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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


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 1/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


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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 60-100V. The rate of emission is measured with a suitable
caljbrated Picoammetey, The usable range of photocurrents is roughly from

10 * to less than 10 amperes. (The, :dark" current-mostly 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


If necessary, adjust slit width by reading residual ion chamber current on
electrometer, setting right hand scanner to #1.

III. Data Acquisition

1.

#1

#2
#3

Both flaps wide open (entrance and exit slits), not tripped.

TC trip settings raised on both pumps

Gate valve closed 2" pump (control panel switch down)

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.

Lower TP using "Ju IS+"

Check both stepping motors on.

Enter "Ju 00", Resume - Mantiesa of preseure 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.

Resume starts run. 60 minutes running time.

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

TP raises, flap valve closes. TP background. 5 sets of sum of 10
readings.

TP test signal, as above.
Ion chamber. TP lowers, gate valve closes. Lets in gas for 90 seconds.

C-2


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


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.3l) 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(Z)
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


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


Form NBS—2s9

(REV. 12-78)

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS
WASHINGTON, D.C. 20234

533.04 duly 1, 1986

REPORT OF TEST

for

ABC Corporation

Samples: Far ultraviolet photodiode, windowless, Al,0, photocathode,
NBS serial no. 123. No previous NBS Report of Test.

Method of Calibration: Direct comparison of sample cathode
photocurrents with NBS-calibrated 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 60.0V (1604-12164) or
100.0V (50A-150A) relative to the photocathode, which was at ground
potential. The beam size on the photocathode was 6 mm x 6 mm
(S19A-1216A) or 2 mm x 2 mm (SOA-500A).


SB 7\S87/ S8.4ESR 5598498598099. 495 5122

SB ESS S935 358.2998 9898.7/98.9959.4 ss.essq

SB4ASB SSE 45B2)58S se.dseq ssa 33.3955.7|

Se cseas7. S5e.0987Ss9.295.4959799.89s335

+-

S8098 e858 3385998 9395.39935499.795. 75355

SEWS98 298.958 6)985S91 |S54939.599.595.5

SE ZIS8 S558 S585 S28 5959. 859.555.3359 8545

9859844587 S5B8)988598.7/951|S543355.5955)

se7ise49e.49e4/9e8e7(/9e.9551 |Sseq55eqceq

5e.438.298.345985/599495.4935.4 sadssi S32.

N= l2l.6nm

SPATIAL UNIFORMITY-CENTER OF EVSD PHOTOCATHODE

Figure 13
44


Instructions for using EVSD Photodiodes

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

Window: Magnesium fluoride

Photocathode: Cesium telluride (on inside of window)
Window Seal: Indium alloy (melts at *70C)

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, spectro 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 7OC will cause seal failure and destruction of
the photocathode.

Maximum total photocurrent should be kept below 1xl0-7A. 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 30004 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.

itgF, WINDOW
PROTECTIVE GLASS RING

INDIUM SEAL

CATHODE PHOTOCATHODE

GUARD ———"

ANODE ——”

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34


Appendix C

THERMOPILE CALIBRATION PROCEDURE

I. Turn on light source

1.
2.
3.

4.

7.

Outgas filament 15 A. (flap valve closed)
Open Kr valve slowly, press.about 2-3 x 107> Torr
Turn up minor arc to over 0.2 A.

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.

Set w.1. on grating drive: He 584.3 angstroms; Ne 735.9 angstroms;
Ar 919.8 angstroms (+ offset). Set on peak of line.

For He: Close Kr valve, let in He to about 0.8 x 107 torr.

(80 V. are voltage)

Ne: Let in Ne to about 8-10 x 1 -5 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.

Ar: Let in Ar slowly to 8-10 x 1075 Torr, then valve off Kr.
Arc about 45 V.

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.

Il. Electronics

. Test calculator when turned on (i.e. 3 x 2 = , etc.).

« Load program 070M @ 00 (4 cards). Enter "Ju 00", then load cards.
Idle light goes out while cards are being read.

. 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


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

iii


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.

6. When stable, valve off Kr, let in hydrogen about 1.1 x 10~* Torr.
Reduce I(major arc) to 1.1 A., 100-130 V. Set filament about 14 A.

III. Initialize data acquisition system

1. When stable, set w.1. 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: rei[][]96

b) Diode card for diode being tested
Permanent data at rcel[][]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 ITP in 10 step increments.
Stop by pressing Reset.

7. "Ju 18+" 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


#1 on paper tape

#4 Shuts gas valve, pumps out ion chamber.

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.

Waits 40 seconds, repeats TP

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.C. Plate #1 1 1
I.C. Plate #2 1 2
TP background 2 1
TP signal 2 1

THERMOPILE-DIODE CALIBRATION

I. Cover on glass window. 23 w.1. 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


After last w.1. (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 rel[][]32, switch to "WRITE", insert A side of diode data
card to update permanent data. Switch back to "READ".

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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.

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19


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 Univereity 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 4s normally grounded.
Maximum photocurrent should be kept at less than 10 amperes and the
magnesium fluoride window mst 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


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


=| 30°
r— .093
las 1.015 DIA
y r— .04 DIA
4 —
<<
-f6l — — eet
125 4
.875+.005
1.50
4-40 UNC 2B
3 PL EOL SP
ON 1.250 DIA BC
2-56 UNC 2B
[> PL EQL SP
| i
= oe
.50 }
NOTES:
|. MAT: TEFLON
2. SCALE 2-1 FIGURE 2c

10


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 fyll scale Keithley range, Resume. Must be not greater than
3x 10 .
Computer prints Keithley range K factor and line of dots.

(Program reads temporary data file to see what starting w.1. 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. When 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.1.

Can go backwards in w.1. by doing "-1", Resume twice. Deletes
QE from preceding w.1.

6. After "0" or "-1", Resume, prints date and diode no., then w.1.,
photoelectric correction, and present F value. Halts to wait for
Keithley range input with flap opeg and TP lowered. Try to keep
diode current midrange on 3 x 10 range for good TP signal.

7. Here can set new w.1. 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, s1
and insert B side of diode data card. Switch to "READ".

tech to "WRITE",

To dump data summary, "Ju IS exp(x)", Resume prints out results
from 1164-2537 Angstroms.
C-5


Results: (EVSD Model No. 54-0-000 photodiode, serial no. XXX-X,
calibrated 9/85.)

Wavelength (A) Quantum Efficiency (x100) 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

Discussion: 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 calibra-
tion does not necessarily imply equality of merit as a transfer standard.

2) The window of the sample must not be modified (e.g., by contamina-
tion, 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 recalibration by NBS is strongly recom-
mended,

A-2


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


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. Iwo 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


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
DIC (Ne) 3-15 10
15-60 40
DIC (Ar) 15-60 50
Mode Wavelengths (nm) Pressure (torr)
SIC (Ne) all 0.01, 0.02
DIC (Ne) 3-15 0.8, 2.0
15-60 0.5-1
DIC (Ar) 15-60 0.5-1

(none critical)

DATA REDUCTION

The program CALRED.BAC is used for all aspects of data reduction and is
menu-driven. The logical sequence in each spectral range follows:

1. Extrapolate 2 SIC runs to 0 pressure.
2. Calculate secondary ionization factor ("C"). Edit as required.
3. Calculate fraction of lst order ("F"). Edit as required.

4, Calculate QE of monitor over entire range (data from 1.).
5. Calculate QE of standard over F=l1 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 DIC
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.


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 ewitches 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.

BA


OPENING BEAM LINE

Before considering opening the BL-9_gate 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.


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 l,
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
I, = radiant flux entering the chamber (zero pressure)
ly = " " " " " (high pressure)

In either pressure mode the flux is calculated from:

_ i
1 SS ee T ”

22


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 line-
if 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: 1. Ju 00, load 071 program (2 cards)
2. Ju .02, load 049 and 050 in sequence

Initializing: 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").

symbol: 0 1 2 3 4 5 6 7 8 9 - £
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)

Plotter: 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. If 056 program left data in calc, ju 00(res)
and wait for idle light to come back on.
(OR)1. If using data from cards instead, rc1l0, load data,
Ju symb sin (res).

2. enter: X min (0)
X max (15)
Y min (0)
Y max (10)
# pixels per line (10)
X1 (4.1)
¥1 (9.5)
X incr (.7)
#¥/#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


TABLE OF CONTENTS

I

Il

Iv

VI

VIL

VIII

Ix

Introduction.......

Description of Service......++.+e eee c cece ececeeseeeeens seecceeee 2
a. Background
b. Goals
c. Present Services
d. Future Directions

Description of Detector Types...seeseececcecscenescceesececceceres 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)

Calibration Techniques....... Serres cece en ecreseees oe 20
a. Calibration Principles
b. Measurement Methods
Calibration Systems........+..6+ ee eeeeeeees eee ececsseees seeeeeee 26
a. SURF-II System
b. Thermopile System
c. Photodiode Intercomparison System
Treatment of Errors... 45
Safety....eseees weeeeeeeees sevcceeecceeececeeececsecccsseseseces 47
Customer Interaction 48
Acknowledgement ....seeececceccscccccccseeeecccsseseeeerees seeeeeee 49
References & Bibliography....ssssececesscccccccccccscssseses seeeee 50
Appendices
A. Sample Report of Test.........-- seeeee 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-l


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.

iy


75

.!20 DIA 3 PL_EQL SP
ON 1.250 DIA BC

NOTES:
|. MAT: TEFLON
2. SCALE 2-1 FIGURE 2d

W


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.


Starting program:
1. Ju 00(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 8864.

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 6994.

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 7358.

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. -10

9. After 1026A, set Keithley to 10x10 scale.

At the completion of 121648, 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 qes).
Ju symb "sin" (res), enter old date, (res).
To prepare for card storage: Ju symb "read", (res), rcl00, “write”,
enter card, "read".
D-6


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
needle 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 7358,
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 lst 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 -


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NBS Measurement Services:

Far Ultraviolet
Detector
Standards

NBS
Special
Publication
250-2

L. Randall Canfield
Nils Swanson

U.S. Department of Commerce
National Bureau of Standards

—


The MgF,-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 came 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 duoplasmatron 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


TIL DESCRIPTION OF DETECTOR TYPES

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

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 photoyield 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 10ma.

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.0.S.A. 39, 532 (1949).


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.


Appendix A

Form NBS—zs9

(REV. 12-78)

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS.
WASHINGTON, D.C. 20234

533.04 REPORT OF TEST September 9, 1985

for
ABC Corporation

Sample:

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-calibrated secondary
standard photodiode of the same type, both detectors housed within the
same vacuum chamber. The beam on the sample photocathode was
approximately 4 x 5 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


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3qg0Y¥19313
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Aa

Q13l4
DILANOVW


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_jon 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 00, load 062 (4 cards)
2. rc180, load wavelengths (077b)(source-pref order)
3. rcl1.10, 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.

J. 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


where f = fraction of total flux which is first order

Q; = absorption term for Ist order

Qe = " "for 2nd order
e = electronic charge
i = measured ion current

K,, Ko = multiple ionization corrections for Ist & 2nd orders

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

(£) is less than 1 are:

Radiant flux at primary wavelength:

Radiant flux at half wavelength:

2
e Qe

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

In =

(11)

(12)

efficiency at the half wavelength (where f=1) to arrive at the efficiency at

the primary wavelength:

in- ely
QE at primary wavelength 0” ©

@~ el;

where i measured photodiode current

o
"

sé

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

photoionization of a rare gas to calibrate a photodiode which can intercept
the incoming light before it passes through a thin foil (preventing the ion

(13)

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


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/mm. 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 100nm are He, Ar, Kr and Ne. For wavelengths longer than
100nm 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


where q

g

= total ion current
= electronic charge
= pressure

= temperature (C)

= gas absorption coefficient (per cm)

= 273.16 | PL
(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:

where a

b

Cy

Ce
my)

M2

r

m2 (1 - rb)

Y (ma - Mb) + M> - Mm (8)

= exponential term at T,P for primary wavelength

= . oom on

for half wavelength

= secondary ionization correction coefficient at primary
wavelength

= secondary " " “at half wavelength
= ¢ (1 - a)
= ¢p (1 - b)
i
ar

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:

FQ i

ion current from lst order io= bk Wt) FO K (9)
- 1M.

from 2nd order _... 0 Ki (10)
6 Kp

23


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
wonitor/filter need not be a part of this measurement.)

B-1


Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure

Figure

1.
2.
2a.
2b.
2c.

2d.

LIST OF FIGURES

Photocathode anodizing arrangement.......seceeeeceeeece
NBS windowless far ultraviolet photodiode.....ssssseeee

NBS windowless photodiode cathode support.......

NBS windowless photodiode anode structure....sssseeeeee
NBS windowless photodiode main body....ssseeeesseeceeee

NBS windowless photodiode cathode support retainer.

Efficiency of typical NBS windowless photodiode........
EVSD photodiode...ceececceccccccecccccecvcssecssensseee

Schematic of thermopile and ionization chamber....

Far ultraviolet detector calibration facility at SURF..
Side view of SURF detector calibration facility........
Photoelectric loss of gold black thermopile coating....

Schematic of thermopile data acquisition.....

Schematic of thermopile calibration system.....sseeeeee
Duoplasmatron light SOUrCE.. cece cece eceeccecescscenes
Schematic of thermopile....ssscscsseeccseceseecvsesvece
Power supply for duoplasmatron....cereeeescsseeeecccece

Spatial uniformity of EVSD photocathode......sseseeeeee

vi

Page

10
1l
12
17
19
27
30
32
34
35
36
38
42

44


OPERATING INSTRUCTIONS FOR NBS WINDOWLESS FAR 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 (150A-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 point--Teflon insulation is recommended.

FEE HHG EHUB HEHEHE IBGE NOTE Jag GHEE EEE EEE EE

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.

ee TTI ETI Tir irtittrititittititiitiritiirisiitititiiiiiiii iil iii ili eerie rr rr oes

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 10E11/sec. Greater flux levels may
require elevation of the anode voltage until plateau operation is obtained.

A significant percentage of the photoelectrons 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 SOA 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

4

CATHODE


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 rum 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--other-
wise, store at main reg 01).

Program will do 5374 and 5848, stopping after 5844 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 amperes.
D-8


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

7. E.B. Saloman, "Typical Photoefficiency Between 20-250 eV of Windowless
XUV Photodiodes with Tungsten and Anodized Aluminum Oxide Photocathodes",
Appl. Opt. 17, 1489 (1978).

8. R.P. Madden, "The Availability and Development of NBS Radiometric Stand-

ards", 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. C.D Moak, H.E. Banks, J.N. Thurston, J.W. Johnson, & R.F. King, "Duoplasma-
tron 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. C.M. 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 Rad-
iometry in the Far Ultraviolet", Appl. Opt. 4, 1574 (1965).

4

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


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
(4908-9208) and with the same working standard pre-calibrated

vs thermopile at 1026A and vs a windowed photodiode at 12164.

Module:

The a-b intercomparison module with provision for mounting two

photodiodes is used (the same as for windowed photodiode calibra-
tions).
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 monochromator:

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


Wavelength (A) Quantum Efficiency (x100) Probable Error (%)

622 16.88 8
639 17.60 8
657 17.83 8
669 17.94 8
683 17.81 8
699 17.42 8
712 17.39 8
735 16.43 8
752 15.57 8
771 14.68 8
800 13.35 8
818 12.41 8
844 11.54 8
865 10.91 8
886 10.32 8
920 9.39 8
1026 5.02 8
1216 1.11 8

Discussion: 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 windowed photodiodes is not likely to exceed
+3% of the quantum efficiency per year in 90% or more of the
photodiodes, periodic recalibration by NBS is strongly recommended.
Since environmental conditions can directly affect the quantum
efficiency of windowless photodiodes, it is most difficult to predict
the anticipated drift, and frequent recalibration of these photodiodes
is urged.

A-6


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 wavelength-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 de mode.

Vacuum UV light is obtained from a l-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


Appendix D

DIODE CALIBRATION PROCEDURE

Initializing system

. Perform light source startup (vacuum)
. Turn on all electronics (left scanner on ch.3)
. Vent experimental chamber
« 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

Sequence:

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.


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32


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-SURF, 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) O-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


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 directly; 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
I a
@ (i, - Ip q)
example
where i; = ion current (plate 1) 1x1071° amp (coul/sec)
iz = ion current (plate 2) ix107!4 amp
e = the electronic charge 1.6x10729 coulomb
I = radiant flux entering chamber 6.944x108 photons/sec

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

la. Direct Calibration of Photodiodes

I£ a windowless photodiode is being calibrated directly by use of the
ionization chamber, the relationship

evel (2)


9.

REFERENCES

L.R. Canfield, R.G. Johnston & R.P. Madden, "Preliminary Report on

Photoelectric Detector Transfer Standards for the Vacuum Ultraviolet",
NBS Report #9897.

J.A.R. Samson, "Absolute Intensity Measurements in the Vacuum Ultraviolet",
J. Opt. Soc. Am. 54, 6 (1964).

J.A.R. Samson & G.N. Haddad, "Absolute Photon-flux Measurements in the
Vacuum Ultraviolet", J. Opt. Soc. Am. 64, 47 (1974).

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

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).

E. B Saloman & D. L. Ederer, "Absolute Radiometric Calibration of Detectors
Between 200-6004", Appl. Opt. 14, 1029 (1975)

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).

E.B. Saloman, J.S. Pearlman, & B.L. Henke, "Evaluation of High Efficiency
CsI and Cul Photocathodes for Soft X-ray Diagnostics", Appl. Opt. 19 ,
749(1980).

L.R. Canfield, R.G. Johnston, & R.P. Madden, "NBS Detector Standards for
the Far Ultraviolet", Appl. Opt. 12, 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).

E.B. Saloman, S.C. Ebner, & L.R.Hughey, "Vacuum UV and Extreme UV Rad-
iometry Using Synchrotron Radiation at the National Bureau of Standards",
Optical Engineering 21, 951 (1982).

E.B. Saloman, S.C. Ebner, & L.R. Hughey, "Radiometry Using Synchrotron
Radiation", Proc. of SPIE Symposia 279, 76 (1981).

H. Kaase, K.H. Stephen, W.M. Burton, A.T. Hatter, A. Ridgely, L.R. Can-
field, & R.P.Madden, "Intercomparison of Radiometric Irradiance Scales
in the 90-250 nm Wavelength Range", Appl. Opt. 19, 2529 (1980).

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


Measurements associated with this calibration were made by

L. Randall Canfield
(301) 921-2031

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

est Now: 123/86-1
Date: July 1, 1986

A-7

Nils Swanson
(301) 921-2031


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 this document for the sole pur-
pose of adequately describing experimental or test procedures. In no event does such 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


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


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 1.3nm
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 um 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 micrgvolt/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


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 C1(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


INCIDENT
BEAM

ANODE

|
La

CATHODE

a

6
YUM
\
LIT.

NBS PHOTODIODE (WINDOWLESS)
(5-122 nm)

Figure 2
7


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


Measurements associated with this calibration were made by:

L. Randall Canfield
(301) 921-2031

Report of this calibration approved by:

Robert P. Madden
Group Leader, 533.04

For the Director

WiTiiam Re Ott
Chief, Radiation Physics Division

Order No.: ABC123
Test Now: XXX-X/85-1
Date: September 9, 1985


NO 51 THRU
3 PL EQL SP

-25

i

a.../_____]

SS

NOTES:
|. MAT: SS TUBING .O10 WALL
2. SCALE 4-1 FIGURE 2b

9


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


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30


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 rc1.92

Running: 1. ju 00,(res),enter: sample #(res)
std #(res)
Ist year of plot (2 digits)(res)
2. load last prev data(res)
3. load newer data(1 or more)(res)
4. after last data, ju symb 0 to plot
(flag if new plot to generate tic marks)
Note: In the 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 rc130,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-rc132
wave lengths-rc100
new data-rc196


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 100nm 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


Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National
Bureau 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 System-for 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- e Photon Physics
tions; conducts theoretical and experimental research with electron,

laser, ultraviolet, and soft x-ray radiation for measurement applica-

tions; determines fundamental mechanisms of electron and photon inter-

actions with matter; and develops advanced electron- and photon-based

measurement techiques. 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 e 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
DeveTops, operates, and improves major NBS radiation facilities
including the electron Linac and race track microtron; develops, Linac Operations

designs, and builds electronic and mechanical instrumentation for Electronic Instrumentation
radiation programs and facilities; provides national leadership @ Mechanical Instrumentation
in the standardization of nuclear instrumentation; and develops

new competence in radiation sources and instrumentation. The

Division consists of the following Groups:

Accelerator Research

Jonizing Radiation Division

Provides primary national standards, measurement services, and basic © Office of Radiation Measurement
data for applications of ionizing radiation; develops new methods of @ Radiation Theory

chemical and physical dosimetry; conducts theoretical and experimental @ Radiation Chemistry and Chemical
research on the fundamental physical and chemical interactions of Dosimetry

ionizing radiation with matter; provides essential standards and * Neutron Measurements and Researc
measurement support services to the National Measurement Support e Neutron Dosimetry

System for Ionizing Radiation; and develops and operates radiation @ Radioactivity

sources needed to provide primary radiation standards, fields, and @ X-Ray Physics
well-characterized beams of radiation for research on radiation © Dosimetry

interactions and for development of measurement methods. The Division -
consists of the following Office and Groups:

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.



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42


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 windowlees 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, moe 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


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where ip = 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
ay be used to determine the quantum efficiency. Combining equations (1) and
2) gives:

i (3)

(If the currents from both the ionization chamber plates and the photodiode
are measured with the same picoammeter, absolute calibration of the
picoammeter is not required.)

1b. 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
j .
p22 Re pepe w
: . so ~ i, -
e (i, - i,) k Rex e (1 - 12
where k = monochromator gas absorption correction

thermopile photoelectric correction

I = radiant flux incident on thermopile or ion chamber
(photons/sec)

F = thermopile calibration factor
d = wavelength (nm)
R = ratio of thermopile signal to test signal
The subsequent calibration of a photodiode using the thermopile is given by:

vo ip
ee

I

=FR'22X 80

(5)

eFR' ga

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

21


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Results: (NBS windowless Far UV Photodiode, serial no. 123, calibrated
April, 1986).

Wavelength (A) Quantum Efficiency (x100) 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 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


Running:

(if new ratios are 1/x, flag)

1, ju 00(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:
rel32(write pos) enter card (return to read pos)

PHOTOCATHODE UNIFORMITY PROGRAM

Loading photodiode:

Initializing:
1.
2.
3.
4,
5.

Note:

Loading program:

Running: Ju 00(
Enter:

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 allen screw.
Make normal electrical connections to photo-
diode (red is anode) and check continuity.
Mount module on experimental chamber and
evacuate.

Start light source as usual.

Set monochromator wavelengthjand slits to give
reasonable current on 3x10 scale

or a little higher if 1216.

Manually find edges of photodiode by scanning

in x direction until current is about half of peak.
Calculate central position based on these

readings, write down, do same for y direction.
Leave in center, leave source flap half closed.

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.

Ju 00, load 056 program (3 cards)

res)

date

sample #

wavelength (normal:1216,1403 & 1608)

# of elements (pixels) per line-usually 10

# of stepping motor steps per increment-150 for Imm incr
ratio of #y lines to #increments-usually 1

Manually scan to starting position: if doing

lem x lem scan, .090" from center on indicator in

both x and y toward the upper right.

Turn on stepping motor power supplies for x and y motors,
and open source flap.

"Resume" will start main program--program uses a ref

D-3


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


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
3
0:

measurement precision (observed)
radiation impurity (estimated)

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) O-1
Probable Error(%) 6-10

The comparison calibration of a thermopile involving a standard lamp and the
ionization chamber cited on p.3l 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


6-32 UNC 2B

BI
.O3 DP
.09
————
34 ili t
1 i
——— .735+.005 DIA

NOTES:
|. MAT: AL 6061
2. SCALE 2-1 FIGURE 2a