NBS Measurement Services:

Neutron

Personnel

Dosimetry

U.S. Department of Commerce

National Bureau of Standards
Center for Radiation Research

The Center for Radiation Research is a major component of the National Measurement Laboratory in the National 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 Systemfor ionizing radiation; conducts research in radiation related

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

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

Atomic and Plasma Radiation Division

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

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

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

or wavelength standards; develops new measurement techniques and

methods for spectral analysis and plasma properties; and collects,

compiles, and critically evaluates spectroscopic data. The Division

consists of the following Groups:

Radiation Physics Division

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

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

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

tions; conducts theoretical and experimental research with electron,

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

measurement 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 • Radiometric Measurement Services

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

spectroradiometric and spectrophotometric standards and calibration

procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Develops, operates, and improves major NBS radiation facilities

including the electron Linac and race track microtron; develops,

designs, and builds electronic and mechanical instrumentation for

radiation programs and facilities; provides national leadership

in the standardization of nuclear instrumentation; and develops

new competence in radiation sources and instrunentation. The

Division consists of the following Groups:

• Accelerator Research

« Linac Operations

• Electronic Instrunentation

• Mechanical Instrunentation

Ionizing Radiation Division

Provides primary national standards, measurement services, and basic

data for applications of ionizing radiation; develops new methods of

chemical and physical dosimetry; conducts theoretical and experimental

research on the fundamental physical and chemical interactions of

ionizing radiation with matter; provides essential standards and

measurement support services to the National Measurement Support

System for Ionizing Radiation; and develops and operates radiation

sources needed to provide primary radiation standards, fields, and

well-characterized beams of radiation for research on radiation

interactions and for development of measurement methods. The Division

consists of the following Office and Groups:

• Office of Radiation Measurement

• Radiation Theory

• Radiation Chemistry and Chemical

Dosimetry

• Neutron Measurements and Research

• Neutron Dosimetry

• Radioactivity

• X-Ray Physics

• Dosimetry

Nuclear Physics Group

Engages in forefront research in nuclear and elementary particle physics;

performs highly accurate measurements and theoretical analyses which

probe the structure of nuclear matter; and improves the quantitative

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

NEUTRON PERSONNEL DOSIMETRY

Robert B. Schwartz

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 July 1987
Library of Congress Catalog Card Number: 87-619842

National Bureau of Standards Special Publication 250-12

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-12, 44 pages (July 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
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 250-12), NBS Measurements Services: Neutron

Personnel Dosimetry, by R. B. Schwartz, is the twelfth to be published

in this new series of special publications. It describes the

preparation and calibration of neutron dosimeters and remmeters with

californium neutron sources, both "bare" and moderated (see test

number 44060C in the SP 250 Users Guide). Inquiries concerning the

technical content of this document or the specifications for these

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

contacts cited in SP 250.

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

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

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

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

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

for new calibration services, special tests, and measurement assurance

programs

.

George A. Uriano Chris E. Kuyatt

Director Director

Measurement Services Center for Radiation Research — i i i
Detailed procedures are given for calibration of neutron dosimeters and rem- meters with californium neutron sources, both "bare" and moderated. Corrections for air scatter, room-return, anisotropic neutron emission, and deviation from the inverse square law are discussed, and specific examples given.

The uncertainties in arriving at the final value for the calibration factor

are also discussed.

Key words: calibration; californium; dose equivalent; dosimeter; neutron;

remmeter.
CONTENTS

1. Description of Service 1

2. Facility Description 3

3. Corrections 5

a. Anisotropic Emission 5

b. Room Return 6 c. Air Scatter 6

d. Deviation from Inverse Square Law 7

Procedures 7

a. Spreadsheet for Dosimeter Irradiations 7

b. Spreadsheet for Remmeter Irradiation 10

c. Performing the Calibration 13

c.l. Dosimeter Irradiations 13

c.2. Remmeter Calibrations 13

5. Uncertainties 14

a. Statistical Uncertainty 15

b. Uncertainty Due to Source Calibration 15

c. Uncertainty Due to Corrections 15

d. Uncertainty in Distance Determination 15

e. Uncertainty in Fluence-to-Dose Equivalent

Conversion 16

6. Internal Quality Control 16

7. Safety 17

8. Acknowledgement 18

References 19

Appendix A. Pocket Calculator Programs A-1

Appendix B. Averaging Program B-1

Appendix C. Sample Test Reports C-1

Appendix D. Neutron Spectra D-1

Appendix E. Definitions E-1

V
TABLE OF CONTENTS, CONTINUED

LIST OF TABLES

Table 1 . Room Return Correction, S, percent per meter-squared 5

Table 2. Air Scatter Correction, percent per meter 7

Table 3- Values of Group Source Strength per Logarithmic Energy D-2

Interval for a ^^^Cf Spontaneous Fission Source

Table Values of Group Source Strength per Logarithmic Energy D-4

Interval for a ^^^Cf Spontaneous Fission Source in the

Centre of a DjO Sphere with a Radius of 150 mm

LIST OF FIGURES

Figure 1 . Simplified View of Calibration Facility

Figure 2. Spreadsheet for Dosimeter Irradiations 8

Figure 3. Spreadsheet for Remmeter Calibration 11

Figure 4. Neutron Spectrum from a ^^^Cf Spontaneous Fiss ion Source D- 3

Figure 5. Neutron Spectrum from a ^^^Cf Spontaneous Fission Source D- 4 in the Center of a D2O Sphere with a Radius of 150 mm

vi
1 . Description of Service

The National Bureau of Standards (NBS) supplies essentially "free-field"

neutron irradiations for calibrating neutron detection instrumentation. This

is done under SP 250 Number iJ4060C (formerly 8.1J)^. By "free-field" is

meant that the irradiations are performed in a relatively large, open room (specifically, the lower level of the Van de Graaff low-scatter room) with a minimum of scattering material, so that the neutron field is close to that

which would exist in the vicinity of an isolated source. Explicit corrections are made for residual scattering effects, and, in fact, these corrections form an important part of our work.

The neutron fields available are those produced by californium-252 sources,

either free in air, or in the center of a 15 cm radius heavy-water sphere

covered with cadmium^. These are referred to as the "bare" and the "moderated" sources, respectively. Whether to use the bare or the moderated source

is determined by the type of device to be calibrated and the neutron spectrum

to vrfqich it (and the radiation worker) are expected to be exposed. For calibrating albedo dosimeters, particularly if they are to be used in the relatively soft spectra found in reactor environments, moderated californium,

with its spectrum rich in low and intermediate energy neutrons, is clearly

preferred^'** (compare Figures k and 5, Appendix D) . We have shown"* that for

other common dosimeter types (NTA film, CR-39, polycarbonate) there are not

very great differences (less than a factor of 2) in calibration factors

between bare and moderated ^^^Cf. For common types of remmeters, the difference in calibration factors is a factor of ~ 1.4. Thus, moderated ^^^Cf is

the more versatile of the two types, since it is appropriate for all of the

commonly used personnel protection instrumentation. It is also the only neutron source given in ANSI N13.11^. On the other hand, the spontaneous fission

neutron spectrum from bare californium, similar to that from ^^^U fission,

has been very carefully evaluated and is better known than that from moderated californium, and higher dose equivalent rates are obtained from bare

californium. In the end, however, the customer usually has his own ideas

about which source to use (sometimes asking that the same device be calibrated with both sources), and we, of course, abide by his wishes.

The differential energy spectra, taken from a recent ISO Draft Standard^, are given in Appendix D.

Most of our work involves calibrating neutron personnel protection instrumentation, both passive (e.g., dosimeters) and active (e.g., remmeters). For

dosimeters, we irradiate to a known dose equivalent (see Appendix E for

definitions) , and return the dosimeters to the customer. The customer then

reads out the dosimeters in his usual manner, and thus obtains a calibration

for his system in terms of our delivered dose equivalent. Remmeters, on the

other hand, are placed in a field of known dose equivalent rate . We read the

remmeter and report the reading, the true dose equivalent rate, and the calibration factor which we define as the ratio of the actual dose equivalent

rate to the remmeter reading. The rest of the discussion will concentrate on these types of calibrations, but the facility can be used for calibrating any

other type of neutron detector, and at times is used for calibrating Bonner

Spheres, tissue equivalent proportional counters, etc.

1
The basic physical principles are straightforward. We start with a californium neutron source of known emission rate, Q, measured at NBS^ with an uncertainty of ~ ±1.3%. (Note that this uncertainty, and all other uncertainties given in this document, are understood to be one standard deviation

(la)). At any point a distance r from the source, the free field fluence

rate is just Q/4irr^ . Using conventional f luence-to-dose equivalent conversion factors®, we calculate the free field dose equivalent rate at that

point. We then put the instrument at the distance r and either read its

output (for a real-time instrument), or expose it for a measured length of

time in the case of an integrating device such as a "film badge".

The conversion factors for the two different spectra are obtained by taking

the energy-dependent conversion factors as given by the International Commission on Radiation Protection (ICRP)® and weighting these factors with the

neutron energy spectra. This is done via a 620 energy group calculation

which approximates a continuous integral of the product of the ICRP factors

and the neutron energy spectra, using log-log interpolation on the given ICRP

values. The calculation is done by C. M. Eisenhauer, using the code DETAN.

The numerical values are given in equations 5 and 6, below.

Note that in this system, the closest thing to a "standard" is the calibrated

neutron source. There are no "standard" instruments in this field. Note

also that once the source is calibrated (they are re-calibrated about every

six years), the principal "measurement" made during the calibration is just

the source-to-detector distance. This is generally done with an ordinary

tape measure, calibrated in meters.

There is, however, more to calibrating than simply putting a source and a detector in the middle of a room and reading the detector output. The neutron field at the detector differs from that due to the source itself due to

nuclear reactions with the walls and floor of the calibration room, with the

air in the room, and with the source encapsulation. These reactions generally result in scattered neutrons with lower energy, different angular distribution, and different variation with distance than the direct source neutrons. Further, since none of the instruments have an energy-independent

response on either a fluence or dose equivalent basis, interpretation of the

instrument reading is not as straightforward as it might at first appear.

All of these effects may be dealt with by appropriate (Monte Carlo) computer

codes, but this seems a somewhat cumbersome procedure, requiring a new calculation for each change in source-to-detector distance, source encapsulation,

and so on. Instead, we treat all of these effects as corrections, and have

developed a semi -empirical approach to making the corrections. While this

approach itself makes use of computer calculations, it offers physical

insight into the problem without requiring further significant calculations.

The corrections are discussed in section 3 of this report.

In order to get a wide range of dose equivalent rates, we use three different

sources (differing in intensity by approximately factors of 10) for the

moderated irradiations, and two sources, differing in emission rate by a factor of 50, for the bare irradiations. By using the different sources at

different distances, we generally cover the range from a few millirem per

hour to a few rem per hour for bare irradiations, and from a few millirem per

hour to - 700 mrem per hour for the moderated irradiations. We are limited

2
in the maximum dose equivalent rate for moderated irradiations by the present

californium source strength and by the fact that there are spectral changes

at close distances (< 30 cm)^''° which limit the minimum distance which can

be used. We plan, however, to get a new californium source with more than

three times the emission rate of our present strongest source, so that we will be able to do moderated irradiations up to ~ 2 rem/hour. We are also

getting a third source for bare irradiations, to give more flexibility.

The calibration uncertainty is generally in the range of - ±5% (1o). The

limitations on accuracy include a ±^%^ uncertainty in the fluence to dose

equivalent conversion factor for the moderated source, and ~ 1.3% uncertainty

in the knowledge of the neutron source emission rate. At low dose equivalent

rates the statistical fluctuations in the instrument readings usually provide

the dominant uncertainty, and at close distances the uncertainty in the

location of the effective center of the instrument is significant. These

uncertainties are discussed in more detail in Section 5.

In addition, the readings of many instruments drift in an uncertain manner

which may amount to several per cent per hour^'. While we do not understand

in detail the physical mechanism for this drift, it appears to be primarily

due to an actual change in the count rate of the BF3 proportional counter,

rather than, say, a drift in some part of the electronics. It is not known a priori which instruments will drift and which won't. Since it is much too

time consuming to measure the drift for each instrument, we consider the calibration to be a sort of "snap-shot" at some stage of the drift cycle. Since

a calibration usually takes somewhat less than an hour, there is thus the

possibility that an instrument reading taken at, say, the end of the calibration is a few percent higher than it would have been at the beginning. Since

we don't know whether an instrument is drifting or not, it is not possible to

assign a realistic uncertainty for drift. On the other hand, any drift will

manifest itself as a variation in the readings, and hence will automatically

contribute to the overall uncertainty in the calibration factor. Since the

instruments are all portable and battery operated, in actual use they are usually left on for a time period no longer than the time it takes us to do a calibration. Hence, we feel that our "snap-shot" is a realistic one. It is

not an ideal situation, but we feel that our approach is a reasonable compromise, given that these are field instruments.

2 . Facility Description

Our facility is located in the lower level of the NBS Van de Graaff accelerator low-scatter room. (See Figure 1.) This area has an 1 1 m x 12 m concrete

floor, and concrete walls which extend to a "ceiling" height of ^.6 m. This

"ceiling" is actually an aluminum grating which serves as the floor of the

low-scatter room above. The sources are stored in a 25 cm diameter, 1.6 m deep, hole in the center of the concrete floor. This hole is sub-divided

into 8 smaller holes, with each source in one of the separate smaller holes.

When not in use, each hole is shielded by a lucite and steel plug, and the

whole arrangement is covered with a polyethylene cap which is kept locked in

place to prevent unauthorized access to the californium sources. A length of

beaded chain is attached to each source; the other end of the chain is

attached to a dacron string which passes through a series of teflon-lined

eyelets and then to a fishing reel mounted behind a shield wall near the

entrance to the low-scatter room. The operator uses the fishing reel to

raise and lower the sources into position. The position of the source is

monitored by closed-circuit television cameras.

3
FIG. 1 Simplified view of Low Scatter Neutron Calibration Facility ( sections

of overhead grating floor shown removed. ). Perspective variation allows

compressed view of what is virtually a large, nearly empty room.

.1 Neutron source being raised into position

.2 T.V. camera for monitoring of source in exposed position.

.3 Remote mobile platform,

.4 PRS-2 neutron meter in position for calibration.

.5 T.V. camera for monitoring of neutron meter read-out.

.6 T.V, camera for monitoring of platform position.

.7 Platform position indicator.

.8 Ball screw and nut assembly.

.9 Source storage facility.

10. T.V. camera for monitoring source movement in and out of

storage facility.

4
The rest of the facility consists of a low mass stand approximately 2.1 m high. The top of the stand is an aluminum platform, approximately 60 cm x 60 cm X 6 mm thick, to support the instruments to be irradiated. The stand is on casters which ride on angle irons set in the floor to serve as tracks.

Another closed-circuit television camera mounted on an arm attached to the

stand enables the instrument being calibrated to be read remotely.

3 . Corrections

Although, as indicated above, our facility is designed to come as close as possible to providing "free-field" irradiations, there are still important

corrections which must be made. We have identified four separate corrections: correction for anisotropic source emission, for neutron scattering

off the walls, floor, and ceiling ("room return"), for air scattering, and

for the deviation from the 1 /r^ law due to the finite size of the source or detector. (It should be noted that our approach does not give consistent

results in very large calibration halls (linear dimensions ~ 20 m or more)

which use grating floors. We think that this is probably due to scattering

off the grating which our model does not take into account, but this has not

been conclusively demonstrated. In any case, the method which we describe

below must be used with caution in these kinds of rooms.)

3. a. Anisotropic Emission

Neutron source emission rates are determined by a manganese bath calibration^, which measures the total emission rate into 4tt . Dividing the emission

rate, Q, by ^irr^ would give the fluence rate at a distance r _if the neutron

emission were isotropic. Neutron scattering within the source encapsulation,

however, usually results in anisotropic emission. Spherical encapsulation

does not give any asymmetry; a right circular cylinder with diameter equal to

the height minimizes the asymmetry, and a long circular cylinder maximizes

the asymmetry. Our bare sources are right circular cylinders, viewed in the

direction perpendicular to the cylindrical axis. For this case, the effect

of the internal scattering is always to increase the fluence rate over that

which would be calculated from Q/^irr^. Since the scattered neutrons have a different energy spectrum from the source neutrons, the effect on any instrument depends upon that instrument's energy response. We use anisotropy

correction factors to correct for the increase in instrument response, and

thus there are different corrections for each instrument-source combination.

The corrections are based on to-be-published calculations and measurements by

C. M. Eisenhauer and J. B. Hunt, and estimation in cases where there are no

data.

For the bare sources and devices such as remmeters and NTA film which respond

primarily to higher energy neutrons, the correction is essentially zero for

the lightly encapsulated NS-series sources, and 3% for the heavier, SR-Cf,

sources. For devices more sensitive to lower energy neutrons, such as the 3"

sphere or albedo dosimeters, the correction is taken to be ^% and 3-7? for

the NS and the SR-Cf sources, respectively. The correction is independent of

the source-detector distance.

Scattering within the moderator makes the field produced by the moderated

source more nearly isotropic, and therefore no anisotropy correction is made

for those irradiations.

5
3. b. Room Return

It has been shown^^'^^ that room scattered neutrons are essentially uniformly

distributed throughout the room, and since the fluence of source neutrons

varies as 1 /r^ , if we first correct for air scattering (see 3.c.), we can, in

general, write for the total response, D, at a distance, r:

D = R+ R= D/r2 + R, (1) o s o s where R is the response of the detector to the source neutrons,

R is the response to the room scattered neutrons, and

D is the response at unit distance to the source neutrons alone.

o

^

The quantity D is the quantity which we are trying to determine. If we define the quan'^ity S as

S = R so/D , (2)

we can re-write eq. 1 as

Dr^ - D (1 + Sr^). (3 o The quantity S is, physically, the fractional room-return correction at unit

source-detector distance. By making measurements of D as a function of r and

fitting to equation 3. we have determined the quantity S for bare and for

moderated californium for the usual devices which we calibrate. The results

are listed in Table 1 , in units of percent scatter correction, per metersquared.

Table 1 - Room Return Corrections, S, percent per meter-squared

Device Bare Moderated

3" Sphere 33^ TJi

Albedo Dosimeter 20 5.4

9" Spherical Remmeter 6 7.9

Andersson-Braun Remmeter 2.^

3.C. Air Scatter

The results of transport calculations show that neutron air in-scattering is

approximately twice the out-scattering, so that the net effect is always to

increase the fluence at the detector. The effect varies linearly with the

source-detector distance. The in-scattered spectrum is, however, shifted to

lower energies due to the kinematics of the elastic scattering from nitrogen

and oxygen. The effect of the in-scattered neutrons is represented by an integral over the energy and angular response of the detector. The calculated corrections, in percent per meter, are listed in Table 2, taken from

references 1M and 15. They are based on unpublished Morse Monte Carlo air

transport calculations by R. C. McCall and energy response measurements by D. Hankins^ ^

.

6
Table 2 - Air Scatter Correction, percent per meter

Device Bare Moderated

3" Sphere

Albedo Dosimeter

Remmeter

NTA Film, Track Etch Dosimeter

1 .1%

1 .

1

1 .0

0.5

3.0

2.3

0.9

3.d. Deviation from Inverse Square Law

For a spherical detector and a point isotropic neutron source, Axton^' showed

that the response of the device is increased by a factor of approximately

above that expected by the inverse square law, where a is the radius of the

sphere, r is the distance from the source to the geometric center of the

sphere, and 6 is a numerical constant less than unity, whose value was estimated to be ~ 2/3. J. B. Hunt'® has shown that 6 depends upon the sphere

size. Following Hunt, we use 6 = 0.^ for a 3" diameter sphere, 0.5 for a 9" or 10" diameter sphere, and 0.55 for an Andersson-Braun remmeter. Ing and

Cross' ^ have shown that the case of a spherical source irradiating a small

detector is formally similar, and we therefore use the same form of correction, with 6 taken to be 0.5. For a spherical source irradiating a spherical detector, we modify the above factor to

where b = 15 cm, the radius of the moderated source.

^ . Procedures

The first step is to decide on the dose equivalent rates, and hence the

distances and sources, to be used for the calibrations. This is done with

the help of spreadsheets. The operator enters the source emission rate and

the distance into the spreadsheets; the software then calculates the free

field dose equivalent rate, the scattering correction, and the other quantities necessary for the calibration. Several of the equations used may also

be conveniently programmed on a Hewlett-Packard Series 10 calculator; these

programs are listed in Appendix A. The procedures for calibrating passive

devices, such as dosimeters, are different from those for calibrating active

devices (remmeters). Hence, we shall consider them separately. The best way

to do this is to consider the entries in the spreadsheets, item by item.

4. a. Spreadsheet for Dosimeter Irradiation

Figure 2 is the spreadsheet used for albedo dosimeter irradiations. (Digital

Equipment Corporation (DEC) Professional 350 computer; C-Calc spreadsheet

software.) This irradiation was done on January 30, 1985 and actually con- sisted of a series of irradiations: 300, 500, and 1000 mrem total dose

equivalents with both bare and moderated californium. (After this calibration was completed, the spreadsheet was copied onto a diskette for a perma-

[1 + 6(a/2r)"]

{1 + 6[(a^ + b^)/(2r)^]}

7
nent record. This spreadsheet is then used as a template for the next dosimeter irradiation. One does not lay out a new spreadsheet for each new calibration.

)

DOSIhETER IRRADIATIONS January 30. 1985

Source Q(n/s) D( cm)

NS-86 5.477e+08 50.8

NS-39

Cf-136

NS-79 2.554e+08 50.8

Cf-132

FFDE CORRECTION EFF.FFDE hrs/mrem

488.1

944.2

4.01 %

6.72 %

507.7

1007.7

0.001970

0 .000992

Source Dose(mrero) Time(hr) Dose Time( hr

)

Dose Time( hr

)

NS-86

NS-39

Cf-136

NS-79

Cf-132

300

300

0 .5909

0 .2977

500

500

0 .9849

0 .4962

1000

1000

1 .9698

0 .9924

Figure 2. Spreadsheet for Dosimeter Irradiations

On this worksheet, the first column gives the identification numbers of the

five sources which we are currently using. These identifications are not

used in any calculations, but are for the convenience of the operator. The

second column in the upper part of the spreadsheet is the source emission

rate, in neutrons/second. Since the sources decay at the rate of 0.5? per

week, the appropriate emission rate must be entered.

For dosimeter irradiations we usually have little choice in the sourcedetector distance, D. According to ANSI N13.11^ i all dosimeters are to be

mounted on the front face of a 40 cm x 40 cm x 15 cm thick rectangular lucite

phantom, the distance between the center of the front face of the phantom and

the center of the source is to be 50 cm, and the distance D is to be measured

between the source center and the point on the front face of the phantom

directly behind the sensitive element of the dosimeter. For a single dosimeter centered on the phantom, clearly D = 50 cm. In general, however, one irradiates more than one dosimeter at a time (up to - six); the dosimeters

are then arranged in a circle centered on the phantom face. In this case the

circle was 9 cm in radius, hence:

D = SQRT(502 + 92) = 50.80 cm. (4)

The free field dose equivalent rate (FFDE) for NS-86 (a moderated source) is

calculated from

FFDE ^ = .0023*Q/D^ mod (5)

8
and for NS-79 , a bare source,

FFDE^ = .00954*Q/D^ (6) bare

where the numerical factors are the f luence-to-dose equivalent conversion

factors discussed in Section 1

.

The correction (percent) for the moderated source (NS-86) is

CORRECTION(?) ^ = 5.4*(D/100)2 + 3*D/100 + 281 3/D% (7) mod

where the first term accounts for room scattering (5.4? per meter-squared),

the second term is air scattering (3% per meter) , and the third term corrects

for the non-1 /r^ behavior of the field. D is in cm. The corrections would

be of the same form for the other two moderated sources. For the bare source

(NS-79),

CORRECTION (5J)^ = 1 + 20*(D/100)2 + 1.1*0/100, (8) bare

where the first term is a ]% correction for non-isotropic emission, the

second term is a 20% per meter-squared room scattering correction, and the

third term is a 1.15^ per meter air scatter correction. The correction for

the other bare source (Cf-132) is of the same form except that the nonisotropic emission correction is 3-7?, rather than ^%.

Now, each of the terms in the scattering corrections result in extra neutrons

reaching the dosimeter, and therefore they increase its reading over a free

field by the amount indicated. But note that this is not the increase in the

dose equivalent; the dose equivalent is actually increased only slightly. As

explained in Section 3, the correction actually represents the increase in

the reading of the dosimeter, whether or not the dosimeter is accurately

measuring the dose equivalent. We then calculate an "effective" dose equivalent rate as

EFF.FFDE = FFDE*(1 + CORRECTION), (9)

and the irradiation time per millirem as

hrs/mrem = 1/EFF.FFDE. (10)

The desired irradiation dose equivalents are then entered in the "Dose"

columns in the lower half of the spreadsheet, and the irradiation times are calculated, simply, as

Time(hr) = Dose*hrs/mrem. (11)

It was not necessary, in this case, to do the calculations for the other

sources, since for the sources selected (the strongest sources) the irradiation times turned out to be convenient. One would only consider using the

weaker sources if the desired dose equivalents were so low that the irradiation times became so short that there was a possibility of significant timing

errors. As a rule-of -thumb , the minimum desirable irradiation time is taken

to be ~ 5 minutes.

9
4.b. Spreadsheet for Remmeter Irradiation

Remmeter calibrations are quite different from dosimeter calibrations. For one thing, in the latter case one never sees the results. The dosimeters are irradiated and sent back to the owner who takes care of the processing and

reading. In addition, the distance is generally fixed (as explained above),

and, of course, each dosimeter only gets irradiated once.

For remmeters, we read the remmeter, apply the corrections, and derive a calibration factor. (Ratio of free-field dose equivalent rate to corrected

reading) It is necessary to calibrate the remmeter over a wide range of dose

equivalent rates, and therefore generally all the sources will be used, at

two or three different distances. We use two computers for remmeter calibrations: the Pro 350 for initially setting up the calibrations and for permanent storage of the results, and a portable computer to actually take to the

calibration room and calculate the results as the calibration progresses. In

that way, one has all the calibration factors and uncertainties calculated a few minutes after the calibration is done. We use the spreadsheet "Lucid" on

the Tandy 200 computer. If several instruments are to be calibrated, a separate spreadsheet should be prepared for each instrument by copying the original worksheet, but giving it a name which will identify the particular

instrument being calibrated. (If there is not much else stored in the Tandy

200 memory, there is room for about 17 spreadsheets.)

The use of two computers is primarily a matter of convenience: the Pro 350

is physically awkward to move back and forth, and the bulk storage available

for the Tandy 200 (magnetic tape cassettes) is not appropriate for archival

storage. As an incidental benefit, the use of two independent computers furnishes a valuable safeguard against "computer error".

Figure 3 shows a spreadsheet for the calibration of a 9-inch spherical rem- meter with moderated californium on April 23, 1986. This calibration was done in six separate "runs"; the first column just identifies the runs. The

second and third columns are the source identification numbers and emission

rates, as described above. The fourth column is the distances used, and the

fifth column is the calculation of the free field dose equivalent rates, also

as described above. To start out, one chooses distances D, either on the

basis of guesses or by choosing the ones which were used last time. The

spreadsheet then calculates the FFDE rates, and then one iterates on the

values of D to get an appropriate range of FFDE rates. In general we do not

go below 3 or 4 mrem/hr for the lowest rate, nor choose a D value smaller

than 25 or 30 cm to get the highest rate. (Since the instrument is - 23 cm

in diameter, the deviations from the 1 /r^ law become large and uncertain at

closer distances.) It is not coincidental that each distance is used three

times, once for each source. This makes the work a little simpler: by doing

the calibrations in the order in which they are listed on the worksheet, we only have two distances to measure. More important, however, is the fact

that this furnishes an important internal check. If one gets two different

values for the calibration factor for two measurements with different sources

at the same distance, one can be quite certain that it's a fault in the

instrument rather than a possible error in distance measurement. On the

other hand, if one distance triad gives a set of calibration factors significantly different from those obtained at the other distance, than the possibility of an error in the distance measurement should be considered.

10
PRS-2 S/N 348 Calvert Cliffs Test 237140

run Source Q(n/s) D( cm) FFDE D' (cm) Corr Read CRead CalF

1 136-z 4.959e+06 60 3.17 33

.

3 4.8 4.38 4.18 0 . 76

2 NS-39 5.007e+07 60 ol . 33. 3 4.8 44.4 42.3 0 .76

3 NS-86 3 . 977e+08 60 254.1 33. 3 4 .

8

344 .

7

328 .

8

0 .77

4 NS-86 3.977e+08 38 633.5 11 . 3 4.5 849.6 813 0 .78

5 NS-39 5.007e+07 38 79.7 11 . 3 4.5 110 .1 105.4 0 .76

6 136-z 4.959e+06 38 7.90 11 . 3 4.5 10 .85 10.4 0 . 76

ratio 80 .2 Apr . 23 , 1986

Stat cal Corr Dist Sum Sum+4 FFDE Cal F

11 1.3 1.0 1.7 11.2 11.9 3.17 0 .758

2.2 1.3 1.0 1.7 3.2 5.1 31 .99 0 .755

0.5 1.3 1.0 1.7 2.4 4.7 254.1 0 .773

0.5 1.3 0.9 2.6 3.1 5.1 633.5 0 .779

1.5 1.3 0.9 2.6 3.4 5.3 79.7 0 .757

6.2 1.3 0.9 2.6 6.9 8.0 7.90 0 .761

Figure 3. Spreadsheet for Remmeter Caibration

The quantity D is defined as the source-to-detector center-to-center

distance. The quantity D' (= D - 26.7) is the source surface to instrument

surface distance, where D and D' are in centimeters, and is the distance

which is actually measured.

The correction, Corr, is of the same general form as for the dosimeter

calibrations

:

Corrii) = 7.9*(D/100)2 + 2.3*D'/100 + 4^446/0^ (12)

where the first term is a 1.9% per meter-squared room scatter correction and

the second term is a 2.3% per meter air scatter correction. The last term is

the correction for deviation from the 1 /r^ law. The quantities D and D' are

in centimeters. The numerical values for 6 (=0.5), a ( = 11.4 cm), and b (15

cm) discussed in section 3.d. are incorporated in the numerical coefficient

of the last term.

The instrument reading is entered into the "Read" column by the operator. If

the instrument has an analog readout, the operator mentally averages any

fluctuations in the reading to get the value to be entered. He then notes

11
the magnitude of the fluctuations, and estimates the la value of these fluctuations (see p. 14), which is then taken to be the statistical uncertainty.

With a digital readout, several readings (typically, 5 to 8 ) are averaged,

using the program "AVERAG" (Appendix B) . For the most common digital instrument (Eberline PRS-2) one switch-selects the number of counts to be accumulated, and thus the statistical precision, for the individual readings. AVERAG then computes the weighted mean, and compares the sample-based weighted

standard deviation with the standard deviation calculated assuming that the

weights approximate the true (reciprocal) variances of the individual

readings. See p. B-1 for a more complete discussion of this point.

The corrected reading, CRead, is just

CRead = Read/(1 + .01*Corr), (13)

and the calibration factor, CalF, as defined above, is

CalF = FFDE/CRead. (14)

The entry marked "ratio = " is the "Q" for NS-86 divided by that for 136-z,

The division is performed by the spreadsheet when new values are entered for

these quantities. Since this ratio is almost independent of the decay of the

sources, the constancy of the ratio from one calibration to the next serves

as a check that the correct emission rates have been entered.

The lower half of the worksheet is used solely to calculate the uncertainty

in the calibration factor. As indicated earlier, all uncertainties are given

as one standard deviation (1o). The first column, "Stat", is the statistical

uncertainty in the instrument reading, as discussed above. The second

column, "cal", is the estimated uncertainty in the source calibration. As

indicated, this is 1.3?. The next column, "Corr", is the uncertainty in the

correction, equal to 205^ of the correction itself which is listed on the top

half of the worksheet (Section 5.b.). The "Dist" column is the uncertainty

in the effective distance due to uncertainty in the location of the effective

center of the instrument. This is taken to be 0.5 cm, and the correction is

then numerically equal to 100/D, with D in cm (Section 5.c.). "Sum" is the sum in quadrature of the preceding four columns, and is taken to be the total

random uncertainty in the calibration factor. The next column, "Sum+4" adds

(in quadrature) the ^% systematic uncertainty in the fluence to dose equivalent conversion factor^ ° to arrive at the total uncertainty. The last two

columns, "FFDE" and "Cal F" are the same as the fifth and tenth columns in

the upper part of the worksheet, and are listed for convenience. The uncer- tainties are discussed in more detail in Section 5.

To sum up the use of this spreadsheet:

Before beginning the actual calibration, one enters the run number, source identification, source emission rate, and uncertainty in the emission rate in

the appropriate cells in the spreadsheet. One then determines the distance,

D, to be used. At this point the spreadsheet calculates FFDE, D'(cm), and

Corr in the upper part of the worksheet, and the uncertainties in the correction and distance in the lower part. As the calibration itself proceeds,

one enters the reading ("Read") and the statistical uncertainty ("Stat") in

the appropriate cells, and the spreadsheet will calculate the corrected

12
reading ("CRead"), the calibration factor ("CalF"), and the over-all uncertainty in the calibration factor ("Sum" and "Sum+^").

Similar spreadsheets have been prepared for the other types of remmeters

which we encounter, for both the bare and moderated sources. The spreadsheets are all of the same general form, but with different numerical values

for some of the quantities.

4.C. Performing the Calibration

Again, the procedure for calibrating dosimeters is quite different from that

for calibrating remmeters, and so we shall discuss them separately.

^.c.1. Dosimeter Irradiations

In general, dosimeters are mounted on a phantom. While a variety of phantoms

are available, the ANSI phantom^ mentioned in section 3.a. is to be used

unless there is a specific reason not to do so. Since it weighs ~ 28 kg,

carrying the phantom up a ladder and positioning it on the stand is not a trivial procedure. This, however, is not considered an adequate reason for

not using it. Two lab jacks are placed on top of the calibration stand, the

phantom is placed on the jacks, and the jacks are used to adjust the height

of the phantom so that its center is on a horizontal line with the center of

the source.

To simplify the mounting of the dosimeters, ^0 cm x ^0 cm x 1/iJ" thick lucite

sheets are available. The dosimeters are mounted on a lucite sheet, and the

sheet then taped to the front face of the phantom. This is much easier than

standing on the ladder and trying to mount the dosimeters directly on the

phantom. The dosimeters are generally held in place with double-backed foam

tape. To avoid possible neutron scattering from one dosimeter to another,

one should not irradiate too many dosimeters at once; the rule-of -thumb limit

is ~ 6. The sensitive element of the dosimeter should not be closer than 10 cm from the edge of the phantom^", and the dosimeters should be arranged on the periphery of a circle of some convenient radius so that they are all at

the same distance from the center of the source. The stand is then moved so

that the front surface of the lucite sheet (which is now considered to be the

front face of the phantom) is 50 cm from the source center.

It is now just a matter of removing the shield plug from the hole containing

the proper source, attaching the beaded chain to the source, pulling the source up out of the hole, and starting the stopwatch. At the end of the irradiation time, the source is lowered back into the hole, the hole covered

with the shield, and the dosimeters taken down.

4.C.2. Remmeter Calibrations

The remmeter is placed on a lab jack on the calibration stand and adjusted so that the remmeter and source centers are at the same height. The instrument

package is positioned so that it can be read by the television camera attached to the stand.

Any necessary range switching is done at this time. The Eberline PNR-4, the

most common type of remmeter, effectively has automatic range switching, so

13
this step is not necessary. The Eberline PRS-2, the next most common instrument, is digital and does not require range switching. It does, however,

have a switch to select the number of counts it will accumulate, and hence

the statistical accuracy, for each update of the reading. The count range

switch has four positions, corresponding to 10, 100, 1000, and 10000 counts.

The lowest (10 counts) position is too few counts to be of any use. Since

the instrument's intrinsic count rate is approximately 50 counts/minute per

millirem/hr, when reading ^ mrem/hr the instrument will require 30 seconds to

reach 100 counts, and that is the position which is used at the lowest dose

equivalent rates. On the other hand, at 2 rem/hr it only takes 6 seconds to

accumulate 10,000 counts, so, clearly, that is the position to use at the

highest dose equivalent rates. Somewhat arbitrarily, we use the 100 range

for the two lowest dose equivalent rates, the 1000 scale for the two intermediate rates, and the 10,000 scale for the two highest rates. No attempt is

made to maintain a constant statistical precision at all dose equivalent

rates, but a minimum of ^ readings is taken at each point.

Most of the other instruments (e.g., "Snoopy", Ludlum Remmeter) do require

conventional range switching.

Once the instrument is set up, the television camera is focused and adjusted.

Then the plug is removed from the hole, the desired source attached to the

chain, the source raised into position, and the instrument reading observed

on the television monitor and written down in the notebook. The rest of the

calibration data are taken by lowering the source, changing either the source

or the distance or both, making any necessary changes in the instrument range

switch, and continuing.

At convenient intervals during the course of the calibration (or perhaps at

the end) the readings and statistical uncertainties are entered in to the

spreadsheet and the results noted. If there are significant differences in

the calibration factors at different dose equivalent rates, the operator

should carefully examine the data to see whether the instrument is faulty (an

all too common occurrence)'^ or v^ether there is a possibility that an error

was made in the calibration itself. It may be necessary to repeat one or two

points. In any case, the operator should satisfy himself that the calibration was done properly before taking the instrument down.

The final report for the instrument calibrated using the spreadsheet in

Figure 3 is given in Appendix C. There were actually three instruments calibrated at this time, as is clear from the report. The spreadsheet in Figure

3 is for just one of them.

5 Uncertainties

It must be emphasized that our systematic approach to the corrections for

neutron scattering and "geometry" effects is a relatively new development,

and is still a subject for research, both here and at our sister laboratories

in the U.K. (NPL) and in Germany (PTB). Thus, while some of the components

of the overall uncertainty are well established and easily defended, other

uncertainty components are (at least for the time being) estimates based on a few specific measurements, and our general experience. With this in mind, we can discuss the error components illustrated on the bottom portion of the

spreadsheet in Figure 3. The numerical values given in the columns marked
"Stat", "cal", "Corr", "Dist", "Sum", and "Sum+^l" in the bottom part of

Figure 3 are the uncertainties expressed as percentages. Again, all uncertainties are given as one standard deviation (1o).

5. a. Statistical Uncertainty ("Stat")

This is discussed in section ^.b,, "Spreadsheet for Remmeter Irradiation",

and in Appendix B. To summarize, for an instrument with digital output,

several readings are taken, and using the code "AVERAG", the weighted mean,

the standard deviation of the mean computed from the weights (i.e., the

counting statistics) , and the sample-based standard deviation of the mean are calculated. Whichever of the two standard deviations thus calculated is the

larger is taken as the statistical uncertainty. For instruments with an analog output, the fluctuations in the meter reading are noted and an estimate is then made of the standard deviation. This is done by estimating the

magnitude of the "swing" of the needle which encompasses most (i.e., 955^) of

the readings, and assigning that a value of 2a. This value is then just

divided by 2, and converted to a percent of the average reading. The uncertainty is thus objectively correct for digital instruments, and, necessarily,

rather subjective in the case of instruments with only an analog reading.

The statistical uncertainty is usually the dominant uncertainty at the lowest

dose equivalent rates, and negligible at higher rates.

There is considered to be nothing analogous to a statistical uncertainty when

irradiating dosimeters.

5.b. Uncertainty Due to Source Calibration ("cal")

The calibration of neutron sources has a long and well documented history at

NBS^ , and the uncertainty in the source calibration has been carefully investigated and is well understood. It is usually ~ ^.3%, as it is in this case.

5.C. Uncertainty Due to Corrections ("Corr")

The general form of the correction factor is the same for all instruments,

but the numerical coefficients will differ from one instrument type to another, and between the bare and moderated sources. We assume that the numerical

values will be the same for all instruments of the same type, for the same

source. An uncertainty of 20% of the value of the correction itself is

assumed. Note that the column marked "Corr" in the upper part of the spreadsheet is the value of the correction factor in percent. In an unfortunate

choice of notation, the uncertainty in the correction is also labelled

"Corr", in the lower part of the spreadsheet. The latter is 20% of the

former. This is based on the estimated uncertainty in our determinations of

the room return correction (Section 3), and to allow for the fact that the

manufacturer sometimes makes changes in the construction of the instrument

without telling anyone (and, at times, without being aware of it himself).

Thus, supposedly identical instruments are not necessarily identical, and

hence the corrections may not always be exactly right. The 20% is an estimate, based on our general experience. It is probably somewhat conservative.

5.d. Uncertainty in Distance Determination ("Dist")

This is not an uncertainty in the measurement of the distance; it is rather

15
an uncertainty due to our lack of knowledge of the location of the effective

center of the instrument. It has been shown by J. B. Hunt^® that for a spherically symmetric instrument, the effective point of measurement can be

taken as the instrument's geometric center. Unfortunately, none of the

instruments in common use in the United States is spherically symmetric.

While the most common types do have spherical moderators, the detector itself

is a cylindrical proportional counter 1.3 cm in diameter by 2.5 cm long.

Based on our general experience, we assume that this introduces a 0.5 cm uncertainty in the effective source-detector distance. This means a percentage uncertainty in D of 0.5/D, where D is expressed in meters. Since the

fluence varies as 1 /D^ , the percent uncertainty in the fluence, and hence the

dose equivalent, is twice the percent uncertainty in the distance, or just

1/D, with D in meters. This is quite Important at the smallest distances.

As indicated above, when irradiating dosimeters, by convention the effective

point of measurement is taken to be the point on the phantom face directly

behind the dosimeter, and, by convention, there is then no uncertainty in D.

5. e. Uncertainty in Fluence-to-Dose Equivalent Conversion

The energy-dependent f luence-to-dose equivalent conversion factors given in

ICRP Publication 21® are taken as "given", in that these are internationally

agreed-upon factors with no uncertainties in any utilizable form being cited.

Therefore uncertainties in the conversion factors are assumed to be solely

due to uncertainties in the energy spectra of the neutron fields. For bare

^^^Cf, the uncertainty is negligibly small since the spectrum is well known

in the energy region responsible for most of the dose equivalent^'.

There is only one published calculation of the moderated californium spectrum^ and we use a conversion factor derived from weighting the Ing and Cross

spectrum with the ICRP 21 data. The conversion factor thus obtained was verified experimentally at NBS'°; the measured value agreed with the calculated value within 2%, with an estimated uncertainty of ±'^%. We therefore assume a value of 4$ for the estimated uncertainty in the conversion factor

for moderated californium.

For each run, the uncertainties listed in the first four columns of the lower

part of the spreadsheet are added in quadrature to obtain the total random

uncertainty, and given in the column marked "Sum". To this is added (again

in quadrature) the estimated ^% uncertainty in the conversion factor, to

obtain the total uncertainty ("Sum+^").

6

.

Internal Qual ity Control

As indicated above, there are no "standard" instruments in this field. In

fact, there are no instruments other than the field instruments of the type

which we calibrate. We have therefore purchased one of these instruments and

carefully calibrated it with both the bare and moderated sources. Since this

instrument has an adjustable, digital, calibration multiplier, we can easily

set the instrument so that its calibration factor is unity for whichever

source we are using. We then "calibrate" this instrument along with other

remmeters. If the calibration factor stays at unity, we can be confident

that we have not made any mistakes.

16
It must be said, however, that the most important form of internal quality

control is just watching the results carefully as they come in. This is

particularly true in the case of instruments with a digital readout. As

indicated in the "Procedures" section, at a given calibration point several

readings are taken, and averaged using the program "AVERAG". This program

computes the weighted mean, the sample-based weighted standard deviation of

the mean, and the standard deviation of the mean computed from the assigned

weights. (See Appendix B). If the former uncertainty is significantly

greater than the latter, it is a signal that something may be wrong; e. g. , a number read incorrectly or entered into the computer incorrectly, an initial

transient when the instrument was turned on, a warm-up problem, etc. In any

case, some judgment is required.

Our experience indicates that for most instrument types, calibration factors

are usually reasonably constant for a given instrument, within the uncertainties, over the range of dose equivalent rates which we use. (The calibration factor is, however, by no means necessarily equal to unity, nor is it

necessarily the same from instrument to instrument. ) ^

^ As indicated in the

"Procedures" section, an outlier should be looked at carefully; two outliers,

if associated with a particular distance or a particular source, should be

looked at very carefully.

Finally, we generally calibrate several instruments of the same type during a calibration session, using the same set-up for each. We then carefully

compare results among the instruments to look for correlations among any

outliers which might suggest a systematic error in some part of the

calibration.

Unfortunately, none of these quality control measures are applicable to a passive device such as a dosimeter. It should be clear that these calibrations are much easier to do than remmeter calibrations, and one just has to

be careful to get it right the first time.

7. Safety

We are concerned with the safety of the operator and of other personnel who

may be in the vicinity of the calibration facility. The safety of the latter

group is assured by simply keeping them out of the calibration area. There

is only one entrance to the calibration area (there are two exits, but one can only be opened from the inside) and the operator does the calibrations

from a position just inside the doorway. The operator thus acts as a guard

to keep people out. For long irradiations, if the operator is not present,

the door is locked and only the operator and Health Physics have keys. As

additional warnings, the door to the Van de Graaff control room is closed

(but not locked), and has a sign posted noting that a calibration is in progress. Additionally, an illuminated warning sign (controlled by a Geiger

counter) at the entrance to the facility is automatically turned on whenever

a californium source is taken out of its hole.

The operator himself must wear a neutron dosimeter issued by Health Physics,

and should have a remmeter with him at all times.

17
8 . Acknowledgement

I should like to thank Charles Eisenhauer for his close collaboration and

expert guidance in making this calibration service a reality.

18
REFERENCES

1. G. A. Uriano, E. L. Garner, R. K. Kirby and W. P. Reed, "NBS Calibration

Services Users Guide 1986-1988," NBS Special Publication 250, July, 1986.

2. R. B. Schwartz and C. M. Eisenhauer, "The Design and Construction of a DjO-Moderated ^^^Cf Source for Calibrating Neutron Personnel Dosimeters Used

at Nuclear Power Reactors," NUREG/CR-1 20^1 , January, 1980.

3. G. W. R. Endres, J. M. Aldrich, and L. W. Brackenbush, "Neutron Dosimetry at Commercial Nuclear Sites - Report on Subtask B," Pacific Northwest

Laboratory, PNL-SA-9201 , January, 1981.

4. R. B. Schwartz and C. M. Eisenhauer, "Use of a DjQ-Moderated ^^^Cf

Source for Dosimeter Testing and Calibrating," Proc. Eighth DOE Workshop on Neutron Dosimetry, Louisville, Kentucky, June 18-19, 1981. PNL-SA-9950, p. 153.

5. "American National Standard for Dosimetry - Personnel Dosimetry Performance - Criteria for Testing," ANSI N13.1 1-1981 (New York: American National

Standards Institute) (1983).

6. Draft International Standard ISO/DIS 8529, "Neutron Reference Radiations

for Calibrating Neutron Measuring Devices Used for Radiation Protection

Purposes and for Determining Their Response as a Function of Neutron Energy",

1986.

7. E. J. Axton, "Intercomparison of Neutron-Source Emission Rates(1979-

198^4)", Metrologia 23, 129-1^4 (1986/87); E. D. McGarry, V. Spiegel, and Li

Linpei, in: BIPM Com. Cons. Etalons Mes. Ray. lonisants (Section I), 7, 1985.

8. ICRP Publication 21 , "Data for Protection Against Ionizing Radiation

from External Sources," Pergamon Press, Oxford, 1973.

9. H. Ing and W. G. Cross, "Spectral and Dosimetric Characteristics of a DjO-Moderated ^^^Cf Calibration Facility," Health Phys. _46, 97-106, ( 198^^).

10. R. B. Schwartz, C. M. Eisenhauer, and J. A. Grundl, "Experimental Verification of the Neutron Spectrum from the NBS DjO-Moderated ^^^Cf Source,"

NUREG/CR-3^00 (1983).

11. R. B. Schwartz, "Experiences in Calibration of Neutron Survey Instruments," in Proc. Workshop on Radiation Survey Instruments and Calibrations,

Gaithersburg, Md. , July 10-12, 198^^. PNL-SA-1 33^6 , pp. F.35-F.43 ( 1985).

12. A. K. Savinskii and I. V. Filyushkin, "An Estimate of the Contribution

by Neutrons Scattered in Enclosed Rooms to a Total Radiation Dose," Proc.

Symp. on Neutron Monitoring for Radiation Protection Purposes, IAEA, Vienna,

December, 1972, Vol. I, p. 271. (In Russian, available in translation as UCRL-Trans-1 0771 from University of California, Livermore, California).

13. C. M. Eisenhauer, R. B. Schwartz, and T. Johnson, "Measurements of the

Neutrons Reflected from the Surfaces of a Calibration Room," Health Phys. 42,

489 (1982).

19
14. R. B. Schwartz and C. M. Eisenhauer, "Procedures for Calibrating Neutron

Personnel Dosimeters," NBS Special Publication 633 (1982).

15. C, M. Eisenhauer, J. B. Hunt, and R. B. Schwartz, "Calibration Techniques for Neutron Personal Dosimetry," Radiat. Prot. Dosim. 1_0, 43 (1985).

16. D. E. Hankins, "Energy Dependence Measurements of Remmeters and Albedo

Neutron Dosimeters at Neutron Energies of Thermal and Between 2 keV and 5.67

MeV," Proc. Int. Radiation Protection Assn. IV International Congress, Paris,

France, April, 1977, p. 563.

17. E. J. Axton, "The Effective Center of a Moderating Sphere When Used as an Instrument for Fast Neutron Flux Measurement," J. Nucl. Energy 2_6, 581

(1972).

18. J. B. Hunt, "The Calibration of Neutron Sensitive Spherical Devices,

"Radiat. Prot. Dosim. 8, 239 (1984).

19. H. Ing and W. G. Cross, "Spectra and Dosimetry of Neutrons from Moderation of and ^^^Cf in H^O," Health Phys. 29, 839 (1975).

20. R. B. Schwartz, G. W. R. Endres, and F. M. Cummings, "Neutron Dosimeter

Performance and Associated Calibrations at Nuclear Power Plants," NUREG/CR2233, 1982.

21. J. A. Grundl and C, M. Eisenhauer, "Fission Spectrum Neutrons for Cross

Section Validation and Neutron Flux Transfer," Proc. Conf. Nuclear Cross

Sections and Technology, Washington, D.C. , March 3-7, 1975. NBS Special

Publication 425, p. 250. J. Grundl and C. Eisenhauer, "Fission Rate Measurements for Materials Neutron Dosimetry in Reactor Environments," Proc. First

ASTM-EURATOM Symp. on Reactor Dosimetery, Petten (Holland), Sept. 22-26,

1975. EUR 5667 e/f Part I, P. 425.

20
Appendix A. Pocket Calculator Programs

The listings on the following page are programs for Hewlett-Packard Series 10

calculators.

The first listing, "DOSE EQUIVALENT RATE", is used for solving equations (5)

and (6), knowing the source emission rate, Q, and either the free field dose

equivalent rate, FFDE, or the distance, D. To use this program, the californium source emission rate, in neutrons/sec, is keyed in and stored in register 1 (STO 1). If the distance is known, it is keyed in and LBL B (f B) will

calculate the dose equivalent rate for the bare source; LBL D (f D) will

calculate it for the DjO-Moderated source. If the desired dose equivalent

rate is known, it may be keyed in and LBL A will give the required distance

from the bare source; LBL C will give the distance from the moderated source.

All distances are in cm and dose equivalent rates are in millirem/hour.

The second listing, "CORRECTION (%)", calculates the distance-dependent

correction for nine-inch (or ten-inch) spherical remmeters, for either the

bare or moderated sources. Steps 37-'42 compute the room scatter, 43-^9

compute the air scatter, and 51-59 compute the non-1 /r^ effect. To use,

first either set flag 0 (g SF 0) if it is the moderated source, or clear flag

0 (g CF 0) if it is the bare source. Then key in the distance in meters , and

LBL E (f E) will compute the correction in per cent. For bare irradiations

using sources with "Cf" identification numbers, 3% must be added as a correction for anisotropic neutron emission (compare equation (12)). The anisotropy correction is - 0 for other remmeter irradiations. The total correction

will be the same (except for rounding errors) as the quantity "Corr" discussed on p. 9, and as used in equation (13).

The labels and subroutine identifications are chosen so that the programs do

not interfere with each other, and hence may be in the calculator at the same

time. These two programs occupy 13 program registers.

A-1
DOSE EQUIk^LENT RATE

Keystrokes Pi splay

f LBL A 001- 42,21 ,11

4 002- 4

003- 48

1 004- 1

4 005- 4

4 006- 4

007- 10

f LBL C 008- 42,21 ,13

GSB 1 009- 32 1

X 010- 11

g RTN 011- 43 32

f LBL B 012- 42,21 ,12

2 013- 2

014- 48

0 015- 0

3 016- 3

6 017- 6

018- 10

f LBL D 019- 42,21 ,14

g X 020- 43 11

GSB 1 021- 32 1

g RTN 022- 43 32

f LBL 1 023- 42,21 ,1

1/x 024- 15

2 025- 2

026- 48

3 027- 3

EEX 028- 26

CHS 029- 16

3 030- 3

X 031- 20

RCL 1 032- 45 1

X 033- 20

g RTN 034- 43 32

CORRECTION (%)

Keystrokes Di splay

f LBL E

1-1 OCT 42,21 ,15

STO 0 U ob — 44 U g X

Ti ~? U o / — i 1

g F? 0 (loo U do- 4o , b ,

U

GSB 7 u oy

0 0 -7 6

n /I n _ U 'f u — b

X U 'tl — 2U

STO 2

n yl o n't 2

RCL 0

n >i o U 4 J- '5 U

g F? 0

n /\ yi U 'f '» — 4o , b ,

U

GSB 6

n /I c: U 'fo- o2 b

• ri yi U4b— 4o

1 n /I "7 U4/— 1

1

n yl o U 'to 1

rt yi G Oft 30

STO + 2 U jU- A A A C\ 0 44 , 40 , 2

RCL 0 051 - AC C\ 45

g X

f-i cr52-o 43 11

1/x n c o 05 J- 1 5

g F? 0 054- yi 0 y A 43 ,6,0

GSB 5

rk c c 0 55- 32 5

05b- yi Q 48

1

n c "7 U5/— 1

6

n cr o58— y- X 59- 20

STO + 2 UbU — /I /I /in 0

RCL 2 Ubl — to 2

g RTN 'f J 32

f LBL 5

n <^ o U bo— /1'(20 0 -1 ^ , 21 , 5

2

ri064- ^ 02

r\ f~ ^ Ub5-

8 bb- 0

X 067- 20

g RTN rk /~ o bo- 43 32

f LBL 7 069- /" Q 42,21 ,7

1 1

0 71- 48

3

r» "772-o 0

X

i-i -7 o 0 73- Oft g RTN 0 74- 43 32

f LBL 6 0 75- 42 , 21 ,

6

• 076- 48

2

0 ~7 ~7 77- 2

2 078- 2

079- 30

2 080- 2

• 081- 48

3 082- 3

X 083- 20

q RTN 084- 43 32
Appendix B. Averaging Program

The listing on the next page is for the program "AVERAG" on the Tandy 200.

The user is first prompted for the number of input data points (line 20) and

then for the manner in which the uncertainties are expressed (lines 30-70).

The program then goes to the appropriate subroutine, prompts for the input

data, and, after the last datum is entered, calculates the weighted mean, the

standard deviation of the mean computed from the weights, and the samplebased standard deviation. These three quantities are listed on the computer

display screen, and whichever of the two uncertainties is the larger is con- verted to a percent uncertainty, and also listed. (The peculiar symbols in

lines 162-2^0 put a frame around the output listing on the Tandy 200 display.) The quantities are calculated from the following formulas:

Weighted Mean = Z ( x^ • w^ ) /Zw^

,

stdev of the mean computed from weights = 1//(Ew^), (B-1)

sample-based weighted stdev of the mean = /Zw . (x . -x) ^ / ( N-1 ) Ew .', (B-2)

where x^ are the individual input data,

are the weights corresponding to each x^,

w.111 = 1/(E,)^, where the E. are the uncertainties in the input data,

X is the weighted mean, and

N is the number of data points.

B-1 and B-2 are both correct formulae for computing the standard deviation

(stdev) of the mean, independent of any assumed statistical distribution.

In the application discussed in the text, we make the usual assumption of the

Poisonness of the counting data in order to assign the uncertainties; i. e.

,

the E.'s are assumed to be the square root of the number of counts, but this

is the only assumption which is made regarding the statistical distribution

of the data.

Formula B-2 is the more "robust" of the two formulae, in that it will give a reasonable estimate of the stdev even if the weights are not quite correctly

chosen. If the weights are properly chosen, B-1 is mathematically equivalent

to B-2 and will give the same answer. By "properly chosen" we mean that the

w. are actually equal to the (reciprocal of the) true data point variances.

In our case, if the counting statistics are the only source of uncertainty

within a given data set, then the weights will be properly chosen and B-1 and

B-2 will give the same answer. This is observed to be the case for most of

our data. If B-2 and B-1 give different results, this is taken to be a "flag", signalling that something else may be causing fluctuations in the

data, as discussed in the text.

B-1
1 REM - "AVERAG.BA"

10 CLEAR :CLS

20 INPUT "Houj many data points are there" iN

25 DIM G(N-1 , 1

)

30 PRINT "Are the uncertainties given as:"

35 PRINT

40 PRINT TAE(2);"1. Absolute standard deviations?"

50 PRINT TAB(2);"2. 7. S.D.i same value for all points?"

60 PRINT TAB(2);"3. '/. S. D. , and not all the same?"

65 PRINT

70 PRINT TAE(5 ); "ENTER 1 , 2 or 3"; TAB (20)

75 INPUT C

77 PRINT

80 ON C 60SUB 300, 500, 700

90 WA ! =WTDT/SUMW

100 U!=SUMW'^-.5

110 FOR A=0 TO N-1

120 SCATR=SCATR+G(A, 1 )*(G( A, 0)-WA I ) -^2 130 NEXT A

140 SPRD ! =SQR(SCATR/ ( (N-1)*SUMW;

;

150 PCT I =U ! *100/WA

!

160 IF SPRD!>U! THEN PCT ! =SPRD 1 * 1 00/ WA

1

162 CLS: PRINT " f"

;

163 FOR N=l TO 38

164 PRINT TAB(N) ; " ""

;

165 NEXT N

166 PRINT TAB(39);"n";

170 PRINT "I " ;TAE(2) ; "Weighted Mean =";WAl;"t

" ; PCT ! ; "/." ; TAB (39) ; " "

;

175 PRINT "I " ;TAB< 39 ) " I" ; 180 PRINT "I"; "Stat. Std. Dev. of Mean=±

" ;U! ; TAB (39) ; " " ; 190 PRINT "";"S.D. from Data Spread =t

" ;SPRD ! ; TAB (39) ; " I" ; 200 PRINT "-";

210 FOR A=l TO 38

220 PRINT TAB( A) ; " -" ; 230 NEXT A

240 PRINT TAB(39) ; "J" ; 250 END

300 FOR A=0 TO N-1

310 PRINT "No.

;A+1 ;TAB(6)

320 INPUT "Datum, un cert.

;D, E

330 (;i( A, 0)=D:G( A, 1 )=£--2

350 WTDT=WTDT+ D*(i(A,l)

360 SUMW=SUMW+G ( A , 1

)

370 NEXT A

380 RETURN

500 1NPUT"ENTER per-cent un cer ta i n ty " ;

P

510 FOR A=0 TO N-1

520 PRINT "No. " ; A+1 ;TAB(6)

530 INPUT "Datum" ;D

540 G(A,0)=D:6^A,1)=<. 01*P*D> "-2

560 WTDT=WTDT + D*G(A,1)

570 SUMW = SUMW + G(A,1)

580 NEXT A

B-2
590 RETURN

700 FOR A=0 TO N-1

710 PRINT "NO. " ; A+1 ;TaE(6)

720 INPUT "Datum,'/. Uncer ta i n ty "

; D , P

730 6(A,0;=D: G ( A , 1 ) = ( . 0 1 P-^D ) -^-2

750 WTDT=WTDT + D*G(A,1)

760 SUMW=SUMW + G(A,1>

770 NEXT A

780 RETURN

B-3

Appendix C. Sample Test Reports

Pages C-2 - C-^ are a typical remmete

shown in the spreadsheet of Figure 3.

by a test report for dosimeter irrad

explanatory.

r test report, generated from the data

This is followed on pages C-5 and C-6

iations. The reports should be selfC-1
rOMM NBS—

iMCV. •-•B)

U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

OAITHERSBURC. MARYLAND 20899

REPORT OF TEST

NBS Test No.

Instruments Submitted by: Nuclear Power Station

Dates of Irradiation: April 2^, 1986

Neutron Sources:

Californium-252 spontaneous fission source NS-79 (neutron

emission rate of 1.86 x 10" n/s ± 1.6$) and SR-Cf-132

(emission rate of 3.91 x 10* n/s ± 1.3JType of Instruments: Eberline PRS-2 Remraeters

Quantity: 3

Serial Numbers: 3^8, 355, 377

The instruments were alternately mounted on a stand and measurements

were made at various distances from one or the other of the two sources

to obtain different dose equivalent rates. Table I lists the source

and distance combinations used, together with the instrument readings

for each source-distance combination.

The second column in Tables II lists the "Free Field Dose Equivalent"

(FFDE) Rates corresponding to the source-distance combinations of Table

I. The FFDE Rate is the dose equivalent rate due to neutrons from the

source alone, in the absence of background caused by neutrons scattered

into the instrument from the walls, the air in the room, and the source

support. The FFDE rate, in mrem/hr, is calculated using the formula:

where Q is the source emission rate (n/s), and r(cm) is the distance

listed in Table I (R.B. Schwartz and C. M. Eisenhauer, NBS Special

Publication 633 (May 1982)).

FFDE Rate = 9.5^ X 10 3 Q/r^

C-2
The next columns In Table II, "Corrected Instrument Readings" are the

remmeter readings from Table I, corrected for the neutron background

mentioned above. (At the smallest distance, there is also a small

correction for the deviation from the inverse square law due to the

finite size of the source and detector.) For most of the data the

corrections are small (~5t), as can be seen by comparing the observed

readings from Table I with the corrected readings in Table II. At the

largest distance, however, the relatively large background of room

scattered neutrons leads to ~10t corrections.

The FFDE, divided by the corrected reading, gives the calibration

factor listed in Table III. The calibration factor is thus the factor

by which the instrument reading should be multiplied to get the true

dose equivalent.

All three instruments showed a random, fluctuating, reading in the

absence of any neutrons. This "background" reading varied between 0

and 0.6 mrem/hr. For the purpose of analyzing the data, a "background"

of 0.3 ± 0.25 mrem/hr was substracted from all readings. These

extraneous counts do not have any appreciable effect on the

calibrations at the higher dose equivalent rates, but they do increase

the uncertainty in the calibration factor at the lowest rates.

The estimated uncertainties (1o) in the calibration factors also

include the uncertainty in the source calibration, statistical

fluctuations in the instrument reading, and the uncertainty of the

location of the effective center of the detector and uncertainty in the

scatter and source anisotropy corrections. Uncertainty in the flux

density to dose equivalent rate conversion is not included.

It should be understood that the uncertainty figures are current

estimates based on the Judgement of the NBS staff and are not

necessarily supported by a long term data base.

Measurements associated with this calibration were performed by:

For the Director,

R. S. Caswell, Chief

Ionizing Radiation Division

Test No.

:

SP250 No. 8.1J

Date; May 7, 1986

C-S
NBS Teat No,

TABLE I - Experimental Data

Distance (r) Instrument Readings, mrem/h

(center-toSource No. center), cm Ser. f3^6 Ser. #355 Ser. #377

SR-Cf-132 99 11.18 3.67 H.6

SR-Cf-132 50 14.9 U.35 16.7

SR-Cf-132 25.1 56.3 56.7 65.8

NS-79 99 175.5 1814 201

NS-79 50 636 686 733

NS-79 25.^1 2»128 2558 2869

TABLE II - Results

Free-Field Corrected Instrument Readings, mrem/h

Dose Equivalent

Rate, mrem/h Ser. #3'<8 Ser. #355 Ser. #377

3.80 3.80 3.34 14.18

1i|.9 U.I 13.6 15.8

57.7 53.1 53.5 62.1

180.7 16H.1 172.1 187.5

708 621 669 715

27^5 2357 21483 2785

TABLE III - Calibration Factor and Estimated Uncertainty

Estimated

Free-Field Instrument Calibration Factor Uncertainty

Dose Equivalent

"Rate, mrem/h Ser. #31^8 Ser. #355 Ser. #377 %

3.80 1 .00 1 .114 0.91 13

1^4.9 1 .06 1.10 0.914 7

57.7 1 .09 1 .08 0.93 5

180.7 1.10 1 .05 .96 3 708 1 .114 1 .06 .99 3

27^45 1.17 1.11 .99 14

Mean 1.11 1.07 0.97 2

Instrument High Voltage: #3148-2052 v #355-20141 V

#377-1953 V

Temperature of Calibration Room: 19°C

Relative Humidity: ^3%

C-4
FORM NBS—259

(REV. e-es)

U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG. MARYLAND 20899

REPORT OF TEST

NBS Test No.

Dosimeters Submitted by:

Date of Irradiation: January 2, 1985

Dose Equivalent Delivered: 750 mrem

Certified Source Strength;

Californium-252 spontaneous fission source NS-BS

Q Emission Rate 5.62 x 10 n/s + 1 . 39J

The californium source was moderated in the NBS 30-cm diameter

heavy water sphere.

Type of Dosimeters: Eberline Albedo Dosmeters

Quantity: 15, plus 4 controls

The dosimeters were irradiated in three groups of five each,

as follows:

Group 1: 0002 0301, 0302, 0303, 0305, 0306.

Group 2: 0002 0307, 0308, 0309, 0310, 0311.

Group 3: 0002 0312, 0313, 0314, 0315, 0316.

Controls: 0002 0300, 0304, 0317.

For each group, the dosimeters were mounted equally spaced

on a 14 cm diam circle centered on the front face of a 40 cm x 40

cm X 15 cm thick lucite phantom. The distance from the center of

the source to the center of the front face of the phantom was set

at 50 cm. All the dosimeters were mounted with the typed identification numbers facing the source.

The free field dose equivalent rate is given by the formula

-3 2

D.E. Rate = 2.30 x 10 xQ/r , where the dose equivalent rate is in millirem/hr, Q is the source

emission rate in neutrons/sec, and r is the source-to-phantom

distance (50 cm). In this case, the free field dose equivalent

rate at the center of the phantom is 517 mrem/hr. Since, how-
ever, "the dosimeters are 7 cm away from the center, the dose

equivalent rate at the dosimeter position is 2% lower. On the

other hand, we estimate that the air and room scattered neutrons

will increase the dosimeter response by approximately 4K. Thus,

the "effective" dose equivalent rate is 530 mrem/hr, giving an

irradiation time of 1 hr 25 min for the 750 mrem irradiation.

The estimated uncertainty in the delivered dose equivalent

is +556 (one standard deviation) , due largely to the uncertainty

in the fluence to dose equivalent conversion factor.

Note that dosimeter 0002 0304 fell open during handling, and

its reading may therefore be suspect.

This irradiation was performed by

R. B. Schwartz, physicist

For the Director,

R. S. Caswell, Chief

Nuclear Radiation Division

Test No.

January 3, 1985

C-6
Appendix D. Graphic and Tabular Representation of the Neutron Spectra

The spectra are represented as plots of ABo/Aln(E/Eo ) (linear scale) versus

neutron energy E (logarithmic scale) with ABg (group source strength) normalized to a total source strength Q = 1 neutron/sec and Eq taken as 1 MeV.

In the tabulations, the energy associated with a value of AB^ /ln( E/E,, ) corre- sponds to the upper limit of the particular interval. This energy also serves as the lower boundary of the next higher energy interval, and so on.

The data are taken from reference 6.

D-1
TabI* 3 — Valu«s of group tourc* ttrvnoth p«r logarithmic arwrgy interval for a 3E2Cf spontanaout fiation tourca

Nautron artargy. £

Aln(£-/£jl M«V • -'

4.14 X 10"'

0.01 >.4U X 10 ' 0.06 2,74 X 10"'

0.10 1,24 X 10"'

0.20 3.33 X 10"*

0,25 X 10"^ O.X 7.90 X 10"^

0.40 1,07 X 10"'

0.50 1.46 X 10*

0.60 1,84 X 10"

0,70 2,21 X 10*'

0.80 2.56 X 10"'

1.00 3.01 X 10"'

1.20 3.53 X 10-'

1.40 3.95 X 10-'

1.50 4.19 X 10-'

1.60 4.32 X 10-'

1.80 4.46 X 10-

2.00 4.58 X 10-'

2.20 4.62 X 10-'

2.30 4.61 X 10-'

2.40 4.59 X 10-'

2.60 4.53 X 10-'

Nautron tnmgy. E

M«V i- '

2.80 4.42 X 10-'

3.00 4.27 X 10-'

3.40 4.01 X 10-'

3.70 3.66 X 10-'

4,20 3.25 X 10-'

4,60 2.78 X 10-'

5.00 2.39 X 10-'

5.50 1.99 X 10-'

6.00 1.61 X 10-'

6,50 1J8 X 10-'

7,00 1.01 X 10'

7.50 7.92 X 10 -J 8.00 6.16 X 10-2

8.50 4 76 * 10 - 2 9.00 3.65 X 10-2

9.50 2.79 X 10-2

10,00 2.13 X 10-2

11.00 1.42 X 10 2 12.00 8.04 X 10-3

13.00 4.51 X 10-3

14,00 2.50 X 10-3

16.00 1,08 X 10-3

18.00 3.20 X 10-*

-2
D-3
Tabic 4 • VakiM of a group aourca •trangth par logarfthmk anargy intarval for a ^Cf apontanaoua fission aouroa in tl>a cantra of a OjO aphara with a radius of 180 mm

Nautron anargy. £

M«V »- 1

4.14 X 10-T

1,0 X to-« 2.15 X 10-2

1.0 X io-» 2.74 X 10-2

6.0 X 10-8 3.75 X 10-2

1.0 X io-« 4.57 X 10-2

2.0 X io-« 4.92 X 10 '

4,.0 X 10-* 5.51 X 1U ^

7.0 X 10-* 5.BD X 10 ^

1,0 X 10-

J

0.S X 10 ^

3.0 X 10-3 £ OA w 0.B8 X 10 *

6,0 X 10-3 7.34 X

%f\ - 3 10 '

1.0 X 10-2 7.42 X in- 2 10 *

2.0 10-J 7.89 X 10 '

4.0 X 10-2 7.38 X 10-2

6.0 X 10-2 7.30 X 10-2

8.0 X 10-2 6.95 X 10-2

1.0 X 10-' 6,52 X 10-2

1,5 X 10-' 6.10 X 10-2

2.0 X 10-' 5,54 X 10-2

2.5 X 10-' 5,12 X 10-2

3.0 X 10-' 4,88 X 10-2

3,5 X 10-' 4.26 X 10-2

4.0 X 10-' 3,66 X 10-2

4.5 X 10-' 2.25 X 10-2

5,0 X 10-' 2,98 X 10-2

5.5 X 10-' 4,41 X 10-2

6.0 X 10-' 4,73 X 10-2

i A8b

AInlf/fpl

7,0 X 10-' 5.08 X 10-2

8.0 X 10-' 5,08 X 10-2

9.0 X 10-' 4.88 X 10-2

1.0 3.39 X 10-2

1,2 4,10 X 10-2

1.4 5,47 X 10-2

1.6 6.84 X 10-2

1.8 7.26 X 10-2

2.0 7.66 X 10-2

2.3 9.57 X 10-2

2.6 1.18 X 10'

3,0 1.04 X 10 -' 3.5 8.01 X 10-2

4,0 6,13 X 10-2

4,5 6.88 X 10 -2

0.21 X 10 ' 6,0 4.77 X 10-2

7,0 3.20 X 10-2

8,0 1.81 X 10-2

9,0 1,10 X 10-2

10.0 7.27 X 10-3

11.0 4.65 X 10-3

12,0 1,86 X 10-3

13.0 1.56 X 10-3

14,0 8,00 X 10-

«

15,0 4.10 X 10-'

-4
D-5

Appendix E. Definitions

Taken from: NCRP Report No. 51 , "Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerator Facilities". National

Council on Radiation Protection and Measurements, Washington, D. C. (March, 1977).

absorbed dose (D): The quotient of dE by dm, where dE is the mean energy

imparted by ionizing radiation to the matter in a volume element of mass

dm.

anisotropic (see also isotropic): Not isotropic; having different properties

in different directions.

dose equivalent (H) : The product of absorbed dose, D, and quality factor, Q,

at the point of interest in tissue.

fast neutrons: Neutrons of energies above a few hundred keV.

fluence (*): The quotient of dN by da, where dN is the number of particles

which enter a sphere of cross-sectional area da.

intermediate neutrons; slow neutrons: Neutrons with energies between about 1 eV and a few hundred keV.

inverse square law: That rule which states that the intensity of radiation

from a point source decreases as 1/r^ from the source in a non-absorbing

medium, where r is the distance from the source.

isotropic (see also anisotropic): A condition in which the properties are

the same in whatever direction they are measured. With reference to

radiation emission, this term indicates equal emission in all directions

from a point source or each differential-sized element of an extended

source.

linear energy transfer (LET): The average energy lost by a directly ionizing

particle per unit distance of its path in a medium.

point source (of radiation): Any radiation source measured from a distance

that is much greater than the linear size of the source.

quality factor (Q): A factor which is used in radiation protection to weight

the absorbed dose with regard to its presumed biological effectiveness

insofar as it depends upon the LET of the charged particles. The quality

factor is a function of the LET of the charged particles which deliver the

absorbed dose.

rem: A special unit of dose equivalent.

E-1
NBS-n4A (REV. 2.ec) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBS/SP-250/12 July 1987

4. TITLE AND SUBTITLE

NBS Measurement Services:

Neutron Personnel Dosimetry

5. AUTHOR(S)

Robert B. Schwartz

6. PERFORMING ORGANIZATION (If joint or other thon NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

Gaithersburg, MD 20899 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 87-619842

I I Document describes a computer program; SF-185, PIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sunrtmary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

Detailed procedures are given for calibration of neutron dosimeters and remmeters

with californium neutron sources, both "bare" and moderated, Corrections for

scatter, room-return, anisotropic neutron emission, and deviation from the

inverse square law are discussed, and specific examples given. The uncertainties

in arriving at the final value for the calibration factor are also discussed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) calibration; californium; dose equivalent; dosimeter; neutron; remmeter

13. AVAILABILITY

["^ Unlimited

I I For Official Distribution. Do Not Release to NTIS

I^vl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

44

15. Price

USCOMM-DC 6043-P80


PUBLICATIONS IN THE SP 250 SERIES FROM THE CENTER FOR RADIATION RESEARCH

SP 250-1 NBS Measurement Services: SP 250-12

Spectral Radiance Calibrations

J.H. Walker, R.D. Saunders, and A.T. Hattenburg

SP 250-2 NBS Measurement Services: SP 250-13

Far Ultraviolet Detector Standards

L.R. Canfield and N. Swanson

SP 250-3 NBS Measurement Services:

Radiometric Standards in the Vacuum Ultraviolet SP 250-14

J.Z. Klose, J.M. Bridges, and W.R. Ott

SP 250-4 NBS Measurement Services:

Irradiation of Ferrous-Ferric Dosimeters

(Fricke- Dosimetry) SP 250-15

C.G. Soares, E.L. Bright and M. Ehrlich

SP 250-5 NBS Measurement Services:

Alpha-Particle Calibrations SP 250-16

J.M.R. Hutchinson

SP 250-6 NBS Measurement Services:

Regular Spectral Transmittance

K.L. Eckerle, J.J. Hsia, K.D. Mielenz, SP 250-17

and V.R. Weidner

SP 250-7 NBS Measurement Services:

Radiance Temperature Calibrations

W.R. Waters, J.H. Walker, and A.T. Hattenburg SP 250-18

SP 250-8 NBS Measurement Services:

Spectral Reflectance

V.R. Weidner and J.J. Hsia SP 250-19

SP 250-9 NBS Measurement Services:

Calibration of Beta-Particle-Emitting

Ophthalmic Applicators

J.S. Pruitt SP 250-20

SP 250-10 NBS Measurement Services:

Radioactivity Calibrations with the "4Tr" Gamma Ionization Chamber, and Other

Radioactivity Calibration Capabilities SP 250-21

J.M. Calhoun

SP 250-11 NBS Measurement Services:

Dosimetry for High-Dose Applications

J.C. Humphreys, D. Hocken, and W.L. McLaughlin

NBS Measurement Services: Neutron Personnel Dosimetry

R.B, Schwartz

NBS Measurement Services:

Activation Foil Irradiation

with CaliforniLTi Fission Sources

G.P. Lamaze and J. A. Grundl

NBS Measurement Services:

Activation Foil Irradiation by Reactor Cavity Fission Sources

G.P. Lamaze and J, A, Grundl

NBS Measurement Services:

Photometric Calibrations

R.L. Booker and O.A. McSparron

NBS Measurement Services:

Calibration of X-Ray and Gamma-Ray

Measuring Instruments

P.J. Lamperti, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

The NBS Photodetector Spectral Response

Calibration Transfer Program

E.F. Zalewski

NBS Measurement Services: Neutron Source Strength Calibrations

E.D. McGarry and E.W. Boswell

NBS Measurement Services:

Calibration of Gamma-Ray-Emitting

Brachytherapy Sources

J.T. Weaver, T.P. Loftus, and R. Loevinger

NBS Measurement Services:

Spectral Irradiance Calibrations

J.H. Walker, R.D. Saunders, J.K. Jackson,

and D.A. McSparron

NBS Measurement Services:

Calibration of Beta-Particle

Radiation Instrimentation and Sources

J.S. Pruitt, C.G. Soares, and M. Ehrlich
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300