this step is not necessary. The Eberline PRS-2, the next most common instru-
ment, 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 4 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 interme-
diate 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 4 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 whether 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 calibra-
tion 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 cali-
brated 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
ean discuss the error components illustrated on the bottom portion of the
spreadsheet in Figure 3. Tne numerical values given in the columns marked

14


Table 4 - Values of a group source strength per logarithmic energy interval for s 22Cf spontaneous fission
‘source in the centre of a D20 sphere with a radius of 160 mm

aa 4B
St Neutron energy. E —
AWE/E,) SintEVE,)
" Mev =
x 7.0 x 10-7 $.08 x 10-?
x 2,15 x 10-2 8,0 x.10-! 5.08 x 10-7
x 2,74 x Wr? 9,0 x 107" 458 x 10-7
x 3,75 x 10-2 10 3,39 x 10-2
x 4,57 x 10-2 12 4.10 x 10-2
x 492 x 10-2 14 5.47 x 10-?
x 5,51 x 10-2 16 6.84 x 10-2
x $,86 x 10-2 18 7,26 x 10-2
x 6,29 x 10-2 20 7,66 x 10-2
x 6.88 x 10-2 23 9,57 x 10-2
x 7,34 x 10-7 28 10°!
x 7,42 x 10-2 3.0 10-'
x 7,89 x 10-2 35 10-2
x 7,38 x 10-2 40 6.13 x 10-2
x 7, x Wrz 45 6.88 x 10-7
x 6,95 x 10-2 5.0 6.21 x 10-2
0 x 6,52 x 10-2 6.0 4.77 x 10-2
15 x 10°! 6,10 x 10-2 7.0
20 x 107? 5,54 x 10-2 8.0
25 x 107? B12 x 10-2 9.0
3,0 x 10-1 4,88 x 10-2 10,0
35 x 10°! 4,26 x 10-2 11.0
40 x 10-1 3,66 x 10-2 120
45 «10-1 2.25 x 10-2 13.0
50 x 10-7 2,98 x 10-2 14.0
55 x 10-! 4 At x 10-2 15,0
60 x 10-7 4.73 x 10-2



ever, the dosimeters are 7 om 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 4%. Thus,
the “effective” dose equivalent rate is 530 mrem/hr, giving an
ivxadiation time of 1 hr 25 min for the 750 mrem irradiation.

The estimated uncertainty in the delivered dose equivalent
is +5% (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

CH6


nent record. This spreadsheet is then used as a template for the next dosim-
eter irradiation. One does not lay out a new spreadsheet for each new
calibration. )

OSIMETER IRRADIATIONS January 30, 1985

Source Q(n/s)> DCem) FFDE CORRECTION EFF .FFDE hrs/mrem
NS-86 5.477e+08 30.8 488.1 4.01 % 507.7 0.001970
NS-39

Cf-136
NS-79 2.554e+08 50.8 944.2 6.72 % 1007.7 0.000992
cf-132
Source Dose(mrem) Time(hr) Dose Time(hr) Dose Time (hr)
NS-86 300 0.5909 500 0.9849 1000 1.9698
NS-39
Cf-136
NS-79 300 0.2977 500 0.4962 1000 0.9924
Cf-132

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 source-
detector distance, D. According to ANST N13.115 , all dosimeters are to be
mounted on the front face of a 40 em 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 dosime-
ter 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(507 + 97) = 50.80 cm. (4)

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

FPO od = .0023*Q/D?, (5)


8. Acknowledgement

I should like to thank Charles Eisenhauer for his close collaboration and
expert guidance in making this calibration service a reality.

18


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CONTENTS

1. Description of Service

n

Facility Description

3. Corrections
a. Anisotropic Emission
b. Room Return
ec. Air Scatter
d. Deviation from Inverse Square Law

4, Procedures
a. Spreadsheet for Dosimeter Irradiations
b. Spreadsheet for Remmeter Irradiation
ec. Performing the Calibration
e.1. Dosimeter Irradiations
c.2, Remmeter Calibrations

5. Uncertainties
a. Statistical Uncertainty
b. Uncertainty Due to Source Calibration
ec. Uncertainty Due to Corrections
d. Uncertainty in Distance Determination

e. Uncertainty in Fluence-to-Dose Equivalent

Conversion
6. Internal Quality Control
7. Safety
8. Acknowledgement
References
Appendix A. Pocket Calculator Programs
Appendix B. Averaging Program
Appendix C. Sample Test Reports
Appendix D. Neutron Spectra

Appendix E. Definitions


NBS Test No.

TABLE I - Experimental Data

Distance (r)
(center-to-

Instrument Readings, mrem/h

Source No. center), cm Ser. #348 Ser. #355 Ser. #377
SR-Cf-132 99 4.18 3.67 4.6
SR-Cf-132 50 14.9 14.35 16.7
SR-Cf-132 25.4 56.3 56.7 65.8
NS-79 99 175.5 184 201
NS-79 50 636 686 733
NS-79 25.4 2428 2558 2869
TABLE II - Results
Free-Field

Dose Equivalent

Corrected Instrument Readings, mrem/h

Rate, mrem/h Ser. #348 Ser. #355 Ser. #377
3.80 3.80 3.34 4.18
14.9 Wat 13.6 15.8
57.7 53.1 53.5 62.1
180.7 164.1 172.1 187.5
708 621 669 715
2745 2357 2483 2785
TABLE III - Calibration Factor and Estimated Uncertainty
Estimatec
Free-Field Instrument Calibration Factor Uncertair
Dose Equivalent
Rate, mrem/h Ser. #348 Ser. #355 Ser. #377 t
3.80 1.00 1.14 0.91 13
14.9 1.06 1.10 0.94 7
57.7 1.09 1.08 0.93 5
180.7 1.10 1.05 96 3
708 1.14 1.06 99 3
2745 1.17 1.11 +99 4
Mean 1.11 1.07 0.97 2
Instrument High Voltage: #348-2052 v
#355-2041 v
#377-1953 v
Temperature of Calibration Room: 19°C

Relative Humidity:

43%


"Stat", "eal", "Corr", "Dist", "Sum", and "Sum+4" in the bottom part of
Figure 3 are the uncertainties expressed as percentages. Again, all uncer-
tainties are given as one standard deviation (10).

5.a. Statistical Uncertainty ("Stat")

This is discussed in section 4.b., "Spreadsheet for Remmeter Irradiation",
and in Appendix B. To summarize, for an instrument witn 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 esti-
mate 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., 95%) of
the readings, and assigning that a value of 20. This value is then just
divided by 2, and converted to a percent of the average reading. The uncer-
tainty 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 uneertainty 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 inves-
tigated and is well understood. It is usually ~ 1.3%, 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 anoth-
er, 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 spread-
sheet 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 esti-
mate, 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


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", 1s 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 califor-
nium source emission rate, in neutrons/sec, is keyed in and stored in regis-
ter 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 D,0-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 ecm 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, 43-49
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 correc-
tion for anisotropic neutron emission (compare equation (12)). The anisotro-
py correction is ~ 0 for other remmeter irradiations. The total correction
will be the same (except for rounding errors) as the quantity "Corr" dis-
cussed 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.


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 uncer-

tainties, over the range of dose equivalent rates which we use. (The calib~
ration 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 calibra-
tions 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 prog-
ress. 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.


FORM NBS~259
(REV. 6-05)

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-Z5Z spontaneous fission source NS-86

Emission Rate 5.62 x 108 n/s + 1.3%
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 Z: 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
om 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 identi-
fication numbers facing the source.

The free field dose equivalent rate is given by the formula

3 xQ/r?,

D.E. Rate = 2.30 x 10
where the dose equivalent rate is in millirem/hr, Q is the source
emission rate in neutrons/sec, and xy 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-

C5,


the magnitude of the fluctuations, and estimates the 10 value of these fluc-
tuations (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 instru-
ment (Eberline PRS-2) one switch-selects the number of counts to be accumu-
lated, and thus the statistical precision, for the individual readings. AV-
ERAG 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. a4)

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 (10). 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 20% 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 om, and the correction is
then numerically equal to 100/D, with D in em (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 colum, "Sum+4" adds
(in quadrature) the 4% systematic uncertainty in the fluence to dose equiva-
lent 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 correc-
tion 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


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.

Neutron source being raised into position

T.V, camera for monitoring of source in exposed position.
Remote mobile platform.

1
2
3
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.

&


roau NUS-290

U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS
GAITHERSBURG, MARYLAND 20699

REPORT OF TEST
NBS Test No.

Instruments Submitted by: Nuclear Power Station

Dates of Irradiation: April 24, 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 + 1.3%.

Type of Instruments: Eberline PRS-2 Remmeters
Quantity: 3
Serial Numbers: 348, 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:

FFDE Rate = 9.54 x 10-* Q/r?
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)).


1 REM — “AVERAG. BA"

1@ CLEAR:CLS

2Q@ INPUT "How many data points are there"jN

25 DIM G(N~1,1)

3@ PRINT “Are the uncertainties given as:"

35 PRINT

4Q@ PRINT TAB(2)3"1. Absolute standard deviations?"

5@ PRINT TAB(2)3"2. % S.D.3 same value for all points?"
6@ PRINT TAB(2)5"3. % S.D.s and not all the same?"

635 PRINT

7@ PRINT TAB(5)3"ENTER iy2 or 3"5§TAB(2Q)
75 INPUT C

77 PRINT

8@ ON C GOSUB 308, 500, 700

90 WA!=WTDT/Suriw

100 U!=SuMWs-.5

11@ FOR A=@ TO N-1

120 SCATR=SCATR+G(A,y 1) #(G(A,Q)-WA! 42

13@ NEXT 4
14@ SPRD!=SQR(SCATR/( (N-1) *SUMW) >
15@ PCT! 1*100/WA!

16@ IF SPRD!>u!
162 CLS:PRINT
163 FOR N=1 To 38
164 PRINT TAB(N) 5 "TM"5
165 NEXT N
166 PRINT TAB(39) 5" "85
17@ PRINT “#"53TAB(Z)
“3PCT!5 "2%" 5 TABR(S9) §
175 PRINT “§"3TAB(39 3
18M PRINT "B"3"Stat. Std. Dev. of Mean=%
"SU! §TABR(B9) 5" 9G
199 PRINT 3"S.D. from Data Spread =2
"3SPRD! 5TAB(39) 5" 85
2@Q PRINT "Me"
210 FOR A=1 TO 38
PRINT TAB(A) 5 "me"
NEXT A
PRINT TAB(39)5"a@"5
END
FOR A=@ TO N-1
PRINT “No. "3A+15TAB(G)
INPUT “Datum,uncert."3D,&
GCA, @)=D:G(A, 1ISE*
WTDT=WTDT+ D¥G(A, 1)
SUMW=SUMW+G( A, 1)

THEN PCT!=SPRD!*100/WA!

Weighted Mean ="gWA!5"t

NEXT A
RETURN
INPUT"ENTER per-cent uncertainty"4P
FOR A=0 TO N-1
PRINT "No. "$A+15 TAB(6)
INPUT "Datum" 5D
GCA, O)=DIGiA, 1) =. OL*ePRDI%-2

WTDT=WTDT + D*¥GiA,1)
SUMW = SUMW + G(A,1)
NEXT A



i. 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 44060C (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 correc~
tions are made for residual scattering effects, and, in fact, these correc-—
tions 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 em radius heavy-water sphere
covered with cadmium?. These are referred to as the "bare" and the "moder-
ated" 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 which it (and the radiation worker) are expected to be exposed. For cali-
brating albedo dosimeters, particularly if they are to be used in the rela-
tively soft spectra found in reactor environments, moderated californiun,
with its spectrum rich in low and intermediate energy neutrons, is clearly
preferred*’* (compare Figures 4 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 differ-
ence in calibration factors is a factor of ~ 1.4. Thus, moderated ?57Cf 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 neut-
ron source given in ANSI N13.115. On the other hand, the spontaneous fission
neutron spectrum from bare californium, similar to that from ?*5U fission,
has been very carefully evaluated and is better known than that from moder-
ated californium, and higher dose equivalent rates are obtained from bare
ealifornium. In the end, however, the customer usually has his own ideas
about which source to use (sometimes asking that the same device be calib-
rated 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 instrumen-
tation, 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 cali-
bration 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, ete.


TABLE OF CONTENTS, CONTINUED

LIST OF TABLES

Table 1. Room Return Correction, S, percent per meter-squared 6
Table 2. Air Scatter Correction, percent per meter T
Table 3. Values of Group Source Strength per Logarithmic Energy D-2

Interval for a ?5*Cf Spontaneous Fission Source
Table 4. Values of Group Source Strength per Logarithmic Energy D-4

Interval for a ?°5*Cf Spontaneous Fission Source in the
Centre of a D,0 Sphere with a Radius of 150 mm

LIST OF FIGURES

Figure 1. Simplified View of Calibration Facility 4
Figure 2. Spreadsheet for Dosimeter Irradiations 8
Figure 3. Spreadsheet for Remmeter Calibration W

Figure 4. Neutron Spectrum from a 25?Cf Spontaneous Fission Source D-3

Figure 5. Neutron Spectrum from a ?5*Cf Spontaneous Fission Source D-4
in the Center of a D,0 Sphere with a Radius of 150 mm

vi


reading ("CRead"), the calibration factor ("CalF"), and the over-all uncer-
tainty in the calibration factor ("Sum" and "Sum+H").

Similar spreadsheets have been prepared for the other types of remmeters
which we encounter, for both the bare and moderated sources. The spread-
sheets 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.

4.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,
earrying 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, 40 cm x 40 cm x 1/4" 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-hacked 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
em 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.

Tt 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 ir-
radiation 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


in the maximum dose equivalent rate for moderated irradiations by the present
ecalifornium source strength and by the fact that there are spectral changes
at close distances (< 30 om)°?’?° 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% (lo). The
limitations on accuracy include a +4%° uncertainty in the fluence to dose
equivalent conversion factor for the moderated source, and ~ 1.3% uncertainty
in the knowledge of the ncutron 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 BF, 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 cal-
ibration 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 calibra-
tion 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 doa
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 compro-
mise, given that these are field instruments.

Fac

2. ty Description

Our facility is located in the lower level of the NBS Van de Graaff accelera-
Lor low-scatler room. (See Figure 1.) This area has an 11 m x 12 m concrete
floor, and concrete walls which extend to a "ceiling" height of 4.6 m. This
"ceiling" is actually an aluminum grating which serves as the floor of the
low-seatter room above. The sources are stored in a 25 om 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


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
D,0-Moderated ?°?Cf Source for Calibrating Neutron Personnel Dosimeters Used
at Nuclear Power Reactors," NUREG/CR-1204, January, 1980.

3. G. W. R. Endres, J. M. Aldrich, and L. W. Brackenbush, "Neutron Dosime-
try at Commercial Nuclear Sites - Report on Subtask RB," Pacific Northwest
Laboratory, PNL-SA-9201, January, 1981.

4, R. B. Schwartz and C. M. Eisenhauer, "Use of a D,0-Moderated 75?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 Perfor-
mance - Criteria for Testing," ANSI N13.11-1981 (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-
1984)", Metrologia 23, 129-144 (1986/87); E. D. McGarry, V. Spiegel, and Li
Linpei, in: BIPM Com. Cons. Etalons Mes. Ray. Ionisants (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
D,0-Moderated ?°*Cf Calibration Facility," Health Phys. 46, 97-106, (1984).

10. R. B. Schwartz, C. M, Eisenhauer, and J. A. Grundl, "Experimental Veri-
fication of the Neutron Spectrum from the NBS D,0-Moderated 7°?Cf Source,"
NUREG/CR-3400 (1983).

11. R. B. Schwartz, "Experiences in Calibration of Neutron Survey Instru-
ments," in Proc. Workshop on Radiation Survey Instruments and Calibrations,
Gaithersburg, Md., July 10-12, 1984. PNL-SA-13346, 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~10771 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. 4a,

489 (1982).

19


NBS MEASUREMENT SERVICES:
NEUTRON PERSONNEL DOSIMETRY

Robert B. Schwartz

Center for Radiation Research
National Measurement Laboratory
National Bureau of Standards
Gaithersburg, MD 20899

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Issued July 1987


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:

= = 2
D R + R, Do/r + Rey (1)
where K_ is the response of the detector to the source neutrons,

R- is the response to the room scattered neutrons, and

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

The quantity D. is the quantity which we are trying to determine. If we
define the quantity Sas

S = Ro/Dos (2)
we can re-write eq. 1 as
Dr? = D1 + Spt), (3)

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 meter-
squared.

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

Device Bare Moderated
3" Sphere 33% 13%
Albedo Dosimeter 20 5.4
9" Spherical Remmeter 6 7.9
Andersson~Braun Remmeter 2.4

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
inerease 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. Tne calcu-
lated corrections, in percent per meter, are listed in Table 2, taken from
references 14 and 15. They are based on unpublished Morse Monte Carlo air
transport calculations by R. C. McCall and energy response measurements by D.
Hankins'®,


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 em 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 correc-
tions: 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 calibra-
tion’, which measures the total emission rate into 4m. Dividing the emission
rate, Q, by 4mr? 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/4nr?. Since the scattered neutrons have a
different energy spectrum from the source neutrons, the effect on any instru-
ment 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-serles sources, and 3% for the heavier, SK-Cr,
sources. For devices more sensitive to lower energy neutrons, such as the 3"
sphere or albedo dosimeters, the correction is taken to be 1% 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.


Appendix C. Sample Test Reports

Pages C-2 - C-4 are a typical remmeter test report, generated from the data
shown in the spreadsheet of Figure 3. This is followed on pages C-5 and C-6

by a test report for dosimeter irradiations. The reports should be self-
explanatory.


590 RETURN

708 FOR A= TO N-1

718 PRINT "NO. “5A+15TAB(6)

720 INPUT “Datum,% Uncertainty"3D,P
730 G(A,O)=D? GCA, 1)=(.O1*P#D) 4-2
758 WIDT=WTDT + D*¥G(As 1)

768 SUMW=SUMW + G(A,1)

77@ NEXT A

780 RETURN

B-3


The basic physical principles are straightforward. We start with a califor-
nium neutron source of known emission rate, Q, measured at NBS’ with an
uncertainty of ~ 41.3%. (Note that this uncertainty, and all other uncer-
tainties given in this document, are understood to be one standard deviation
(10)). At any point a distance r from the source, the free field fluence
rate is just Q/4nr?. Using conventional fluence-to-dose equivalent conver-
sion 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 Commis-
sion 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 neu-
tron 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 general-
ly result in scattered neutrons with lower energy, different angular distri-
bution, and different variation with distance than the direct source neu-
trons. 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 calcu-
lation 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


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 ecm long.
Based on our general experience, we assume that this introduces a 0.5 em
uncertainty in the effective source-detector distance. This means a percent-
age uncertainty in D of 0.5/D, where D is expressed in meters. Sinee 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 fluence-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
252cf, 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 spec-
trum® and we use a conversion factor derived from weignting 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 calcu-
lated 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 4% uncertainty in the conversion factor, to
obtain the total uncertainty ("Sumt4"),

6. Internal Quality 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.


Table 3 — Values of group source strength per logarithmic energy interval for @ 32Cf spontaneous fission source

Neutron energy. E Ae Neutron energy. E So
AWIE/E,) AIntE/EQ)
Mev ot Mev a
4.14 x 10-7 280 442 x 10-!
0.01 4.40 x 10-8 3.00 427 «10-1
0.05 2.74 x 10-3 340 4.01 x
0,10 124 x 107? 3,70 3.66 x
0.20 3.33 x 10-2 420 3.25 «
0,25 6.04 x 10-2 4.00 2.78 «x
0,30 7,90 x 10-2 5,00 2.39 x
0.40 1,07 x 107! 5.50 1.98 «
0,50 1.46 x 10-1 6,00 1,61 x
0.60 1,84 x 10-7 6.50 1.28 x
0,70 221 x 10-1 7,00 1.01 x
0.80 2.55 x 10" 7,50 7.92 *
1,00 3,01 x 10°! 8,00 6.16 «
1,20 3.53 x 107! 8,50 476 x
1,40 3,95 x 10°! 9,00 3.65 x
1,90 419 x 10! 9.50 2.79 «
1.60 42 x 10°! 10,00 2.13 x
1,80 4.46 x 10°? 11,00 142 x
2,00 4.58 x 10°" 12,00 8,04 x
2.20 4.62 x 10°! 13,00 451 x
2.30 4.61 x 10°! 14,00 2.50 x
2.40 4,59 x 10°" 16.00 1.08 x
2.60 453 x 10"! 18,00 3,20 x



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And

CORRECTION (%)
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2

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3

20

43 32


Detailed procedures are given for calibration of neutron dosimeters and rem-
meters with californium neutron sources, both "bare" and moderated. Correc-
tions for air scatter, room-return, anisotropic neutron emission, and devia-
tion 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.

iv


and for NS-79, a bare. source,

= *Q/D2
FEDE, sno 7 *00954*Q/D?, (6)
where the numerical factors are the fluence~-to-dose equivalent conversion
factors discussed in Section 1.

The correction (percent) for the moderated source (NS-86) is
CORRECTION(E) 94 = 5-4¥(D/100)% + 3*D/100 + 2813/07, (7)

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(%) 4 = 1 + 20*(D/100)? + 1.1*D/100, (8)

where the first term is a 1% correction for non-isotropic emission, the
second term is a 20% per meter-squared room scattering correction, and the
third term is a 1.1% per meter air scatter correction. The correction for
the other bare source (Cf~-132) is of the same form except that the non-
isotropic emission correction is 3.7%, rather than 14%.

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 equiva-
lent 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. a1)

It was not necessary, in this case, to do the calculations for the other
sources, since for the sources selected (the strongest sources) the irradi-
ation times turned out to be convenient. One would only consider using the
weaker sources if the desired dose equivalents were so low that the irradia-
tion 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.


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

un Source Q(n/s) Dem) FFDE D’(em) 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 31.99 33.3 4.8 44.4 42.3 0.76
3 NS-86 3.977et 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
5S 4.5
6 4.5

1398)

Tr

NS-39 5.007et07 38 79.7 11.3 : 110.1 105.4 0.76
136-z 4.959e+06 38 7.90 11.3 . 10.85 10.4 0.76
ratio = 80.2 Apr. 23, 1986

Sum+4 FFDE Cal F

Stat cal Corr Dist Sum
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 3.1 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:

Corr(%) = 7.9%(D/100)? + 2.3*D'/100 + 4uu6/D?, (12)

where the first term is a 7.9% 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 om), and b (15

em) 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

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 wilt 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
Personne] Dosimetry, by R. B. Schwartz, is the twelfth to be published
jin 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

iii


Appendix D. Graphic and Tabular Representation of the Neutron Spectra

The spectra are represented as plots of AB,/Aln(E/E,) (linear scale) versus
neutron energy E (logarithmic scale) with AB, (group source strength) normal-
ized to a total source strength Q = 1 neutron/sec and E, taken as 1 MeV.

In the tabulations, the energy associated with a value of AB,/1n(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.


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 Tech-
niques for Neutron Personal Dosimetry," Radiat. Prot. Dosim. 10, 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, 19/77, 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 26, 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 Modera-
tion of ?*5U and ?5*cf in H,0," 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/CR-
2233, 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 Measure-
ments 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


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


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 (~5%), 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 ~10% 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/nr. For the purpose of analyzing the data, a "backgrouna"
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 (10) 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:

Ve MY egZE- the Director,
R. B. Schwartz, Physicist

4 . 5
TT (aq;
h as Casi Cf
R. S. Caswell, Chief
Ionizing Radiation Division

Test No.:
SP250 No. 8.1d

Date: May 7, 1986


Appendix E. Definitions

Taken from: NCRP Report No. 51, "Radiation Protection Design Guide-
lines 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.


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 sample-
based 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 162-240 put a frame around the output listing on the Tandy 200 dis-
play.) The quantities are calculated from the following formulas:

Weighted Mean = TCxpews )/EWi
stdev of the mean computed from weights = WC Ew) (B-1)
sample-based weighted stdev of the mean = VEw (x 5x) ?/(N-1) EW, (B-2)

where xX; are the individual input data,

ay are the weights corresponding to each Xie

=
u

1/(E,)?, where the Ey are the uncertainties in the input data,

xX 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 tne same answer. By “properly chosen" we mean that the
Ww. 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 8-° 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


Table 2 - Air Scatter Correction, percent per meter

Device Bare Moderated
3" Sphere 1.7% 4.5%
Albedo Dosimeter 1.1 3.0
Remmeter 1.0 2.3
NTA Film, Track Etch Dosimeter 0.5 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

{1 + 6(a/ar)?]

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 esti-
Mated to be ~ 2/3. J. B. Hunt!® has shown that 6 depends upon the sphere
size. Following Hunt, we use 6 = 0.4 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 cor-
rection, with 6 taken to be 0.5. For a spherical source irradiating a
spherical detector, we modify the above factor to

{1 + 6{(a? + b?)/(2r)?]}
where b = 15 cm, the radius of the moderated source.
4. 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 quanti-
ties 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-Cale 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 calibra-
tion was completed, the spreadsheet was copied onto a diskette for a perma-


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 calibra-
tions: the Pro 350 for initially setting up the calibrations and for perma-
nent 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 sepa-
rate spreadsheet should be prepared for each instrument by copying the origi-
mal 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 fur-
nishes 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 caloulation 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 om 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 signifi-
cantly different from those obtained at the other distance, than the possi-
bility of an error in the distance measurement should be considered.

10