelength (with quantum

efficiency defined as the number of photoelectrons per incident photon). A

typical Report of Test is shown in Appendix A. Recalibration of each type is

available after customer use.

Additionally, special detectors which do not lend themselves to convenient

on-site cross-calibration may be calibrated at NBS if the detectors merit

radiometric application and if the NBS facilities are suitable for the

particular device.

The person to contact in regard to such calibrated detectors is:

Mr. L. R. Canfield

A-251 Physics Bldg.

National Bureau of Standards

Gaithersburg, MD 20899

2
d. Future Directions

It is hoped that eventually all far ultraviolet detector calibration

activities can be conducted at the NBS Synchrotron Ultraviolet Radiation

Facility (SURF-II), using its calculable flux as the radiometric base. This

would eliminate the weakest link in the calibration chain, the thermopile, and

might also lead to the elimination of the need for working standards. Even

before this is achieved there are several areas in which improvements may be

made. Materials studies may lead to windowless detectors with better spatial

uniformity, temporal stability and improved quantum efficiencies. It is hoped

that the short wavelength limit of calibrations can be extended and that the

uncertainties of calibrations at all wavelengths can be reduced.

3
Ill DESCRIPTION OF DETECTOR TYPES

a. The NBS Windowless Photodiode (5-122nin)

The NBS Windowless Photodiode transfer standard detector is the result of

temporal stability and spatial uniformity studies which were conducted at NBS

in the 1960s on several likely choices for a photocathode material suitable

for use in an open vacuum photodiode (1). The spectral region of interest was primarily at wavelengths shorter than the transmission limit of magnesium

fluoride, so as to extend the range covered by sealed photodiodes.

1. Photocathode Material

The material of choice for the photocathode was and is aluminum with the

natural oxide thickness artificially increased. Vacuum deposited aluminum

(99.999% pure) samples on quartz substrates are anodized to increase the

natural oxide thickness for use as photocathodes in NBS far uv windowless

transfer standard photodiodes. The increased oxide ( about 15nm thick)

improves the stability of the phctoyield over that of the deposited aluminum

by preventing any further oxide development, and achieves more complete

absorption of the incoming radiation, thus reducing wavelength-dependent

variations in the photoyield caused by optical interference. (The aluminum has

a very low coefficient of absorption in much of the spectral range and hence

acts like a transparent film bounded on each surface by an absorbing film.) A

description of the method used to accomplish this follows.

The anodizing method used has been described in the literature (a) . The

general layout of the components is shown in Fig.l. A pH 5.5 bath of tartaric

acid is used and the cathode used in the bath is 99.999% purity aluminum.

The bath is prepared by dissolving 3% (by weight) powdered tartaric acid in

distilled water. The pH is measured and adjusted by the addition of small

amounts of either extra tartaric acid or ammonium hydroxide, depending on whether more or less acidity is needed.

A simple Teflon fixture holds the sample in contact with a pure aluminum wire

at the edge of the circular substrate, so that most of the sample may be

lowered into the bath without wetting the contact point. Electrical connection

is made to this wire (+) and to the cathode wire (-) from a well-regulated

power supply which has been preset to 10. 5v and current-limited to about lOma.

The basic procedure is to lower the fixture into the bath until the liquid is

just short of wetting the contact point, then connect the power supply and

leave connected for 1 minute. At this time the cable is disconnected and the

sample removed from the bath. Each sample is rinsed in distilled water to remove traces of the bath, dried by forced air or nitrogen, and finally baked

at about 110 C for at least an hour. Each sample should be inspected for signs

of blistering or any appearance less than a quality reflector and rejected if

such signs are present. (This appearance is usually the result of insufficient

cleaning of the substrate prior to deposition of the aluminum.)

a. G. Hass, "On the Preparation of Hard Oxide Films with Precise Control of

Thickness on Evaporated Aluminum Mirrors", J.O.S.A. 39., 532 (1949).

4
5
2. Photodiode Assembly

Figure 2 shows the configuration of this photodiode. A cylindrical anode is

suspended very near the cathode, with a machined Teflon component providing

support for both. The photocathode is electrically connected to a machined

piece of aluminum on the opposite side of the substrate by aluminum foil.

Provision for physical location of the whole device is made by a threaded hole

in the rear of the machined aluminum piece. This mounting is at cathode

potential and must be well insulated from ground. The entire device is

intended for use in vacuum. A description follows of the steps in the assembly

of a finished photodiode.

The NBS windowless photodiode assembly consists of four fabricated pieces plus

the photocathode/substrate . Figures 2a-2d are the construction sketches from

which the fabricated pieces are made. Not shown is a triangular piece of

kitchen aluminum foil with a hole approximately 1 cm in diameter in the

center. This piece is simply cut from bulk foil so that the corners of the

triangle protrude 2-3mm beyond the cathode substrate.

Final assembly begins by placing the substrate, film side up, on the

triangular aluminum foil previously prepared. The corners of this foil are bent over the edges of the quartz so that they will form redundant conducting

paths to the rear of the substrate. The Teflon body piece is placed with the

internal ledge facing up and the substrate placed on this ledge with the film

side down.

The tapped hole in the aluminum backup piece must be staked before assembly to

prevent a rear mounting screw from being driven into the cathode substrate.

After the photocathode ID number is scribed on the rear of the substrate (in

place), the backup piece is placed on the rear of the substrate, followed by

the securing Teflon ring. The rear of the aluminum backup piece should also be

scribed with the ID number. The final item of assembly is to secure the anode

to the main body with 2-56 stainless steel screws at the three tapped holes

provided. Store the photodiode in an ultra-high vacuum system or face down on

a clean paper towel in a closed cabinet.

3. Operating Characteristics

Incident far uv photons cause the photocathode to emit low-energy electrons,

which are accelerated away by the electric field established by the anode

potential of 60-lOOV. The rate of emission is measured with a suitable

calibrated picoammetej^ The usable range of photocurrents is roughly from

10 to less than 10 amperes. (The^^'dark" current-most ly thermionic

emission-is known to be less than 10 amperes, but has not been measured.)

From a table of quantum efficiencies (electrons per incident photon) given in

the NBS Report of Test which accompanies each photodiode (see Appendix A), the

flux rate at the surface of the photocathode may be derived. Typical

efficiencies range from a few percent at 5nm to a peak value of about 20

percent in the 60-70nm region, then back down to about 1 percent at 122nm (see

Fig. 3). The photocathode surface, being unprotected from outside

contamination, may change in efficiency due to such exposure, so it is

important that potential sources of contamination be recognized and

controlled. One should also ensure that the photocathode is protected from

6
INCIDENT

BEAM

NBS PHOTODIODE (WINDOWLESS)

(5-l22nm)

Figure 2
NOTES:

I . MAT : AL 606

1

2. SCALE 2-1

8

FIGURE Zo
NOTES:

1 . MAT: SS TUBING .0 10 WALL

2. SCALE 4-1

9

F I GURE
J " " "

NOTES:

I . MAT: TEFLON

2. SCALE 2-1

10

FIGURE 2c
1,11 II '

1 1 1 1 III 1 1 1 1 1

' 11

NOTES

I . MAT : TEFLON

Z. SCALE 2-1 FIGURE 2d

n
CM

i I I L

Oo

in -p

E

c

c

0)

0)

>

(0

in

in

CM

» I I r I I I I I |_j [ I I in

o

o

o

12
charged particles during operation, since any such particles arriving at the

photocathode would result in an incorrect assessment of the radiant flux. The

electrical characteristics and operational considerations on the following

page are supplied with each outgoing new NBS photodiode.

13
OPERATING INSTRUCTIONS FOR NB5 MINDQWLES5 FftR UV TRANSFER STANDARD PHOTODIODES

The photodiode has two operating elements: the anode, which is the stainless steel

cylinder projecting from the Teflon body, and the cathode, which is the mirror-like

surface seen through the anode. The cathode consists of a quartz substrate supporting an aluminum evaporated film which has been anodized to an oxide thickness of approximately

150A. The cathode is electrically in contact with the machined aluminum piece having the

central tapped hole.

The anode should be operated at +60V (150&-1216a) or +100V (<150A). Connection may be made

by clip at any of the three screws which support the anode. Connection to the cathode must

be made at the rear 6-32 tapped hole. Care must be taken to maintain a very high

resistance path to ground at this poi nt--Tef 1 on insulation is recommended.

NOTE ^t*****************************************

The quantum efficiency of any open photoemitter may be altered by very small amounts of

surface contaminants. In order to maintain the NBS calibration, it is imperative that the vacuum system in which the photodiode is used be well-trapped and free of contamination.

It is most advisable to minimize the time which the photodiode spends in any oil-pumped

system. Prolonged storage in an ultra-high (oil-free) vacuum system should not in itself

present any problem. However, it has been found that users sometimes overlook the

possibility that a nearby hot filament may deposit evaporated material onto the

photocathode , or that cycling the vacuum system with voltage applied to the anode may

result in contamination by sputtered material.

Charged particles reaching the photocathode will obviously result in spurious current

readings. (A nearby ion gauge may well provide such particles!). If this is suspected, a guard of some sort may have to be used. The linearity of photocathodes of the type

employed has been proven at photon flux levels to about lOEll/sec. Greater flux levels may

require elevation of the anode voltage until plateau operation is obtained.

A significant percentage of the photoel ectrons leaving the cathode will not reach the

anode, hence the photocurrent measured should be the emitted (or cathode) current.

The efficiency of the photodiode at wavelength short of 50A is not known, but the efficiency

at wavelengths >1216A should follow an extrapolation of the curve of q.e. vs wavelength for

several orders of magnitude.

The diagram below depicts the wiring configuration used during calibrations at NBS.

INCIDENT BEAM

ANODE

CATHODE

14
b. Windowed Photodiode (116-254nm)

During the history of the NBS far ultraviolet transfer standard detector

program several suppliers of windowed detectors have been used. Initially

satisfactory photodiodes were obtained from the Stanford Electronics

Laboratory at Stanford University and from EMR Photoelectric in New Jersey,

but in recent years both of these sources have ceased production of

satisfactory devices. We now obtain windowed photodiodes from the Electronic

Vision & Systems Division (EVSD) of Science Applications International

Corporation, which acquired the necessary competence as a result of a NASA

contract to fabricate Digicon detectors for use in spacecraft.

1. Physical Characteristics

The photodiode supplied by EVSD (Fig. 4) is known as their model 54-0-000 and

utilizes a semi-transparent cesium telluride photocathode deposited on the

inner surface of a magnesium fluoride window. The device is fabricated in

ultra-high vacuum with the photocathode being formed remote from the

photodiode body, and the window/photocathode joined to the body by an indium

alloy seal in the same vacuum. The body is made of OFHC copper electrodes

brazed to ceramic spacers, with the center electrode isolating the cathode

from anode leakage, and the rear electrode serving as the anode.

2. Operating Characteristics

A supply of 150v is attached to the anode and a calibrated picoammeter

measures cathode photocurrent . The center electrode js normally grounded.

Maximum photocurrent should be kept at less than 10 amperes and the

magnesium fluoride window must be maintained free of contamination. Frequent

cleaning of the window may also degrade its transmittance , and should be

avoided as a routine measure. The operational instructions on the following

page are included with each new outgoing EVSD photodiode.

15
Instructions for using EVSD Photodiodes

The EVSD model 54-0-000 photodiode incorporates the following materials:

Window: Magnesium fluoride

Photocathode: Cesium telluride (on inside of window)

Window Seal: Indium alloy ( melts at "^700

Tube Body: Alumina and OFHC copper, brazed

The photodiode should be operated entirely in vacuum with the anode +150V,

guard grounded and photocurrent measured in the cathode circuit- The

electrode nearest the window is the cathode. The magnesium fluoride window

should not be cleaned routinely; if contamination is suspected, spectra grade

acetone may be used with degreased cotton as a swab. We recommend storing the

photodiode, when not in use, either in an oil-free vacuum system or face down

in a relatively clean atmospheric environment, but not in an oil-pumped

vacuum system.

Please remember that the window seal is an indium alloy bond, and exposure to

temperatures of approximately 70C will cause seal failure and destruction of

the photocathode.

Maximum total photocurrent should be kept below 1x10—7A. The possibility of

damage from exposure to high intensity pulsed radiation has not been tested.

Exposure to intense radiation at wavelengths short of the 1130A window cutoff

can be expected to induce color centers in the window, which would reduce

total device efficiency at some wavelengths.* The efficiency at wavelengths

above about 3000& can vary from tube to tube; if response to stray light in

this region is important, it would be wise to investigate the particular tube.

The configuration of the photodiode is shown below.

MgF2 WINDOW

PROTECTIVE GLASS RING

INDIUM SEAL

CATHODE PHOTOCATHODE

GUARD

ANODE

16
17
c. Rare Gas Ionization Chamber

The absolute detector which forms the basis for all present far ultraviolet

detector calibrations at NBS is the rare gas ionization chamber. The basic

operational principles and demonstration of the absolute nature of this

detector have been discussed in the literature (2,3,5). The design used at NBS

(seen in Fig. 5b) has two 10.16cm ion collector plates with a shorter guard

plate at the rear to prevent field fringing in the ion collection region. The

chamber which is used at wavelengths short of 58nm has the anode extended to

very near the ion collectors to reduce the field acting on electrons resulting

from ionization events (3). Direct calibrations of photodiodes are possible

from 5-92nm.

d. Thermopile

A windowless thermopile (see Section IV) is used to transfer the capability of

absolute detection by the ionization chamber to longer wavelengths. (In the

region above lOOnm the photon energy is insufficient to ionize any practical

rare gas.) The thermopile employed is an extended junction, windowless device

which uses very thin gold leaves coated with thin gold black to form a detecting area which is effectively much larger than the actual thermocouples.

Such a detector is in no way absolute and must be calibrated by reference to an absolute detector, in our case, the rare gas ionization chamber. Detection

of relatively weak uv energy with a thermal detector is extremely difficult,

and to achieve reasonable signal-to-noise it is necessary to use ac phase-locked techniques. Further discussion of this application of a thermopile detector is given in Section V and in references in the

Bibliography

.

18
in

<D3

iL

19
IV CALIBRATION TECHNIQUES

Although the exact calibration procedures differ in various portions of the

spectral region covered, in all cases the absolute reference standard is the rare gas ionization chamber (2,3). At wavelengths at which the photon energy

is sufficient to ionize a rare gas, calibration of a photodiode can be

accomplished directlyj otherwise a windowless thermopile is used to transfer

the calibration in wavelength. The mathematical treatment of the absolute

calibrations follows.

a. Calibration Principles

1. Ionization Chamber (long wavelengths)

The use of the rare gas ionization chamber in the 50-92nm region is fairly

straightforward, since the light source and grating used in this region

preclude the existence of second or higher orders from the grating, and the

photon energies are insufficient to ionize the appropriate rare gas more than

singly and the electrons resulting from ionization events have insufficient

energy to cause secondary ionization of the gas. The so-called double

ionization chamber is used exclusively in this region, obviating the need for

pressure and temperature measurements, or knowledge of the cross section of

the gas.

The theory is treated in Ref.(2). The fundamental equation from which the

radiant flux may be calculated is:

2

•l

' ^ = e (i, - i^) (1)

example

where ii = ion current (plate 1) 1x10 amp (coul/sec)

^2 - ion current (plate 2) 1x10 amp

-19

e = the electronic charge 1.6x10 coulomb

I = radiant flux entering chamber 6.944x10 photons/sec

The numbers shown in parentheses are examples of a "typical" hypothetical

case.

la. Direct Calibration of Photodiodes

If a windowless photodiode is being calibrated directly by use of the

ionization chamber, the relationship

^ = -rr C2)

20
where Iq = the emissive photocurrent from the photodiode

e = the electronic charge

I = the radiant flux incident on the photocathode

e = the quantum efficiency (electrons/photon) of the photodiode

may be used to determine the quantum efficiency. Combining equations (1) and

(2) gives:

e ^

ll

(If the currents from both the ionization chamber plates and the photodiode

are measured with the same picoaimneter , absolute calibration of the

picoammeter is not required.)

lb. Calibration of Thermopile

There is no need to obtain an absolute thermopile calibration if the response

of the thermopile to incident radiant flux can be determined relative to a stable reference. Such a reference, a mercury battery, is used to do this,

enabling easy relative calibration of the ac thermopile system with the dc

ionization chamber.

The general relationship describing the calibration of the thermopile is:

2 2

s° ^

" R I X e (ii - iz)

I =

- ^ F R £ A

where k = monochromator gas absorption correction

thermopile photoelectric correction

I = radiant flux incident on thermopile or ion chamber

(photons/ sec)

F = thermopile calibration factor

X = wavelength (nm)

R = ratio of thermopile signal to test signal

The subsequent calibration of a photodiode using the thermopile is given by:

l'=^=FR'£X so s = (5)

e F R' £ X

(Here I' and R' have the same meaning as above, but are different values.)

21
2. Ionization Chamber (short wavelengths)

The use of the rare gas ionization chamber in this region is complicated by

the fact that photon energies are high enough to create both multiple

ionization (more than one ion-electron pair per event) and secondary

ionization from electrons with sufficient energy to ionize the gas. There is

also an unrelated complication which the double ionization chamber helps to

deal with: high order impurity in the diffracted light from the gratings used.

Ref.(9) describes a method for using the double ionization chamber to

determine the fraction of the incident light which is first order. Since the

range of photon energies used and the distribution of the continuum of SURF-II

are now different than those described in the reference, the technique now

must be somewhat different.

In order to proceed, certain assumptions must be made. It is assumed that if

there are high order impurities, they are only second order. It is also

assumed that there exists a portion of the region in which there is only first

order (appropriate choices of filters and machine energies can ensure this and

the first assumption)

.

In regions where the fraction of first order is expected to be less than 1,

determination of the incident radiant flux (normalized by use of a monitor) is

made with both high and low pressure ion chamber measurements. The current

from each of two equal length ion collector plates is measured independently

in the high pressure mode (the ratio of currents will be seen to be needed);

pressure and temperature data are measured in both modes. Two low pressure

measurements are made at about 0.010 and 0.020 torr pressure and the

normalization is extrapolated to zero pressure to eliminate any effects of

secondary ionization (which cannot occur at zero pressure). The relationship

which describes the amount of secondary ionization present during the high

pressure measurement is then:

C

.

. - — (6)

where C = secondary ionization coefficient

= radiant flux entering the chamber (zero pressure)

I = " " " " " (high pressure)

H In either pressure mode the flux is calculated from:

^ e (1 - exp{-yk }) ^7)

9

22
where ^ = total ion current

6 = electronic charge

P pressure

T = temperature (C)

y = gas absorption coefficient (per cm)

k = 273.16 P g (T + 273.16) * 760

To determine the fraction of first order "f" at a given wavelength, the above

values of "C" are used with the data from the two ion collector plates (high

pressure) in the relationship:

^

^ m2 (1 - rb)

r (mid - m2b) + m2 - mi (8)

where ^ = exponential term at T,P for primary wavelength

b = " " " " for half wavelength

Cj = secondary ionization correction coefficient at primary

wavelength

C2 = secondary " " " at half wavelength

mi = ci (1 - a)

mo = Co (1 - b)

r

=

Only low pressure ionization chamber data are used to determine photodiode

quantum efficiencies in the 5-50nm region. The equations describing the

contributions to total ion current are:

f Qi i ion current from 1st order = K2 (1 - f) + f Qi Ki

" " from 2nd order . _

"* " ^

(10) ^

k:

23
where f = fraction of total flux which is first order

Ql = absorption term for Ist order

Q2 = " " for 2nd order

e = electronic charge

i = measured ion current

Kit ^2 ~ multiple ionization corrections for 1st & 2nd orders

The relationships for the general case in which the fraction of first order

(f) is less than 1 are:

Radiant flux at primary wavelength:

Radiant flux at half wavelength;

I9 = (12)

e Q2

The above calculations are used, with a prior measurement of the quantum

efficiency at the half wavelength (where f=l) to arrive at the efficiency at

the primary wavelength:

ir. - £0e 1

2

QE at primary wavelength = — (13)

e li

where Iq = measured photodiode current

€0 = photodiode efficiency at second order

> b. Measurement Methods

1. 5-50nm Region

Calibrations in this region are done at the SURF-II facility, which is

described in detail in Section V(a.). The basic technique is to use the

photoionizat ion of a rare gas to calibrate a photodiode which can intercept

the incoming light before it passes through a thin foil (preventing the ion

chamber gas from flowing into the storage ring) into the ionization chamber.

The calibration of the monitor photodiode is transferred to a working standard

which is then used to calibrate outgoing photodiodes by intercomparison.

The calibration of the monitor photodiode is in terms of response per unit

24
flux passing through the ionization chamber foil (as a function of

wavelength). (It is, of course, impossible to operate a photodiode in the

presence of ionization chamber gas.) The calibration of the monitor photodiode

must be transferred to a working standard which is illuminated through the same foil(s) that were used during ionization chamber measurements.

2. 50-122nm Region

In this region calibrations are done using a measurement system (A-251 Physics

Bldg.) incorporating a normal incidence monochromator and a duoplasmatron

light source. This system is described in detail in Section V(c.). Absolute

calibrations of a working standard windowless photodiode are done by direct

interchange with a rare gas ionization chamber in the region 50-92nm. Two

additional wavelengths, 102. 6nm and 121. 6nm cannot be done using the

ionization chamber directly, so the thermopile technique described in the next

Section is applied directly to a working standard at 102. 6nm, and the 121. 6nm

point is gotten from intercomparison with a windowed standard. It is not

necessary to correct for any of the sources of spurious ionization chamber

data described in a. since the photon energy is always less than twice the

ionization potential and there is no possibility of second order contamination

in the exit beam (due to the use of a line source and a monochromator grating

with very low efficiency at wavelengths below 50nm) . After a working standard

has been calibrated, it may be used to calibrate outgoing photodiodes by

direct intercomparison.

3. 116nm-254nm Region

Calibrations in this region require the use of a thermopile to transfer the

calibration from the ionization chamber (which cannot be used at wavelengths

longer than 102nm) to windowed photodiodes (which cannot be used at

wavelengths shorter than 113nm). Early studies proved that the appropriate

thermopile appears to have the same sensitivity (probable error 3%) throughout

the spectral region 58-92nm (4,5). Therefore it is proper to calibrate such a thermopile at ionization chamber wavelengths (several should give equal

sensitivity), and then use the calibrated thermopile to calibrate a photodiode

at longer wavelengths. This basic procedure is described in detail in Section

V(b.).

As an additional check on the thermopile-derived results, a low pressure Hg arc filtered lamp is used to determine the quantum efficiency of photodiodes.

This lamp was calibrated by the Radiometric Physics Division by techniques

traceable to blackbody radiometry, and is used as a single wavelength

(253. 7nm) irradiance source, with the irradiance specified at 2m distance from

the source. Two such calibrated lamps are available.

Outgoing calibrations of photodiodes are accomplished by intercomparison of

known and unknown, measuring the ratio of photocurrents in a constant

intensity monochromatic beam from the monochromator mentioned in b.

25
V CALIBRATION SYSTEMS

a. SURF-II Detector Calibration System (5-50nm)

HISTORY

The capability of calibrating transfer standard photodiodes at wavelengths

requiring grazing incidence optics was established at NBS in the mid-seventies

making use of the SURF-II electron storage ring synchrotron radiation

facility. This facility served as an extension to shorter wavelengths of the

program which was already in place, in which both windowless and windowed

transfer standards were made available to interested users. In both facilities

the absolute reference detector was a rare gas ionization chamber.

Initially the SURF-II facility was used at wavelengths from about 20-50 nm(6);

eventually the range was extended to 5 nm. The transfer standards which were

calibrated and issued were exclusively of the windowless NBS design, but on occasions the SURF-II facility was used to calibrate other special detectors

for special needs of users (7,8).

In 1983 the existing facility was dismantled to make way for an incoming

experiment, and the construction of a new apparatus was begun. A new, dual

toroidal grating monochromator optimized for the 3-60 nm region was designed

and constructed, as were the toroidal gratings for it. Also designed was a new

experimental system incorporating the experiences of the original SURF-II

detector facility. The new system was to provide greater flux levels at the

experiment, coverage to shorter wavelengths, mo^e accurate results, and faster

throughput of calibrations.

EXPERIMENT

The basic configuration of the experimental system is shown schematically in

Fig. 6. A pneumatic gate valve isolates the calibration facility from the

storage ring. The central portion of this valve is fitted with a window to

allow transmission of visible light in the closed position as an aid in system

alignment. A pneumatically actuated shutter intercepts the incoming beam, and

manually adjustable vertical and horizontal masks limit the illumination of

the monochromator grating as appropriate. Detailed operating instructions are given in Appendix B.

Monochromator

A sine bar drive monochromator with two interchangeable toroidal gratings is

used. Dispersion is in the vertical plane, with the center 2 meters from the

electron orbit tangent point. The angle of incidence at zero order is 83.5

degrees. The ruled gold gratings may be interchanged manually from outside the

vacuum housing. At the shortest wavelengths (3-20 nm) a 1200 line/mm grating

is used; a 300 line/mm grating is used at longer wavelengths. The spectral

scanning range of the monochromator is from slightly below 0 to 20 nm (1200/mm

grating) and ultimate resolution is .02 nm. The image of the electron beam is

focused on an axially adjustable exit slit, with interchangeable prealigned

slits available. A small pneumatic gate valve isolates the monochromator from

the experimental chamber portion of the system.

26
27
Foil Chamber

A bellows couples the monochromator assembly to the balance of the system,

with a 4mm diameter capillary limiting gas flow toward the monochromator.

Radiation emerging from the capillary may be intercepted by a windowless

monitor photodiode (of the same type that is ordinarily calibrated) with

interchangeable polypropylene or aluminum filters. This assembly (not shown in

Fig. 6) provides the intensity level reference for all calibration activities

via photocurrent from the photodiode.

Ion Chamber

When the monitor detector and filter are not in the beam, radiation falls into

the rare gas ion chamber through one of two manually interchangeable filters

(as above) which also serve as gas seals during ion chamber activities, when

the gas pressure may be as high as 2 torr. These filters may be interchanged

from outside the vacuum system. When non-ion chamber measurements are in

progress, the chamber plates remain in place and have, in the absence of

interacting rare gas molecules, no effect on the radiation beam.

... Intercomparison Module

At the rear of the apparatus is an easily removable flange containing an internal mounting wheel for up to 6 photodiodes, which may be used to make

intercomparison measurements. Electrical contact to the photocathode in the

beam is made by an external pneumatic actuator, allowing photocurrent from

this detector to be measured.

Data Acquisition

A Digital Equipment LSI-11/23 computer and CAMAC interface modules accomplish

most data acquisition activities remotely from the SURF-II control room. Stepping motors control both the wavelength drive and diode intercomparison

wheel rotation, with encoders monitoring shaft positions. Pneumatic actuators

control the shutter, monitor detector and filter positions, and photocathode

contact at the intercomparison wheel. Status switches monitor the positions of

shutter, monitor detector and filter, gas seal filters, and photocathode

actuator. The SURF beam current monitor line is connected to enable

integration periods to be normalized regardless of beam intensity level.

Incoming data from either emissive photocurrent or ion current are converted

to pulses whose frequency is proportional to the current intensity. The pulses

are then counted for the period determined by reference to the house beam

monitor. Three electrometers are used, with each calibrated daily using a standard current source. Acquisition programs are written in BASIC.

The concept of system operation is that the monitor diode package (diode + filter) monitors the beam intensity and provides a reference for all ion

chamber measurements. In other words, the package response is related to the

radiation passing through the ion chamber filter. However, when the

calibration of the package is transferred to a diode on the rear wheel, the

same filter is in the beam, so it is thus possible to have knowledge of the

magnitude of radiation reaching the rear detector, and to arrive at an absolute calibration of the detector. (The calibration of the monitor package

is, of course, only relative.)

28
VACUUM SYSTEMS

The vacuum components associated with the SURF-II detector calibration system

are shown schematically in Fig. 7. Pumping in the monchromator section of the

apparatus is accomplished by two 110 1/sec triode ion pumps, with an open

cycle liquid helium cryopump over a gate valve available if needed. A nude ion

gauge monitors pressure and a residual gas analyzer head is resident for

SURF-II beam line acceptance studies. Beneath the foil chamber is a closed

cycle cryopump with an isolation valve and ion gauge. The diode

intercomparison/ion chamber region is pumped by another 110 l/sec triode ion

pump. Both this region and the foil chamber have ion gauges. A dedicated

roughing stand consisting of carbon vane mechanical and liquid nitrogen

sorption pumps is connected to the system through a valve, as is the rare gas

supply system.

b. Thermopile System (100-320 nm)

1. Introduction

At wavelengths below 102 nm, a rare gas double ionization chamber, which

will count individual absorbed photons by the ion-electron pair produced in

each absorption, can be used as an absolute photon detector (2,3). Working

standard photodiodes in this wavelength range can therefore be calibrated

using this ionization chamber (although they will not operate properly in the

gas environment of the ionization chamber)

.

At wavelengths above 102 nm, the photon energy is not sufficient to ionize

xenon, the rare gas with the lowest ionization potential (12.13 eV) . In this case a method must be found for transferring the absolute calibration of the

ionization chamber to the longer wavelength range. Thermal detectors measure

the heating of an absorbing element in proportion to the power of the incident

radiation and have a response that is generally independent of wavelength.

Thus a thermal detector can be calibrated against an ionization chamber in the

short wavelength range and then used in turn to calibrate a working standard

photodiode in the long wavelength range.

2. Sources of Error

Thermopiles in particular have been studied in the Far UV Physics group as potential standards in the 100-300 nm range (4,5). The assumption that a thermopile will develop a given voltage for a specific level of incident

radiation, independent of wavelength, was tested with regard to four possible

sources of error:

1) The thermopile may not absorb the same percentage of incident photons

independent of wavelength.

2) The thermopile may have energy carried away by photoejected electrons.

3) The thermopile may have a wavelength-dependent time constant when a.c. response to chopped radiation is measured. This effect could cause wavelength-dependent variations in a.c. sensitivity.

4) The thermopile may have a wavelength-dependent spatial sensitivity.

29
ITOD

in (T L

30
Possibility (1) was checked for two typical gold blacks used on a Reeder

thermopile. The reflective scattering at three wavelengths in the vacuum UV

was compared to that in the visible and near infrared and found to be the same within 1% of the incident intensity.

Possibility (2) was tested by measuring the thermopile signal with an electric

or magnetic field either switched on to return photoejected electrons to the

thermopile or switched off to let them escape. A correction curve (Fig. 8)

for the region 58-170 nm was derived to take account of this loss mechanism.

Possibilities (3) and (4) were tested by scanning a thermopile through a narrow beam in the far uv and visible with both dc and ac techniques. Both

possibilities were found to have a negligible effect on the results.

The issue of whether such a thermopile indeed had wave length- independent

sensitivity was investigated by calibrating one both in the far uv (with a rare gas ionization chamber as an absolute detector), and in the visible/near

infrared (using an NBS carbon filament lamp calibrated for total irradiance)

.

The comparison agreed to about 3%, using the appropriate photoelectric

corrections for the far uv data.

3. Calibration Technique

After the demonstration that the thermopile is a reliable detector at longer

wavelengths when calibrated by an ion chamber below 100 nm, a UV spectrometer

and detector chamber were built to perform UV calibrations on working standard

diodes from 100-300 nm (9). The detection system used ac signal detection and

processing, since the magnitude of the thermopile emf in the far uv is very

weak compared to changes in the background blackbody radiation, which is also

detected by a thermal detector in a dc mode.

Vacuum UV light is obtained from a 1-m normal incidence grating monochromator

,

equipped with a duoplasmatron light source, A 13 Hz chopper, located between

the light source and the entrance slit, is used during thermopile measurements

to interrupt the light entering the monochromator. Attached to the exit arm of

the monochromator is a vacuum chamber which houses the ion chamber and

thermopile

.

The Reeder thermopile used is of the extended junction variety, employing

three such junctions to give a sensitive area 1 mm x 6 mm. It is essentially

identical to the one used in the earlier study of thermopiles mentioned above.

A rectangular mask with an opening slightly smaller than the area of the

thermopile is used. A three point mounting, designed kinematically for

accurate relocation, positions the thermopile relative to the surface of the

mask, about 0.5 mm from its surface. The thermopile signal is amplified by a 13-Hz amplifier, rectified synchronously with the chopping frequency and

recorded using calibrated picoammeters . Net emf readings are taken and

referenced to a stable ac test voltage that is applied across the thermopile

in the absence of radiation. Immediately following thermopile data

acquisition, the thermopile is rotated away from the mask, and the ion chamber

or diode being calibrated is exposed to the monochromatized light. The ion

chamber or diode currents are then measured. (A gas manifold and set of

pneumatic valves are programed to initiate the appropriate flow of gas before

31
32
the ion chamber current measurements are made. After the measurements the gas

is pumped away.) The thermopile is then rotated back onto its kinematic mounts

behind the mask for another measurement. Alternating measurements are made in

this way until enough data have been accumulated to overcome the

signal-to-noise problems of the thermopile in the far UV.

An additional, independent calibration of the standard diode is made at 253.7

nm. At this wavelength a stabilized low pressure Hg lamp, calibrated for

spectral irradiance at this wavelength by blackbody radiometry techniques, is

used as a spectral irradiance standard. The diode quantum efficiency is

determined from the known flux falling on the diode and the measured diode

current

.

4. System Description

Detailed operational instructions for the thermopile system are given in

Appendix C. The system as it is presently operated, shown schematically in

Fig. 9 and 9a, consists of three distinct parts:

1) Duoplasmatron light source

2) Grating Monochromator

3) Detector chamber

(1) The duoplasmatron light source is a three electrode device, composed of a Pt mesh filament coated with an emitting material, a baffle electrode, and an anode (see Fig. 10). A low pressure arc discharge is constricted by the

funnel-shaped baffle and the anode. An axial magnetic field produced by a cylindrical permanent magnet surrounding the outer shell of the light source further constricts the discharge to a narrow plasma beam along the axis. He is

used for 58.4 nm, Ne for 73.6 nm, Ar for 92.0 nm, and hydrogen for the 116-320

nm region. The first three gases are used in the ion chamber-thermopile

calibration, and hydrogen for the standard diode vs. thermopile calibration.

The major arc currents are normally 1.5 A for the rare gases, and 1.1 A for

hydrogen.

A chopper blade is located between the light source and the monochromator

entrance slit, and is driven in a reciprocating motion through a bellows by a small motor immediately below the light source housing. A flap valve, operated

by a stepping motor, is placed after the chopper blade and can be rotated

either to seal off the light source from the monochromator, to block the light

but allow pumping of the light source through the entrance slit, or to let the

light enter the monochromator. The entrance slit is vertical and has a micrometer adjustment of 200 microns per turn.

(2) The grating monochromator is a 1 meter normal incidence monochromator with

a spherical grating of 600 lines/mm and a dispersion at the exit slit of 1.67

nm/mm. The dispersion plane is horizontal. The monochromator is pumped by a 6"

oil diffusion pump with a freon-refrigerated baffle to reduce oil

contamination in the monochromator chamber. A pneumatic gate valve above the

freon baffle is electrically connected to a pressure-sensing relay, and closes

automatically if the system pressure rises above a preset value. This

33
cr 1 o LU o

-J o

cr Q_ LU

LU ZD 1- X O LU

h- o Q

cro«(D

crro(DZ<oE

cr

LU

LlCO —1 CL CL o

ro < o >- cr

1- o LU

FAGE

QUEN VERT

-J LU o cr

O > o

34


arrangement protects the system from unforeseen pressure bursts or leaks. A

separate roughing valve allows the monochromator to be pumped down from

atmospheric pressure without turning off the diffusion pump.

(3) The detector chamber contains the thermopile in its rotatable mount, and

has provision for placing the double ionization chamber or the diode to be

calibrated in the light beam passing through the exit slit (see Fig. 5). The

exit slit is a fixed vertical slit 0.8 mm wide, corresponding to about l.Bnm

band width.

The thermopile (Fig. 11) is a series connection of three elements which are composed of bismuth-tellurium and bismuth-antimony alloys. The thermal

junctions are extended using gold flakes of 1 mm x 2 mm area. Together they

form a sensitive area 1 mm x 6 mm. The thermopile is compensated, designed for

a 13 Hz chopping frequency and constructed entirely of non-magnetic materials.

The time constant of the thermopile is about 0.04 sec, and when calibrated in

the visible had a sensitivity (in vacuum) of 1 microvolt/microwatt and an ENI

(equivalent noise input) power of approximately 10 W. An impedance-matching

transformer is used between the thermopile and a preamplifier.

The signal from the preamplifier is fed to a 13 Hz broadly tuned amplifier.

The output of this amplifier is rectified synchronously with the chopping

frequency by a mechanically coupled rectifier. The signal is then filtered,

further amplified, and fed into a V-F converter. A small computer is used to

record the data and perform statistical evaluation of the results.

A flap valve is mounted 6 cm in front of the mask, and when closed will allow

the detector chamber to be let up to atmospheric pressure without affecting

the rest of the system. The rear section of the detector chamber is bolted to

the rest of the chamber using a Viton gasket as a vacuum seal. Entrance to the

detector chamber is obtained by removing this rear section.

The thermopile is attached to a rotatable mounting so that, under computer

control, it may be brought into registration with the kinematic mount, or rotated between the plates of the ion chamber out of the beam.

The ion chamber used is a double ion chamber similar to that described by

Samson (2)(see also Section III). The length of each collector plate is 10.2

cm, and the forward end of the first plate is positioned in the plane of the

mask. The axis of the light beam is 0.8 cm from the positive plate, which,

with the mask, is kept at +22 V with respect to the collector plates. The ion

chamber assembly is a self-contained unit with Teflon bars used as a framework

to support the various polished stainless steel plates. The assembly is

positioned behind the mask reproducibly by a locating screw and machined

recess in the floor of the chamber.

The thermopile is calibrated against the ion chamber in the following manner:

The thermopile housing is rotated into position against the (grounded) mask.

With the source chopper operating but the light source blocked by the flap

valve, the signal is recorded by the 13 Hz detection system. Five consecutive

measurements of thermopile background are made. The thermopile is then

connected to a chopped stable test voltage (normally a 1.35 V. mercury

battery) and the thermopile response to this ac test signal is measured. This

measurement normalizes the thermopile response from one set of measurements to

another.

37

The thermopile is rotated down, clear of the ion chamber plates, and the

chopper stopped. Argon is admitted to the ion chamber for 58.4 and 73.6 nm

measurements, and xenon for the 92.0 nm measurement. The gas pressure is

adjusted to give a current ratio between the first and second plates of about

10:1. This pressure in the detector chamber is about 130 microns for Ar and 50

microns for Xe. When the the ion chamber currents have been measured, the gas

flow is shut off by computer-controlled solenoid valves, and the gas is pumped

out at the exit slit by the 6" diffusion pump. When the detector chamber

pressure reaches about 20 microns, a pneumatic gate valve opens to a 2"

diffusion pump directly under the chamber, and the remaining gas is pumped

away.

The thermopile is then raised into position by a computer-controlled stepping

motor, and the chopper started. The flap valve in front of the light source is

programmed through another stepping motor to block the light from the

duoplasmatron for thermopile background measurements, and to open for signal

measurements. After five background and five signal measurements, the

thermopile is lowered again by the stepping motor.

The ion chamber measurement is repeated and the process continues until three

ion chamber measurements are sandwiched around two thermopile measurements.

The thermopile test signal response is then remeasured.

The following corrections are applied to the data as it is recorded (i.e. in

real time)

:

1) The thermopile signal is corrected for losses through photoemission,

amounting to 4.4% at 58.4 nm, 5.3% at 73.6 nm, and 5.1% at 92.0 nm (see

Fig. 8). 2) The reduction in flux entering the ion chamber due to absorption

by the argon or xenon leaking into the monochromator through the exit slit is

corrected for by making a separate measurement of attenuation vs. monochromator pressure over a range of pressures. Thus by monitoring the

monochromator pressure the appropriate correction, typically 4%, can be

applied. (Note that the ion gauge attenuation calibration must be checked

frequently.)

The average flux from two successive ion chamber measurements is compared to

the thermopile signal between them to give a thermopile response factor. The

response factors from the two thermopile measurements are compared to make

sure no drifts have occurred during the entire measurement, which takes about

60 minutes. These measurements are repeated several times at each of the three

wavelengths and the results averaged. The response factors of the thermopile

are typically within a few percent of each other over a set of measurements.

However long-term drifts can reach 25% over several years. (This number does

not contribute to the total error budget, since the thermopile is recalibrated

each time a photodiode is to be calibrated, and only short-term drift is

important .

)

When the thermopile response has been calibrated by the ion chamber

measurements, the detector chamber is let up to air with the flap valve

closed. The ion chamber is removed through the rear section, and the diode to

be calibrated is mounted on a fixture and positioned in the path of the flux

through the exit slit. The chamber is then roughed down and the gate valve

above the 2" diffusion pump opened to pump the chamber to high vacuum.

39
The MgF2~windowed diodes are calibrated against the thermopile at 23

wavelengths from 116.4 to 253.7 nm. The light source gas is hydrogen over the

entire wavelength range. From 116.4 to 160.8 nm, the hydrogen line spectrum is

used. Above 160.8 nm the molecular hydrogen continuum light is used. Also, a quartz filter is placed in front of the light source when wavelengths longer

than 160.8 nm are used to prevent second order radiation from reaching the

thermopile and diode.

For the 116.4 to 160.8 nm range, the light source slit opening is set to 50

microns. After the quartz filter is inserted, the slit must be opened to about

200 microns for adequate light intensity.

At each wavelength the thermopile test signal response is measured first, just

as in the ion chamber measurement. The thermopile is rotated down and the

diode background current and signal current are measured. The light source intensity is adjusted to keep the diode current below 3x10 A. The

thermopile is then rotated back into position and the background and signal

measurements are made as before. In this manner three diode and two thermopile

measurements are made, and then the thermopile test signal response is

remeasured. From each pair of diode measurements around a thermopile

measurement and the thermopile response factor determined from the ion chamber

measurements, the diode quantum efficiency is determined.

If the two results are in satisfactory agreement, the spectrometer wavelength

drive is scanned manually to the next wavelength. Each set of measurements

takes about 40 minutes

.

c. Photodiode Intercomparison System (50-254nm)

History

In 1968 funds became available for the purchase of the necessary

instrumentation to build a system whose primary purpose would be to calibrate

outgoing transfer standard detectors by intercomparison with NBS working

standards which had been calibrated by the already existing thermopile system

(4). Initially this system lacked any control or data reduction capabilities;

this was added in the early 1970s and upgraded in an evolutionary fashion to

the present configuration, in which nearly all data acquisition and reduction

functions are automated. With this evolution catae the additional capability of

performing absolute calibrations of windowless photodiodes in the 50-92nm

spectral region, which greatly expanded the coverage possible and considerably

reduced the time required. In the present form, intercomparison-type

calibrations are conducted from 50nm to 254nm, and absolute calibrations from

50nm to 92nm.

Experiment

The basic configuration of the experimental system is very much the same as that of the thermopile system, (see Fig. 9a). A duoplasmiatron light source illuminates the entrance slit of a normal- incidence vacuum monochromator , and

the experimental chamber is attached to the exit slit flange. Various

experiments are permanently attached to modular flanges, which can be

exchanged on the otherwise empty chamber. Data acquisition and reduction is

under the control of a programmable calculator. Detailed operational

instructions and descriptions are included in Appendix D.

40
Monochromator

A one meter normal- incidence McPherson model 225 monochromator is used with

its own vacuum pumping system. A 15cfm mechanical forepump is used for both

rough pumping and for backing a 7" diffusion pump, which uses DC705 silicone

pumping fluid. A refrigerated chevron trap and pneumatic gate valve are attached to the top of the pump. A zeolite molecular sieve trap is used in the

foreline to minimize the migration of forepump oil vapors into the high vacuum

system. Both exit and entrance slits are laterally adjustable, and both the

exit and entrance flanges may be isolated from the monochromator vacuum by

manual flap valves. The diffraction grating is a 600/mm replica, with nominal

blaze at 150nm. The dispersion at the exit slit is thus 1.67nm/min. A modified

sine-bar drive provides wavelength scanning by both rotation and translation

of the grating.

Light Source

A duoplasmatron source, fabricated in-house, is used for all spectral regions

in this system. A manifold allows the introduction of any of several gases as appropriate for the emission desired, and a roughing line is attached to the

foreline of the experimental system. This type of source uses a hot filament,

three electrodes and a pinching magnetic field to create a rather dense plasma

which is on the optic axis of the monochromator. Because it is only about

1.5mm in diameter and is several cm from the entrance slit, only the central

portion of the grating is illuminated by the plasma. Gases routinely used at

wavelengths short of lOOnm are He, Ar, Kr and Ne. For wavelengths longer than

lOOnm hydrogen is used. Cooling of the source is by forced air both in the area where the plasma strikes the anode and around the exterior of the

filament end. Separate unregulated power supplies are used for the filament

(ac) and the minor and major arcs (dc). A cross section of the duoplasmatron

is shown in Fig. 10, a schematic of the power supply is Fig. 12.

Experimental Chamber

A cylindrical stainless steel chamber with provision for rear mounting of

modular flanges is attached to the exit slit flange. This chamber is provided

with an independent vacuum system, consisting of a 2" pneumatic gate valve

over a refrigerated chevron trap over a 2" diffusion pump using polyphenyl

ether fluid. The 2" pump is backed with a trapped mechanical forepump, which

is also used for roughing. The usual valves isolate and protect. Experiments

are attached to the chamber mounted on a flange, which attaches to the rear of

the chamber.

Experimental Modules

Three such modules are routinely used: an intercomparison module, a mapping

module and an ion chamber module.

The intercomparison module contains a rotating wheel, on which two photodiodes

may be mounted for intercomparison measurements. In general, the photocathodes

of the two photodiodes are shorted to ground, the exception being that when

either is in the beam, its photocathode is connected by means of a gold wiper

41
aa3

0)

k.$o

Q.co

(0E

(0«ao3Q

csi T0)

Ulo

a.<

42
to a vacuum feedthrough. The anodes are tied together and at all times

connected to another feedthrough. Provision is made for rotational relocation

of either photodiode about its center, and quartz or Vycor filters may be

manually inserted in the beam to aid in eliminating overlapping orders from

the grating.

The mapping module flange has orthogonal linear motions on the atmospheric

side driving a shaft going through the flange with a bellows seal. The shaft

is supported inside the vacuum by a gimbal mount, so that displacement of the

outside end of the shaft will result in a proportional displacement of the end

inside the vacuum. Thus an object may be scanned in space inside the vacuum

system in a controlled manner. The normal applications of this module would be

either to map the response of a photocathode (with the beam stopped by a fixed

small aperture) or to map the intensity distribution of the beam (with the

small aperture attached to a photodiode). The external motions can be driven

by stepping motors, and the exact position may be monitored by linear

displacement transducers. An example of the normalized result of such a scan

is seen in Fig. 13.

The ion chamber module is used to directly calibrate a windowless photodiode

by means of a rare gas double ion chamber(2 ,3) . Attached to a rotary motion on

the vacuum side of the flange are an ion chamber with two 10cm plates with a guard plate and a photodiode. Either one or the other may be placed in the

beam by a stepping motor. As this module is placed into the experimental

chamber a small Teflon flange near the exit slit provides a seal so that the

only leakage path for gas from the chamber into the monochromator is through a small aperture in the exit beam. High purity Ar and Xe are individually

brought into the chamber on demand with needle valves limiting the flow so as to balance the influx and leakage into the monochromator and give the desired

gas pressure. The gate valve beneath the chamber may be program controlled to

open and remove the residual rare gas before photodiode readings are taken and

to close so that fresh gas may be admitted before making ion current readings.

Data Acquisition

Almost all system operations are controlled by a programmable calculator with

an external CPU and interfaces to drive stepping motors, actuate solid state

relays and hence valves, etc., switch between sources of very low current or analog voltages and actuate and read a counter-timer . Analog voltages are converted to proportional pulses, which are counted for fixed periods by the

counter-timer. The control programs also contain the data reduction routines,

so that the final results of an experiment are available at the conclusion of

the data acquisition sequences. Storage of programs or data is on magnetic

cards, each card holding .5K bytes. Printout is on paper tape.

43
3S.7 3B.Q 3B.B 3B.-^ 33.

E

33.21 33.4 33.

E

11212

3B.G SB.B 3G.3 3B.B 3B.B 33.7 3B.3 33.4 33.

B

3 3.3

B.^ 3B.B 3S.0 3B.I21 3B.B 3B.B 3B.3 33.3 33.3 3 3.7

B.E 3Q.B 37.3 3B.IZ1 3B.Z 33.

B

33.B 33.7 33.

B

3 3.3

B.B 3Q.B 3S.3 3B.5 3B.3 33.

B

33.^ 33.7 33.7 3 3.B

9B.E 3Q.B 3Q.5 3B.E 3B.E 33.1 35.-=} 33.E 33.E 33.

E

3B.I2] 3B.B 33.

S

3B.3 333 33.12 3 3.B 33.3 33.E 3 3.5

3B.^ 3Q.7 3B.B 3B.B 3B.7 3 3.1 33.3 33.3 3 3.B

3B.^ 3B.-^ 3Q.3 3 3.1 33.3 33.3 3 3.B

3B.^ 3B.B 3B.2 3B.B 33.CZ) 33.-^ 33.4 33.12 33.1 33.E

\= 121, 6nm

SPATIAL UNIFORMITY-CENTER OF EVSD PHOTOCATHODE

Figure 13

44
VI TREATMENT OF ERRORS

Uncertainties associated with the measurement of quantum efficiencies are reported in each Report of Test as "probable errors", meaning that there is a probability of 0.5 that the true quantum efficiency values lie within the

stated error range about the values quoted. The values represent the expected

accuracy of the sample photodiode when properly used in a customer's system,

without regard for any systematic errors present in that system. Each Report

of Test includes a Discussion section, in which the user of the sample

photodiode is cautioned that the temporal stability of the efficiency cannot

be positively predicted, particularly in the case of the windowless

photodiodes with their air-exposed photocathodes

.

The probable error values are determined by summation, in quadrature, of

estimates of the effect of all sources of systematic errors in the NBS

measurements, including the precision of repeated measurements, on the

determination of the quantum efficiency of a photodiode. The analysis of the

calibration of windowed photodiodes in the thermopile system follows.

THERMOPILE-BASED CALIBRATIONS

Error Source Estimated Uncertainty(%)

Calibration of thermopile (58-92nm)

;

ion current (electrometer manufacturer) 2

gas absorption (estimated) 1 photoelectric correction (estimated) 0-1

measurement precision (observed) 3

radiation impurity (estimated) 0-0.5

Photodiode calibration by thermopile (116-253nm):

photocurrent (electrometer manufacturer) 2

photoelectric correction (estimated) 0-1

cathode uniformity (observed) 2

thermopile wavelength sensitivity variation

(estimated) 3

measurement precision (observed) 2-8

radiation impurity (estimated) 0-1

Probable Error(%) 6-10

The comparison calibration of a thermopile involving a standard lamp and the

ionization chamber cited on p. 31 was a test of the wavelength independence of

the sensitivity of a thermopile (far ultraviolet vs near infrared) and is not

routinely done, but it did build confidence in the ability to produce

meaningful data from an ionization chamber calibration of a thermopile. The

agreement found is not, however, directly relevant to these analyses.

Cross-checks of the quantum efficiency of photodiodes using a calibrated

mercury lamp (253. 7nm) cited on p. 33 serve as a test of the accuracy of the

thermopile calibration results at the single wavelength where such a comparison is now possible.

45
In the above sources of possible error, those associated with current

measurements are due primarily to the probable error of the current source used to calibrate the measuring picoammeters . The gas absorption uncertainty

involves the repeatability of ion gauge measurements (the calibration of such

gauges has been observed to drift in time). The photoelectric correction is an empirically determined quantity (see p. 31) which may vary from the samples

originally measured. (Since this correction is zero at many wavelengths above

the range of the ion chamber, the uncertainty of the correction is a function

of wavelength.)

The cathode uniformity is measured on each photodiode and units with more than

5% variation in pixel spatial response over the central 1 cm diameter area are rejected. (Should a photodiode be found to have a discontinuous uniformity

function, even within the 5% acceptance criterion, it would not be issued as a transfer standard.) The portion of the measured area actually used in the NBS

measurements is the central 4x5mm section, and the intensity distribution in

the incident beam is relatively uniform over much of this area. The ultimate

user of a photodiode will also illuminate the central area of the

photocathode , but with a generally different intensity distribution and,

perhaps, a somewhat larger area illuminated. This would result in an error in

the transfer of the NBS calibration, but not approaching the peak variation of

5% allowed in device acceptance. Our estimate is a probable error of 2% from

this source.

The variation of thermopile sensitivity (with wavelength) is an estimate based

on the variation seen within the 58-92nm range and the cross-calibration of

detectors using a calibrated Hg source (Section IV). The measurement precision

(repeatability) is a function of wavelength in the above case in the upper

portion of the windowed photodiode calibration range because of deteriorating

signal-to-noise in the thermopile measurements as the efficiency of the optics

declines and the intensity of the beam is thus reduced.

The above example covers the case of uncertainties in the calibration of

windowed photodiodes. The probable error currently being quoted for windowless

photodiode calibrations is 8% in the 50-122nm region and 10-15% in the 5-50nm

region. These errors are determined from the following analyses.

WINDOWLESS PHOTODIODE CALIBRATIONS AT SURF (5-50nm)

Error Source Estimated Uncertainty(%)

gas cross sections (data reference) 3-4

multiple yield (data reference) 0-1

gas temperature (estimated) 0-0.5

gas pressure (transducer manufacturer) 0-3

photocurrent precision (observed) 2-6

ion current precision (observed) 5-7

radiation impurity (estimated) 8-12

Probable Error(%) 10-15

(Note that the individual errors are wavelength dependent, and therefore the

larger quadrature sum is less than the sum of the largest uncertainties.)

46
In the above analysis, the cross section and multiple yield data are taken

from published sources with interpolation between published wavelengths where

necessary. Since, in this region, the reduction of ionization chamber data

entails the use of gas constants, including the pressure and temperature, the

contribution to the error budget from these must be included. Errors arising

from current measurements are primarily from the variance within sets of three

consecutive conversions of currents permitted in the acquisition program. The

radiation impurity contribution can only be estimated from observation of

system performance, and could include improper assessment of overlapping

orders from the diffraction gratings, stray light from masks, baffles, walls,

etc., the "zero order tail" from the gratings, and imperfections in the

filters used to minimize this source of error. This error would be wavelength

dependent, of course.

WINDOWLESS PHOTODIODE CALIBRATIONS (non-STJRF, 50-122nm)

Error Source Estimated Uncertainty(%)

photodiode current (estimated) 2

ion current (estimated) 2

gas absorption correction (estimated) 0-1

measurement precision (observed) 3

stability of quantum efficiency (estimated) 7 radiation impurity (estimated) 0-1

Probable Error(%) 8

All current measurements in this region are conducted with a single

picoammeter, and since it is the ratio of photodiode to ion chamber currents

that is used to calculate quantum efficiency, an absolute calibration of this

picoammeter is not necessary. The estimated uncertainties associated with

current measurements would be largely due to ionization of residual gases and

other spurious effects. The gas absorption correction is necessary only during

ion chamber measurements to account for the radiant flux absorbed in the

monochromator (typically 10% or less) so the likely error from this source is

minimal. The stability of the quantum efficiency of working standard

photodiodes is felt to be a possible source of error only in the longer

wavelength region (50-122nm), since in the 5-50nm region (at SURF-II) the

photodiodes are calibrated and stored in ultrahigh vacuum, whereas in the

other facility calibrations are in an oil-pumped system, and storage is in

air.

VII SAFETY PRECAUTIONS

The systems used in the far uv detector calibration program present no unique

safety hazards beyond those encountered in a typical laboratory environment.

At the SURF-II facility liquid nitrogen is frequently used in vacuum pumping

applications, and in both the thermopile lab and the photodiode

intercomparison lab hydrogen gas (at very low pressure) is used to supply the

light source over part of the wavelength ranges. Standard prudent care in the

handling of these substances is important.

47
VIII CUSTOMER INTERACTION

Interaction with potential users of NBS far uv detector services generally

leads to the supplying (by NBS) of calibrated transfer standard photodiodes or

in the subsequent recalibration of them. In response to inquiries regarding

such standards, descriptions of the photodiodes (see III), operating

instructions, sketches, price schedules, etc. are sent as appropriate, and

technical questions are answered by telephone or letter.

Purchase orders are accepted for new photodiodes or for recalibration of

previously supplied ones, and work scheduled in a timely fashion. Windowless

photodiodes are fabricated in-house in batches, and windowed photodiodes are purchased from Science Applications International Corporation, Electronic

Vision Systems Division using a reimbursable cost center. Calibration

activities are scheduled to meet customer requirements and the results are given in Reports of Test (see Appendix A). While Reports of Test for new photodiodes conform to an established format, a Report dealing with a recalibration which revealed an abnormality in the device efficiency would

include a portion of the Discussion section treating this. Should the

photodiode under study be no longer suitable for use as a transfer standard,

it would be returned uncalibrated with the recommendation that it be replaced.

48
IX ACKNOWLEDGEMENT

The assistance of Drs. Robert P. Madden, Edward B. Saloman, David L. Ederer, and

Mr. Lanny Hughey is gratefully acknowledged. Dr. Madden has been Group Leader of

the Far Ultraviolet Physics Section throughout the period during which the far

ultraviolet detector calibrations program has been active. Dr. Saloman was responsible for setting up the original SURF detector calibration system, with the

technical assistance of Mr. Hughey. Dr. Ederer, with Dr. Saloman, conducted the

analysis which led to the capability of interpreting data from short wavelengths.

49
REFERENCES

1. L.R. Canfield, R.G. Johnston & R.P, Madden, "Preliminary Report on Photoelectric Detector Transfer Standards for the Vacuum Ultraviolet",

NBS Report #9897.

2. J.A.R. Samson, "Absolute Intensity Measurements in the Vacuum Ultraviolet",

J. Opt. Soc. Am. 54, 6 (1964).

3. J.A.R. Samson & G.N. Haddad, "Absolute Photon-flux Measurements in the

Vacuum Ultraviolet", J. Opt. Soc. Am. 64, 47 (1974).

4. R.G. Johnston & R.P. Madden, "On the Use of Thermopiles for Absolute

Radiometry in the Far Ultraviolet", Appl. Opt. 4, 1574 (1965).

5. L.R. Canfield, R.G. Johnston, K. Codling, & R.P. Madden, "Comparison of an Ionization Chamber and a Thermopile as Absolute Detectors in the

Extreme Ultraviolet", Appl. Opt. 6, 1886 (1967).

6. E. B Saloman & D. L. Ederer, "Absolute Radiometric Calibration of Detectors

Between 200-600A", Appl. Opt. 14, 1029 (1975).

7. R.H. Day, P. Lee, E.B. Saloman, & D.J. Nagel, "Photoelectric Quantum

Efficiency and Filter Window Absorption Coefficients from 20 eV to 10

KeV", J. Appl. Phys. 52., 6965 (1981).

8. E.B. Saloman, J.S. Pearlman, & B.L. Henke, "Evaluation of High Efficiency

Csl and Cul Photocathodes for Soft X-ray Diagnostics", Appl. Opt. 19. , 749(1980).

9. L.R. Canfield, R.G. Johnston, & R.P. Madden, "NBS Detector Standards for

the Far Ultraviolet", Appl. Opt. H, 1611 (1973).

Other References on file:

1. J.G. Timothy & R.P. Madden, "Photon Detectors for the Ultraviolet and

X-ray Region", Handbook on Synchrotron Radiation, Vol. 1 pp. 315-366

(North-Holland 1983).

2. E.B. Saloman, S.C. Ebner, & L.R.Hughey, "Vacuum UV and Extreme UV Radiometry Using Synchrotron Radiation at the National Bureau of Standards",

Optical Engineering 2i, 951 (1982).

3. E.B. Saloman, S.C. Ebner, & L.R. Hughey, "Radiometry Using Synchrotron

Radiation", Proc. of SPIE Symposia 279 . 76 (1981).

4. H. Kaase, K.H. Stephen, W.M. Burton, A.T. Hatter, A. Ridgely, L.R. Canfield, & R.P. Madden, "Intercomparison of Radiometric Irradiance Scales

in the 90-250 nm Wavelength Range", Appl. Opt. H, 2529 (1980).

5. E.B. Saloman, "The Use of Synchrotron Radiation for Detector Calibrations",

Proc. of the Nat. Conf. on Synchrotron Radiation Instrumentation", Nucl.

Instr. Meth. 172 . 79 (1980).

50
6. R.P. Madden, "Availability of NBS Radiometric Standards for Solar Irradiance Studies", Proc. Workshop on Solar UV Irradiance Monitors, 79 (1980).

7. E.B. Saloman, "Typical Photoeff iciency Between 20-250 eV of Windowless

XUV Photodiodes with Tungsten and Anodized Aluminum Oxide Photocathodes"

,

Appl. Opt. 11, 1489 (1978).

8. R.P. Madden, "The Availability and Development of NBS Radiometric Standards", The Solar Output and Its Variation (Colorado Assoc. Univ. Press

1977)

.

9. E.B. Saloman, "Time Response of NBS Windowless XUV Radiometric Transfer

Standard Detectors", Appl. Opt. 14, 1764 (1975).

10. E.B. Saloman, D.L. Ederer, & R.P. Madden, "Radiometry in the EUV Spectral

Region: Standard Sources and Detectors" Proc. IV Inter. Conf. on VUV

Radiation, 798 (1974).

11. R.P. Madden, "Absolute Detectors and the Transfer Standard Problem in

the Vacuum Ultraviolet", Calibration Methods in the UV and X-ray Regions

of the Spectrum Symposium, Munich (1968).

12. L.R. Canfield & R.P. Madden, "Status Report: NBS Far Ultraviolet Transfer

Standard Detector Program Middle Atmospheric Program Handbook"

BIBLIOGRAPHY

DUOPLASMATRON LIGHT SOURCES

1. CD Moak, H.E. Banks, J.N. Thurston, J.W. Johnson, & R.F. King, "Duoplasmatron Ion Source for Use in Accelerators", Rev. Sci. Instr. 30^, 694

(1959).

2. J.A.R. Samson & H. Liebl, "Duoplasmatron as a Vacuum Ultraviolet Light

Source", Rev. Sci. Instr. 33, 1340 (1962).

3. CM. Braams, P. Zieske, & M.J. Kofoid, "Composition of Noble Gas Ion Beams

Produced with a Duoplasmatron", Rev. Sci. Instr. 36^, 1411 (1965).

4. J.A.R. Samson, "Techniques of Vacuum Ultraviolet Spectroscopy", Chap. 5.

(Wiley 1967).

THERMOPILES

1. R.G. Johnston & R.P. Madden, "On the Use of Thermopiles for Absolute Radiometry in the Far Ultraviolet", Appl. Opt. 4, 1574 (1965).

2. L.R. Canfield, R.G. Johnston, K. Codling, & R.P. Madden, "Comparison of an Ionization Chamber and a Thermopile as Absolute Detectors in the Extreme

Ultraviolet", Appl. Opt. 6., 1886 (1967).

51
ION CHAMBERS

1. J.A.R. Samson, "Absolute Intensity Measurements in the Vacuum Ultraviolet",

J. Opt. Soc. Am. 54, 6 (1964).

2. J.A.R. Samson & G.N. Haddad, "Absolute Photon-flux Measurements in the

Vacuum Ultraviolet", J. Opt. Soc. Am. 64, 47 (1974).

52
Appendix A

FORM NBS—259 |REV. 12-781

U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D C. 20234

533.04 REPORT OF TEST September 9, 19B5

for

ABC Corporation

Sampi

e

:

Far ultraviolet photodiode, EVSD model 54-0-000, serial no. XXX-X. No

previous NBS Report of Test.

Method of Calibration :

Direct comparison of photocurrents with NBS-cal ibrated secondary

standard photodiode of the same type, both detectors housed within the same vacuum chamber. The beam on the sample photocathode was approximately 4x5 mm, centrally positioned at normal incidence. The

sample photodiode was operated with the anode 150 V positive relative

to the cathode and guard rings; photocurrent was measured in the

cathode circuit.

A-1
Results : (EVSD Model No. 54-0-000 photodiode, serial no. XXX-X,

calibrated 9/85.)

Wavelength (A) Quantum Efficiency (xlQO) Probable Error (%)

1164 4.02 6

1180 4.06 6

1216 6.61 6

1254 8.86 6

1354 9.17 6

1403 8.00 6

1441 7.62 6

1487 7.30 6

1545 7.98 6

1608 9.82 6

1648 11.40 6

1700 : 13.84 6

1750 16.10 6

1823 18.11 6

1879 18.76 6

1937 18.98 6

2000 18.63 6

2067 18.19 8

2138 17.77 8

2214 17.00 8

2296 16.30 8

2385 15.18 10

2537 14.04 10

Pi scussion

:

The true value of quantum efficiency is within the "probable

error" of the quantum efficiency quoted, with a probability of 0.5. The

probable error is an indication of the accuracy of this calibration when

transferred from NBS only if the following conditions are met.

1) The sample photodiode must be one of the NBS transfer standard

photodiode types. Acceptance by NBS of an alternative type for calibration does not necessarily imply equality of merit as a transfer standard.

2) The window of the sample must not be modified (e.g., by contamination, polishing, etc.) and the device must be used in accordance with

experimental procedures which will not tend to degrade the calibrations.

In addition to the probable errors given above, an additional error may

arise from drifts of quantum efficiency after the NBS calibration. While

experience has shown that the magnitude of such drift at any wavelength

is not likely to exceed 3% of the quantum efficiency per year in 90% or more of the photodiodes, periodic recal i brati on by NBS is strongly recom- mended.

A-2
Measurements associated with this calibration were made by:

L. Randal 1 Canf ield

(301) 921-2031

Report of this calibration approved by:

Robert P. Madden

Group Leader, 533.04

For the Di rector

Wi 1 h am K. Utt

Chief, Radiation Physics Division

Order No .: ABC123

Test No .: XXX-X/85-1

Date : September 9, 1985

A-3
FORM NBS—259 (REV. 12-781

U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D C. 20234

533.04 July 1, 1986

REPORT OF TEST

for

ABC Corporation

Samp] es

:

Far ultraviolet photodi ode, windowless, AI2O2 photocathode

,

NBS serial no. 123. No previous NBS Report of Test.

Method of Calibration : Direct comparison of sample cathode

photocurrents with NBS-cal i brated secondary standard windowless

photodiodes, with sample and standard housed within the same vacuum

chamber. The sample cathode was not cleaned before calibrations. The

anode of the sample photodiode was operated at 60. OV (160A-1216A) or

100. OV (50A-150A) relative to the photocathode, which was at ground

potential. The beam size on the photocathode was 6 mm x 6 mm (519A-1216A) or 2 mm X 2 mm (50A-500A).

A-4
Resu1 ts : (NBS windowless Far UV Photodi ode, serial no. 123, calibrated

April, 1986).

Wavelength (A) Quantum Efficiency (xlOO) Probable Error (%)

50 3.66 15

55 4.99 10

60 6.25 10

65 7.19 10

70 8.07 10

75 8.80 10

80 9.43 10

85 9.85 10

90 9.99 10

95 10.37 10

100 10.56 10

110 10.31 10

120 11 .55 10

130 11.07 10

140 8.98 10

150 X \J \J 9 .04 10

160 8.61 10

170 7.48 15

180 7.54 10

190 7.59 10

200 7.56 10

210 7.84 10

220 8.02 10

230 8.33 10

240 8.53 10

250 8.76 10

260 9.04 10

280 9.29 10

300 10.03 10

320 10.53 10

340 11.22 10

360 11.68 10

380 11.90 10

400 11.82 10

420 11.76 12

440 11.76 12

460 11.80 12

480 12.05 12

500 12.60 12

519 13.68 8

537 14.58 8

556 14.38 8

584 15.75 8

599 15.92 8

A-5
Wavelength (A) Quantum Efficiency (xlOO) Probable Error (%)

622

639

657

669

683

699

712

735

752

771

800

818

844

865

886

920

1026

1216

16.88

17.60

17.83

17.94

17.81

17.42

17.39

16.43

15.57

14.68

13.35

12.41

11.54

10.91

10.32

9.39

5.02

1.11

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

8

Pi scussi on

:

The true value of quantum efficiency is within the

"probable error" of the quantum efficiency quoted, with a probability

of 0.5. The probable error is an indication of the accuracy of this

calibration when transferred from NBS only if the following conditions

are met.

1) The sample photodiode must be one of the NBS transfer photodiode

types. Acceptance for calibration by NBS of an alternative type does

not necessarily imply equality of merit as a transfer standard.

2) The sample photocathode or window must not be modified (e.g., by

contamination, polishing, etc.) and the device must be applied in

accordance with experimental procedures which will not tend to degrade

the calibrations.

In addition to the probable errors given above, an additional error may

arise from drifts of quantum efficiency after the NBS calibration.

While experience has shown that the magnitude of such drift at any

measured wavelength from wi ndowed photodiodes is not likely to exceed

±3% of the quantum efficiency per year in 90% or more of the

photodiodes, periodic recal 1 brati on by NBS is strongly recommended.

Since environmental conditions can directly affect the quantum

efficiency of w1 ndowl ess photodiodes, it Is most difficult to predict

the anticipated drift, and frequent recal i brati on of these photodiodes

is urged.

A-6
Measurements associated with this calibration were made by

Report of this calibration approved by

Robert P. Madden

Group Leader, 533.04

For the Director

William R. Ott

Chief, Radiation Physics Division

Order No. : ABC123

Test No. : 123/86-1

Date: July 1, 1986

L. Randall Canfield

(301) 921-2031

Nils Swanson

(301) 921-2031

A-7

Appendix B

DETAILED OPERATION OF SURF-II DIODE CALIBRATION SYSTEM

Calibration Sequence

In order to be able to calibrate outgoing photodiodes, a calibrated working

standard must exist. The procedure enabling one to achieve this involves the use of the rare gas ionization chamber. Once such a working standard has been

calibrated, outgoing photodiodes may be calibrated by direct comparison with

the working standard.

Ion Chamber

The rare gas ion chamber is operated in two modes: as a double ionization

chamber with two equal length ion collector plates, and as a single ionization

chamber with these two plates connected together. Two rare gases are used: Ar

for wavelengths longer than 15 nm, and Ne for all wavelengths, giving some redundancy of data. The sequence of data acquisition is logically SIC first

(at two low pressures-10 and 20 microns, for example) with one of the two

gases, then DIC with the same gas. (Since it is necessary to change isolation

filters, and hence break the previous gas seal when changing wavelength

ranges, only one spectral region - determined by grating choice - can be done

with a single gas filling.) In all modes of ionization chamber operation the

actual calibration is that of monitor photodiode with its filter relative to

the flux entering the ion chamber (through the gas sealing filter).

Working Standard Photodiode

A photodiode located on the intercomparison wheel behind the ion chamber

region is designated as the working standard, and the calibration of this

detector is accomplished by intercomparison with the monitor/ filter

combination which has previously been calibrated via the ion chamber. The same

gas sealing filter which was used during ion chamber operations must, of

course, be in place.

Outgoing Photodiode Intercomparisons

The working standard is located in position 1 on the intercomparison wheel

with the gas sealing filter in place, and the ratio of the photocurrents of

sample(s) to the standard as a function of wavelength is determined. (The

monitor/ filter need not be a part of this measurement.)

B-1
VOLTAGES & PRESSURES

The following have been established as "normal" operating levels for bias

voltages and ion chamber pressures:

Unit Wavelengths (nm) Bias (v)

monitor 15-60 60

5-15 100

wkg std 15-60 60

5-15 100

samples 15-60 60

5-15 100

mon fltr all 140

SIC (Ne) 5-6.5 335

7-8.5 300

9-15 150

15-20 80

21-60 50

Die (Ne) 3-15 10

15-60 40

DIG (Ar) 15-60 50

Mode Wavelengths (nm) Pressure

SIG (Ne) all 0.01, 1 DIG (Ne) 3-15 0.8, 2

15-60 0.5-1

DIG (Ar) 15-60 0.5-1

; (none critical)

DATA REDUCTION

The program GALRED.BAG is used for all aspects of data reduction and is

menu-driven. The logical sequence in each spectral range follows:

1. Extrapolate 2 SIG runs to 0 pressure.

2. Calculate secondary ionization factor ("G"). Edit as required.

3. Calculate fraction of 1st order ("F"). Edit as required.

4. Calculate QE of monitor over entire range (data from 1.).

5. Calculate QE of standard over F=l portion of range.

6. Calculate QE of standard over balance of range.

7. Calculate QE of samples over entire range.

Steps 1-7 must be done in order. The results of step 1 are needed (with DIG

data) to arrive at "C", which in turn is needed to arrive at "F". The results

of step 4 are needed to accomplish step 5; steps 4 & 5 are needed to

accomplish step 6. Likewise the results of steps 5 & 6 are needed for 7.

B-2
INITIALIZING

Wavelength Drive

Note: before running this routine, grating to be used must have been selected

and masks set to give proper illumination.

Menu item #29. After selection of item, at experiment turn on wavelength drive

power supply, press command button. Program sets trial shaft encoder zero, scans to what it thinks is zero wavelength and stops. Turn off wavelength

drive supply, manually scan into zero (as observed by monitor diode) from

below and press command button. Program records correct encoder zero, scans to

about 80A (1200/mm grating) and stops. Note proper electrometer scale at

manually-selected peak, press command button and sequence terminates. Encoder

zero is Kl.

Diode Wheel

Menu item #22. Before selecting this item, preset diode wheel a little

clockwise from #1 position ("1/4") and turn on diode wheel power supply.

Select item and program will microstep wheel until proper registration for #1

diode is reached (max 300 steps). Displays "#1 diode in beam" after locating

it - if no display, registration is incorrect.

CALIBRATING

Electrometers

Menu item #24 (uses overlay subroutine). Select item, enter as prompted,

printout will give detailed instructions for ranges and no. of electrometers

selected, (Note that channel allocations must agree with program channel

assignments.) Variables are Cl(2)...

Barytron

Menu item #27. Select item, screen prompts. Variable for calibration is

Bl(counts/torr) . Note that if both SIC and DIC runs are to be made in sequence

that the Barytron will have to be pre-calibrated on two ranges, which involves

two iterations of this routine. The appropriate value for Bl will have to be

entered manually.

VARIABLE STORAGE & RECOVERY

Calibration variables may be displayed, stored or recovered from menu. After

initializing & calibrating, it is good practice to store these.

CHANNEL ASSIGNMENTS

SIC ch 2: SIC (both plates together)

ch 3 : mon diode

DIC ch 2: plate 1 ch 3: plate 2

ch 4 : mon diode

cal std vs mon ch 2: mon diode

ch 3: std diode (wheel)

cal diodes

ch 2: diode wheel

B-3
FILTERS

Filters are used at two places in the system: just ahead (toward mono) of the

monitor diode and at the entrance to the ion chamber area. The monitor filters

(Al and polypropylene) are pneumatically inserted by the program, but the

selection of the proper filter for the wavelength range in use is done

manually. A u-shaped stop can be snapped over the actuator shaft to limit its

travel; this will give the Al foil when actuated. Otherwise, polypropylene is

in. The ion chamber filter (which must also be used for anything involving

diodes on the rear wheel) is manually inserted. The actuator has three

positions: fully up is an Al filter, fully down is polypropylene, and the

midpoint (with the handle resting on a snap-in aluminum angle support) allows

light past with no filter. Status switches monitor the position of this

actuator. It is important that this actuator be handled carefully, and that

the motion in the external guide be with gentle force toward the rear of the

system. It is also very important that the actuator be left in the mid

position when not actually in use. Failure to do so may result in a ruptured

filter.

GAS HANDLING

To prepare for any ion chamber measurements, the gas supply system connected

to this housing must be charged with pure gas. The experimental system is

prepared for ion chamber runs by placing the filter to be used in position and

sealing it by use of the linear feedthrough on the front flange of the foil

chamber. The pneumatic valve over the CTI cryopump is opened and the manual

gate valve over the ion pump is fully closed. The G-P metering valve is then

used to bring the chamber region to the desired pressure (as monitored by the

Barytron)

.

To return the system to high vacuum following a run, close the metering valve

("30" on the readout is fully closed), close the pneumatic valve over the

cryopump, close the 2" valve on the mono exit arm and turn off the ion gauge

in the foil chamber. Place the manual foil changer in the mid position,

allowing the ion chamber gas to diffuse into the entire experimental system.

Remove gas from the system using roughing pumps attached. In the case of Ne,

it is necessary to flush the system with dry nitrogen several (at least 3)

times to reduce the partial pressure of Ne and avoid swamping the Vacsorbs.

(This technique is also necessary when purging Ne from the supply lines.) It

still may be necessary to bake the Vacsorb after this use. When the thermal

gauge on the roughing system reads about 2 microns, it should be possible to

begin to open the gate valve over the ion pump carefully. Crack the valve,

watching the pressure scale_on its supply and not allowing the indicated

pressure to rise above 4x10 torr. When the roughing gauge pressure is

observed to be dropping further the roughing valve on the side of the ion

chamber may be closed, and it should be possibe to gradually open the gate

valve. As soon as this is possibe, the cryopump valve should be opened to

reduce the load on the ion pump.

B-4
OPENING BEAM LINE

Before considering opening the BL-9 |ate valve (controls on interlock panel)

the monochromator must be on the 10 range and must have passed RGA since

last being open for test. A spot check of the hydrocarbon peaks may be made at

any time via the integral mass spectrometer in the monochromator; the peak at an indicated mass reading of 40.7 tends to be the greatest magnitude. The nude

ion gauge in the monochrgmator provides the interlock for the BL-9 valve, and

should be set on the 10 range.

STEPPING MOTOR SUPPLIES

Both the wavelength drive and the diode wheel are driven by stepping motors

with separate supplies. These are under computer control and the front panel

overrides on each supply should not be used, since it would be easy to drive

either beyond the physical limits which should be observed. There are no limit

switches which act directly on the supplies. These supplies should be turned

off during SURF-II injection periods, or any other time they will not be used

for an extended period.

B-5

Appendix C

THERMOPILE CALIBRATION PROCEDURE

I. Turn on light source

1. Outgas filament 15 A. (flap valve closed)

2. Open Kr valve slowly, press. about 2-3 x 10 ^ Torr

3. Turn up minor arc to over 0.2 A.

4. Start major arc. Raise Variac quickly to get current started. Set

about 1.5 A., voltage about 40 V. Lower filament about 2 A. Turn off

minor arc.

5. Set w.l. on grating drive: He 584.3 angstroms; Ne 735.9 angstroms;

Ar 919.8 angstroms (+ offset). Set on peak of line.

-4

6. For He: Close Kr valve, let in He to about 0.8 x 10 Torr.

(80 V. arc voltage)

Ne: Let in Ne to about 8-10 x 1^"^ Torr, then valve off Kr. Adjust

pressure to about 1.1 x 10 Torr (corresponds to 9.5 Torr on

G.P. gauge in gas manifold). Arc about 65 V.

_5

Ar: Let in Ar slowly to 8-10 x 10 Torr, then valve off Kr.

Arc about 45 V.

7. Let stabilize - readjust major arc, filament.

NOTE: Don't run He next after hydrogen. Try Argon. Slit should be open 1 1/4

turns = 250 microns. Slit closes when micrometer knob under light source

is turned cw looking from above.

II. Electronics

1. Test calculator when turned on (i.e. 3x2= , etc.).

2. Load program 070M @ 00 (4 cards). Enter "Ju 00", then load cards.

Idle light goes out while cards are being read.

3. Open butterfly valve on 2" pump.

4. Turn on: Kepco P.S. to 22 V. (ion chamber voltage).

Two scanners, both on remote

Sin/cos stepping motor drive

0.05 microvolt test signal (batgery off)

Left hand Keithley on 0.3 x 10 range + mode

Anadex 10 V. range

Monsanto counter Hz-sec range, divide by 5, square wave,

Freq. A

Chamber flap valve open

C-1
If necessary, adjust slit width by reading residual ion chamber current on electrometer, setting right hand scanner to #1.

III. Data Acquisition

1. Both flaps wide open (entrance and exit slits), not tripped.

2. TC trip settings raised on both pumps

3. Gate valve closed 2" pump (control panel switch down)

4. Manually place TP up, close to switch. Turn on motor. Subroutine

at "Chg Sgn 00" will step up TP in 10 step increments. Press RESET

to stop TP when it is just touching switch.

5. Lower TP using "Ju IS+"

6. Check both stepping motors on.

7. Enter "Ju 00", Resume - Mantissa of pressure is read and printed.

Halts.

Enter date (2 digits for day of month). Resume. Gate valve opens,

enter wavelength. Resume. Enter base pressure w/ exponent. Resume.

Halts for system check.

8. Resume starts run. 60 minutes running time.

9. Also, lowering TP by "Ju IS+" will start program at 00.

A. Clear X or Reset closes all valves.

B. Lower TP by using "Ju IS+"

C. Repeat a run or start a new run using previous TP background

and test signal data. Enter "0", Resume. Calculator prints date,

opens gate valve. Enter wavelength. Resume. Enter base pressure

w/exponent. Resume, and calculator prints averages of

previous TP background and test signal data, closes gate valve,

and begins ion chamber measurement with flap valve closed.

(can do "Ju 00" to get a new background pressure, then "0",

Resume to reuse previous TP background and test signal)

.

If ERROR comes up (light flashes on keyboard), push "Clear X"

and retype entry. Gate valve is closed for next step anyhow.

PROGRAM 070M - THERMOPILE-ION CHAMBER CALIBRATION

Running Time 60 minutes

#1 TP raises, flap valve closes. TP background. 5 sets of sum of 10

readings

.

#2 TP test signal, as above.

#3 Ion chamber. TP lowers, gate valve closes. Lets in gas for 90 seconds.

C-2
#1 on paper tape

#4 Shuts gas valve, pumps out ion chamber. Waits 40 seconds, repeats TP

background

.

#5 Flap valve opens - TP signal

#3 Ion chamber

#2 on Tape

#4 Pumps out chamber, TP background

#5 TP signal

#3 Ion chamber

#3 on Tape

#4 Pumps out chamber, TP background

#2 TP test signal.

Flap valve closed at finish of run. Program prints out summary, F values,

Mantissa of base pressure.

LH Scanner RH Scanner

Channel Channel

TP background 2

TP Test 2

I.e. Plate #11 1 I.e. Plate #2 1 2

TP background 2 1 TP signal 2 1

THERMOPILE-DIODE CALIBRATION

I. Cover on glass window. 23 w.l. in 1164-2537 angstrom range.

Open 2" butterfly valve, turn on electronics.

Open gate valve.

Raise main chamber TC gage trip setting.

Check TP down, light source flap closed to light.

Open TP flap valve.

Set diode voltage 150 V. EMR (5 digit no.) or EVSD (4 digit no.);

60 V. SEL (3 digit no.)

C-3
II. Turn On Light Source (exact procedure may vary)

1. Outgas filament 15 A.

2. Let in Kr 2-3 x 10"^ Torr.

3. Turn up minor arc to above 0.2 A.

4. Start major arc. Raise variac quickly to get current started.

May need maximum minor arc current or higher filament current.

Set to 1.5 A., about 40 V. Filament about 14 A.

5. Turn off minor arc.

-4

6. When stable, valve off Kr, let in hydrogen about 1.1 x 10 Torr.

Reduce Kmajor arc) to 1.1 A., 100-130 V. Set filament about 14 A.

III. Initialize data acquisition system

1. When stable, set w.l. and Keithley range. Diode current read for

R.H. Scanner on #1. Proper diode voltage on. (TP signal for #2

Scanner). L.H. Scanner chooses output to Anadex: #1 for Keithley,

#2 for Perkin-Elmer amplifier.

2. Check slit width is proper. For 1164-1608 Angstroms, slit open

1/4 turn = 50 microns. For quartz region above 1608 Angstroms, open

slit 1 to 1 1/2 turns = 200-300 microns.

3. Turn on calculator, test by 2 x 3 =, etc.

4. Load data cards:

a) 1164-2537 Angstroms as data: rcl[][]96

b) Diode card for diode being tested

Permanent data at rcl[][]32 A side

Temporary data at rcl[][]64 B side

5. Load 5 program cards @ "Ju 00" (Program 068M)

6. "Ju Chg Sgn 50" (address 2500) raises TP in 10 step increments.

Stop by pressing Reset.

7. "Ju IS+" lowers TP.

8. "Ju IS Read" opens flap as part of "0 Resume". See #5 below.

IV. Program 068M (Runs 43 minutes)

1. TP lowered, both flaps open, scanners remote, gate valve open,

stepping motors on.

C-4
2. Start with "Ju 00, Resume".

Enter date(2 digits for day of month)

Computer returns diode no., wavelength and photoelectric correction.

Enter F value. Resume.

Enter full scale Keithley range. Resume. Must be not greater than

3 X 10"^ A.

Computer prints Keithley range K factor and line of dots.

(Program reads temporary data file to see what starting w.l. should be.

Can start at any w.l. by inserting a QE into the data register for

the previous w.l., if registers for longer w.l. are empty.)

(Permanent data card contains diode no. and photoelectric correction).

3. Calculator halts for check of system parameters (idle

light comes on)

.

4. Press Resume to start run. Takes data

1) TP background 2) TP test signal 3) Diode background

4) Diode signal 5) TP background and signal.

Does 3 diode measurements around 2 TP measurements. Finishes with

TP background and test signal, with TP lowered and flap valve

closed at end.

5. vnien finished and ready for next wavelength "0 Resume"; will reuse

last TP background and test signal. To repeat a wavelength, enter "-1

Resume". Will proceed as for "0 Resume", but at the old w.l.

Can go backwards in w.l. by doing "-1", Resume twice. Deletes

QE from preceding w.l.

6. After "0" or "-1", Resume, prints date and diode no., then w.l.,

photoelectric correction, and present F value. Halts to wait for

Keithley range input with flap o£|g and TP lowered. Try to keep

diode current midrange on 3 x 10 range for good TP signal.

7. Here can set new w.l. and check Keithley diode current reading.

8. Enter Keithley range. Resume. TP raises, then flap valve closes,

then prints a line of dots.

9. Waits for a Resume to start data collection. TP lowers, prints

averages of previous TP background, test signal, and net test signal.

10. Takes diode background, then diode signal, continues.

NOTE: TP signal should be over 10% of TP test signal for adequate S/N

ratio. 20% is better.

At the end of a day:

Enter new temporary data on card; do rcl[][]64, switch to "WRITE",

and insert B side of diode data card. Switch to "READ".

To dump data summary, "Ju IS exp(x)". Resume prints out results

from 1164-2537 Angstroms.

C-5
After last w.l. (2537 A), update permanent data card:

1) "Ju IS (x-y axes), Resume" prints out for each w.l. the

% difference between previous average Q. Eff. and the present

Q. Eff., the new average Q. Eff., and the new standard deviation.

2) Then rcl[][]32, switch to "WRITE", insert A side of diode data

card to update permanent data. Switch back to "READ".

C-6
Appendix D

DIODE CALIBRATION PROCEDURE

Initializing system

Sequence

:

1. Perforin light source startup (vacuum)

2. Turn on all electronics (left scanner on ch,3)

3. Vent experimental chamber

4. Load samples (blow off dust)

(if sample diode from customer, clean window)

5. Evacuate experimental chamber

6. Connect cables, turn on diode voltage(s)

7 . Perform light source startup

8. Load data acquisition programs and data

9. Initialize experimental module

flap open (don't trip limit switch)

standard in beam (line up red arrows)

filter(s) out of beam

10. Set monochromator wavelength and slits

11. Open light source flap, unlock picoammeter input

12. Fine-adjust first wavelength

13. Turn on stepping motor supplies (3)

14. Check that all above have been done

Running program

Sequence: See instructions for particular program

Terminating

Sequence

;

1. Turn off light source power (not cooling)

2. Turn off light source gas(es)

3. Turn off stepping motor power (3)

4. Lock picoammeter input

5. Turn diode voltage(s) to zero

6. Perform associated program routines (such as printout, analysis, graphics, data storage)

7. Vent experimental chamber and remove diodes

8. (Reload) replace diode module and evacuate

9. If running again, re-enter initializing

sequence at item 6

.

D-1
PROGRAMS FOR DIODE INTERCOMPARISON ANALYSIS

5-JUN-84

Plotting running ratios vs time

Cards: 075

049

Ref ratio data card (# lower left of plot)

Data card for last point previously plotted

Data card(s) for subsequent data

Prep: 1. calculator & plotter on

2. appropriate plot aligned on plotter

3. (x,y)min,max set on plotter

4. load cards: 075-ju 00 (2 cards)

049-ju .02

ref data rcl.92

2.

3.

4.

Note: In the

Running: 1. ju 00, (res) , enter : sample #(res)

std #(res)

1st year of plot (2 digits)(res)

load last prev data(res)

load newer dataCl or more) (res)

after last data, ju symb 0 to plot

(flag if new plot to generate tic marks)

case of diodes calibrated using 2 standards, it

may be necessary to manually change the standard id on the card to agree with that entered at the start

if two lines of dots after feeding in data card, you

can inspect the exact format of the reference id by

rcl6,Prx. The format of the id on the card just fed

in can be inspected by rcl30,Prx. If they don't agree

due to 2 standards, changing the format of reg 30

so that it agrees with reg 6 will cure the problem.

Analyzing ratios with previous history

Cards: 072 (in anal loose-leaf)

appropriate intercomparison data card

appropriate new data card

023 (wavelengths)

Prep: 1. calculator on

2. load cards: 072-ju 00

intercomparison data-rcl32

wave lengths-rc 100

new data-rcl96

D-2
Running: (if new ratios are 1/x, flag)

1. ju O0(res)-auto to end

(if either data card disagrees with the other, error -if so, clear, re-store both data cards

2. at end, re-record intercomparison card:

rcl32(write pos) enter card (return to read pos)

PHOTOCATHODE UNIFORMITY PROGRAM

Loading photodiode:

1. Mount photodiode to be studied on x-y module

holder slide holder toward mask with pinhole

until face of photodiode is about 3/4" from

mask, lock holder in position with alien screw.

2. Make normal electrical connections to photodiode (red is anode) and check continuity.

3. Mount module on experimental chamber and

evacuate

.

Initializing:

1. Start light source as usual.

2. Set monochromator wave lengtl^^and slits to give

reasonable current on 3x10 scale

or a little higher if 1216.

3. Manually find edges of photodiode by scanning

in X direction until current is about half of peak.

4. Calculate central position based on these

readings, write down, do same for y direction.

5. Leave in center, leave source flap half closed.

Note: Stepping motor cable connector (large) must be

changed to "xym" pairing fo x-y module, and limit

stop connector to "xyls" pairing. Also clip grounding

wire to convenient ground.

Loading program: Ju GO, load 056 program (3 cards)

Running: Ju 00 (res)

Enter: date

sample #

wavelength (normal : 1216 , 1403 & 1608)

# of elements (pixels) per line-usually 10

# of stepping motor steps per increment-150 for 1mm incr

ratio of #y lines to #increments-usually 1

1. Manually scan to starting position: if doing

1cm X 1cm scan, .090" from center on indicator in

both X and y toward the upper right.

2. Turn on stepping motor power supplies for x and y motors,

and open source flap.

3. "Resume" will start main program—program uses a ref

D-3
point (the starting position) after each line

to check for light source drift and make any

necessary corrections. The magnitude of the change

relative to the original value is printed out after

an identifier giving the line number. The magnitude

of the change shouldn't be as large as .01 per lineif it is, start over. All data is stored for plotting

at 00 (can be transferred to cards for later plotting

or used right away)

.

4. After the final line is scanned, the drive returns to

nominal center and the normalized data is printed.

5. Close source flap half way and turn off stepping

motors. If no more data will be taken, turn off source.

6. To record the data from 100 pixels (10x10) it is

necessary to use both sides of 2 cards.

®00, "write", feed in cards, "read".

PLOTTING PHOTOCATHODE UNIFORMITY

Loading program:

Initializing:

1. Ju 00, load 071 program (2 cards)

2. Ju .02, load 049 and 050 in sequence

1. Load wavelength code: ju 93("load") enter code

from table (back to "run")

.

2. Load diode code: ju 94("load") enter code from

table, terminate "0" (back to "run").

Plotter;

symbol: 012 3, 45678 9 - f

code: PrX PrA Rst Clx 064 065 066 067 Sin Sin-1 Sqrt 106

(or) 060 061 062 063 070 071 055

(3-digit codes are entered by "enter code( )()()" button, followed

by the 3 digits)

Set up plotter with 8 1/2 x 11 xerox paper, adjusted

so that bottom is in normal position, and left edge

is at 3.4" on scale. No need to adjust pen position.

Running: 1.

(0R)1.

If 056 program left data in calc, ju OO(res)

and wait for idle light to come back on. If using data from cards instead, rcllO, load data,

Ju symb sin (res).

2. enter: X min (0)

X max (15)

Y min (0)

- Y max (10)

# pixels per line (10)

XI (4.1)

Yl (9.5)

X incr ( .7)

#Y/#X (1)

Draws grid and plots data, wavelength & id

("x" before "=" should be greek lambda-use

white-out .. ."a" after wavelength should be

symbol for angstrom units-add small "0" over it).

D-4
Note that the values shown for entries are correct

for the evsd photodiode routinely scanned .. .other

detectors will, in general, require different values.

CALIBRATION OF WINDOWLESS NBS PHOTODIODES

BY A-B INTERCOMPARISON (490A-1216A)

Function:

The determination of quantum efficiency as a function of wavelength

of an nbs windowless photodiode by intercomparison with a working

standard photodiode previously calibrated with the ion chamber

(490A-920A) and with the same working standard pre-calibrated

vs thermopile at 1026A and vs a windowed photodiode at 1216A.

Module:

The a-b intercomparison module with provision for mounting two

photodiodes is used (the same as for windowed photodiode calibrations) . Loading module:

Windowless photodiodes, sample and standard, are mounted at the

usual locations on the rotating wheel. It is necessary to use teflon bushings to insulate each at its rear, since this is

electrically the same as the cathode. Cathode connections are made by clip to the screw heads on the rear of the wheel, and

anode connections to any one of the screws holding the cylindrical

anode in place. Check continuity to this cylinder and to the outer

edge of the cathode surface.

Loading program:

1. Main program is 065 (load @00-4 cards)

2. Load wavelengths (077b) @00 and @72

3. Load wavelength scan increments (077a) @.10

4. Load standard efficiencies (d-715)@32

Initializing module & source and monochroma tor:

Note: the sigma stepping motor power supply is used with this program

for wavelength drive—mount stepping motor attached to the bottom

power supply. Do not use the sine-cosine power supply.

1. start source on Kr, switch to He, wait until stable.

2. Set monochromator on the 537A line of He.

3. The standard should be in the beam, the flaps open, and the

instrumentation ready as for windowed photodiodes,

4. Set slits at 150 microns.

D-5
Starting program:

1. Ju OO(Res), enter: date

sample #

standard # (loops to 00 if wrong card)

Running program:

After successful completion of 584A data, program will halt

for a change of source gas

.

1. Switch source gas to Kr, set major arc to 2 amperes.

2. When stable, (res) to continue.

Next halt will be after completion of 886A.

3. Switch source gas to Ar, set major arc at 2 amperes.

4. When stable, (res) to continue.

Data will be taken at the Ar lines, the next halt will be after

completion of 699A.

-4

5. Switch source gas to Ne, set mono pressure at about 1x10 , and major arc at about 1.5 amperes.

6. When reasonably stable, (res) to continue.

The next halt will be after completion of 735A.

7 . Switch source gas to pure hydrogen (no Kr)

.

8. When stable at about 1.2 amps major arc, (res) to do the

last 2 wavelengths.

9. After 1026A, set Keithley to lOxlO" scale.

At the completion of 1216A, the program will work for a minute

rearranging the sequence of the data to be in the order of

increasing wavelengths, then will print out the results.

Note: to restart individual data set, replace flap and standard as initially, "reset", Ju symb 0.

If S.D. over 1%, program will halt for operator decision.

To repeat data set, Ju symb 0. To not repeat, "resume".

(1% redo seed is at program step 1034 with reentry @1040.)

Terminating program:

1. Turn off stepping motor supplies (3).

2. Turn off source (power supply and gas).

To dump data for typist: Ju symb "out" (res), set d.p., (res).

To compare to old data: load old data rcl.28 (flag if comparing qe

Ju symb "sin" (res), enter old date, (res)

To prepare for card storage: Ju symb "read", (res), rclOO, "write"

enter card, "read".

D-6
ION CHAMBER/ DIODE CALIBRATION

Functions

:

The calibration of a windowless (NBS) photodiode in the wavelength

range 496-920A by the use of a rare gas double ionization chamber

to determine the absolute flux from the monochromator . The two

detectors are alternately placed in the beam; the ionization

chamber calibrates the flux which then produces photocurrent from

the photodiode, allowing calculation of the photodiode efficiency.

The program is arranged to proceed through the wavelengths so that

each duoplasmatron source gas is used only once, minimizing source

delays. The order in which the gases are used is: He,Kr,Ar,Ne.

The program pauses at each wavelength requiring a gas change.

The ion chamber uses Kr or Xe depending on wavelength, with the

program selecting the correct gas.

Note that the program reads the ion gauge in the monochromator to

enable correction for absorption losses. Therefore the_^on gauge

must be in good working condition and reading on the 10 scale

during the ion chamber sections of the program.

Loading program: 1. Ju oo, load 062 (4 cards)

2. rcl80, load wavelengths (077b) ( source-pref order)

3. rcl.lO, load scan increments (077a) ( source-pref order)

4. Turn on scanners and 780 control unit (interface box)

Initializing module:

1. Mount diode to be calibrated on arm in ion chamber

module and connect cathode and anode (anode is just

lip lead to anode of ion chamber ) -don ' t move arm.

2. Mount module on exp chamber and connect gas supply line,

3. Turn on calc, interface box and scanners.

4. Make sure both Xe and Ar supply tanks are

shut off at tanks, then open each needle valve (at

module) several turns.

5. Turn on the middle sigma stepper power supply

temporarily & open the two solenoids by program

instruction: Ju chg sgn 04(res).

6. Now pump down the exp chamber in normal fashion.

7. Leave solenoids open until high vac valve has been

open several minutes, then close by clearx

or reset and turn off sigma supply.

8. Set module wheel so that diode is in beam.

Note: The wavelength drive for this program uses the lowest

sigma power supply. The motor attached to it must be

installed in place of the sine-cosine drive motor.

Starting source: He is the first gas used in this program, start the source as usual on Kr, when it is stably on at about

D-7
2 amp major arc current, open the He shutoff valve

at the manifold. The source will very likely go out.

after the pressure has recovered to the previous Kr

level, try the procedure again. It will take several

tries before the major arc will stay on with He.

When that has been achieved, turn off the Kr and try

to get it to run on pure He. Unless the source has

been running recently on He, it will probably take a few tries to get it stable on He only. (If it has to

be restarted, just open & close the Kr to start, then

open He—this minimizes the time required to pump out

the Kr each time.) eventually it will run on pure He.

Starting program: gate (high vac) valve closed

exit flap closed (to pencil mark)

butterfly open

voltage set @22.5 volts

ion chamber gas needles preset to

give current ratio of 3-5

diode in position

wavelength: 537A

stepping motors: flap, wavelength (see

above) and rotation—turn all on

top: flap & power for gate valve

middle: wheel rotation & power for gas valves

bottom: wavelength drive

Running: 1. Ju 00 (res)

2. Enter: date

diode #

# runs at each wavelength (^1)

mono base pressure

(note that any time the source pressure is

changed, it is necessary to reenter the mono

pressure— if a normal change point in program, it

will be stored by the program after entry—otherwise, store at main reg 01).

Program will do 537A and 584A, stopping after 584A for

a source gas change.

3. Shut off He and open Kr, setting major arc to 2 amperes.

4. Open slits to 150 microns, enter new monochromator pressure

and hit resume to continue.

If all goes well, the next program pause will

be after 886A (it may be necessary to reduce the slits to

100 microns or to raise the picoammeter scale setting at

the last several wavelengths before 886A)

.

5. Cut off Kr and open Ar, readjusting

major arc to 2 avspexes.

D-8
6. Readjust slits to 150 microns, enter

new monochromator presure and hit resume. After several more wavelengths, the program will stop

for a switch to Ne in the source. Ne is regulated with a needl^ valve only—after it is first opened and set (about

1x10 mono) the pressure will drop for quite a while. Keep

opening the needle to keep the pressure from getting below

5x10 until it is falling fairly slowly if at all.

7. Enter new monochromator pressure, then resume to do 735A,

the final wavelength.

To store data on card:rcl20, "write", feed in card, "read".

Printout options:

Dump data: (set d.p.) ju symb [ Compare to old data: (store old data rcl.28) ju symb =, enter date of

old data, res.

Restart options:

To restart 1st data set at present wavelength: first make sure flap,

gate valve, wheel and gas handling valves

are in initial position, then Ju symb +

To restart current data set: settings as above, Ju symb -

D-9
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. fsIBS/SP-250/2

2. Performing Organ. Report No 3. Publication Date

June 1987

4. TITLE AND SUBTITLE

j^^^^^^^^j^^j^^ SERVICES:

Far Ultraviolet Detector Standards

5. AUTHOR(S)

L.R. Canfield, N. Swanson

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Same as item 6

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619832

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) h description is given of the NBS program in which special photodiodes for the far

ultraviolet spectral region (5-254nm) are made available as transfer standards.

Ihese detectors are calibrated in terms of quantum efficiency ( photoelectrons per

incident photon) as a function of wavelength. Descriptions are also given of the

calibration principles, calibration systems, and photodiode types involved in this

program. Calibrations reference to the photoionizatiai of rare gases.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Far ultraviolet; photodiodes; photoionization; quantum efficiency; rare gases;

standards.

13. AVAILABILITY

XI Unlimited

I I For Official Distribution. Do Not Release to NTIS

["XI Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

86

15. Price

USCOMM-DC 6043-PeO

I
PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 NBS Measurement Services: SP 250-12

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

SP 250-2 NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

SP 250-3 NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet SP 250-14

J.Z. Klose, J.M. Bridges, and W.R. Ott

SP 250-4 NBS Measurement Services:

Irradiation of Ferrous-Ferric Dosimeters

(Fricke- Dosimetry) SP 250-15

C.G. Soares, E.L. Bright and M, Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations SP 250-16

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz, SP 250-17

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg SP 250-18

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J, Hsia SP 250-19

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "4it" Gamna Ionization Chamber, and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

SP 250-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

NBS Measurement Services: Neutron Personnel Dosimetry

R.B. Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with CaliforniLm Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor Cavity Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and D.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R, Loevinger

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrumentation and Sources

J.S. Pruitt, C.G. Soares, and M. Ehrlich
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300