600 nm

605 nm

610 nm

600 nm

605 nm

610 nm

FIGURE 5 Examples of FWT Ag scans, from 610 nm down to 600 nm,
showing various oscillations, for different orientations.

NOTE: Spectrophotometer settings:

RANGE: 0.1 ABS; S.B.W.: 3.5 nm; PERIOD (sec):
1.0; CHART DISPLAY: 5 nm/cm; SCAN RATE: 0.2 nm/sec.

A. First position: oscillations are too large and are not
acceptable; film rotated 90°.

B. First rotation (second position): oscillations are very
small and can be ignored; this is the most acceptable reading
of all.

C. Second rotation (third position): oscillations, as in A, are

not acceptable.

D. Third rotation (fourth position): oscillations are within
specification; however, taking an average between the highest
and lowest readings around 606 nm would be required for an
accurate reading of low absorbance values of this film for
this orientation.

11


U.S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MARYLAND 20099 Test 536/ 111111-86

NBS DB 100/100
XRG- 322
December 26, 1985

REPORT OF CALIBRATION

of
Far West Technology, Inc. Type 60-00 Radiochromic Dosimeters

for

Kilorad Corporation
89 Irradiator Avenue
Moline, IL 60609

Attention: Jeffrey Doe
Chemistry Lab Supervisor

Ref. P.O. No.: 111

Far West Technology radiochromic dosimeters, type 60-00, supplied by Kilorad
Corporation, were irradiated, ten at each dose, under controlled conditions using
gamma radiation from a standard ©°Co source. During irradiation the dosimeters
were held between 5,0-mm thick blocks of polystyrene, which were sealed inside
polyethylene packets. The relative humidity at the time of sealing was 56%. The
temperature during irradiation was.24°C, The dates of irradiation and values of

absorbed dose were as follows:

Dosimeter No. Date of Irradiation Absorbed Dose in Water
(kGy)* (Mrad)

101-110 19 Dec. 1985 10.0 1.00
111-120 20 Dec. 1985 15.0 1,50
121-130 20 Dec. 1985 20.0 2.00
131-140 19 Dec. 1985 25.0 2.50
141-150 23 Dec. 1985 30.0 3.00

*) kGy = 0.1 Mrad

APPENDIX A
A-1


6.3 Control limits may be chosen for future check standard dosimeter
observations (where each observation is the mean of n = 5 readings) using the
following:
3s,
Upper control limit = A, +

Ja

Lower control limit =

In this case, a safety factor of 3 was chosen; a factor of 2 is sometimes used,
or for a near-normal response distribution, the appropriate percent point of
the Student’s t distribution, t,,2(v), may be used, where l-a is the specified
confidence level.

6.4 The control procedure applied to each future calibration depends on a test
statistic, T, calculated from the value of the check standard, cy, (the mean of
n= 5 readings) by the following:

T=

If T <3, then the process is in control. If T= 3, then the process is “out
of control" and remedial action is required.

6.5 Control charts are used to determine if the calibration irradiations are
being performed in the proper way. Initial measurements are made to determine
the mean dosimeter response and variability for each absorbed dose level. Once
the dosimeter system response is adequately characterized, then control charts
are drawn up that provide a baseline value for each absorbed dose level and
appropriate control limits as discussed in 6.3. All subsequent readings are
plotted as a function of time on these charts. All the readings should fall
within the control limits except for the few that would fall outside due to
pure chance (i.e., about one in a hundred for a value of a = 0.01 or the 99%
confidence level). If readings fall outside the control limits more often than

would be expected by pure chance, then the process is "out of control" and must
be investigated and corrected.

6.6 The radiation response of the check standard dosimeter batch may change
(drift) slowly with time. An “out of control" situation on the control chart
may indicate such a drift in response. In that case, it is necessary to
completely characterize the batch response again as was done initially in
setting up the control chart.

6.7 Quality control procedures are used in other areas such as data reduction
and report generation. All irradiation data, such as source decay corrections
and calculated irradiation times, are double-checked for accuracy. An
experienced, highly-trained technician performs all calibration irradiations
and analysis of dosimeters. Senior scientific staff review all results and

28


Test 536/222222-86
NBS DB 100/001
XRG-321

October 10, 1985
Page 2 of 5 pages

The dosimeters were read in terms of optical absorbance before (Ag) and after (Ay)
irradiation, using a Cary Mqdel 219 spectrophotometer (3.5 nm S.B.W.),-at two
optical wavelength settings! 605 nm and 510 nm. Dosimeter film thicknesses were
read using a Mitutoyo Model DGS-E thickness gage. Values of increase in optical
absorbance per unit thickness (A/mm, where 4A = Ay - Ay) are also included.

Date,
Dosimeter Time
of Ay A Thickness bA/mm
(605 an}ts10 nm) Reading (605 nm) {510 nm) __ (mm) (605 am)(510 am)

696 0.1516 0.0591 9/9/85 0.1516 0.0591 0.050
697 0.1570 0.0787 11:15 0.1563 0.0781 0.050
698 0.1437 0.0597 “ 0.1435 0.0596 0.049 --Controls--
699 0.1496 0.0605 . 0.1487 0.0599 0.051
700 0.1504 0.0599 “ 0.1507 0.0604 0.051

701 0.1425 0.0591 9/9/85 1.4947 0.2256 0.050 27.04 3.330
702 0.1500 0.0601 11:25 1.5237 0.2281 0.050 27.47 3.360
703 0.1399 0.0631 . 1.4697 0.2246 0.051 26.07 3.167
704 0.1504 0.0603 . 1.6352 0.2380 0.053 28.01 3.353
705 0.1514 0.0627 “ 1.6253 0.2392 0.052 28.34 = 3.394

706 0.1498 0.0600 9/9/85 0.8804 0.1483 0.051 14.33, 1.731
707 0.1526 0.0712 14:35 0.8906 0.1587 0.052 14.19 1.683
708 0.1472 0.0598 “ 0.8893 0.1476 0.050 14.84 = 1.756
709 0.1551 0.0628 . 0.9297 0.1537 0.051 15.19 = 1.782
710 0.1487 0.0638 " 0.9136 0.1549 0.051 15.00 1.786

71 0.1625 0.0800 9/9/85 1.4276 0.2314 0.052 24.33 2.912
712 0.1650 0.0742 14:45 1.4719 0.2291 0.053 24.66 2.923
713 0.1563 0.0722 . 1.3659 0.2209 0.052 23.26 2.860
714 0.1489 0.0598 . 1.4138 0.2149 0.051 24.80 3.041
715 0.1555 0.0604 . 1.4299 0.2168 0.052 24.51 3.008

716 0.1547 0.0640 9/9/85 1.0597 0.1742 0.052 17.40 2.119
77 0.1419 0.0586 15:00 1.0341 0.1673 0.051 17.49 2.131
718 0.1494 0.0600 “ 1.0486 0.1675 0.050 17.98 2.150
719 0.1570 0.0646 “ 1.0935 0.1753 0.051 18.36 2.171
720 0.1550 0.0643 “ 1.0759 0.1732 0.050 18.42 2.178

APPENDIX ©

C-2


3.1.2 The Gammacell 220 irradiator, illustrated in Figures 2 and 3 is a
commercially supplied unit (manufactured by AECL of Canada), with 24 ®°Co
source rods of about 20 kCi total activity in a stationary annular array. The
source rods are located inside a massive lead-steel shielding structure that
provides personnel protection. A motorized "drawer" assembly, consisting of an
open irradiation chamber and shielding plugs above and below, can be moved up
and down through the cylindrical source array. Dosimeters are placed in the
irradiation chamber (with the drawer up), then the drawer is lowered so that
the chamber is positioned reproducibly in the geometrical center of the source
array. Control of the drawer is by means of a preset timer so that
irradiations may be performed automatically. The irradiation chamber has
inside dimensions of 15 cm diameter by 20 cm high. The absorbed-dose rate in
water in May 1986 was 0.21 kGy-min'+. This rate was calibrated by means of
potassium dichromate reference transfer dosimeters from NPL in the U.K. and
verified by NBS radiochromic dye film transfer dosimeters on an annual basis.
The estimated overall uncertainty in this rate is about +3% at a 95% confidence
level [4, 5]. Discussion of uncertainties associated with absorbed dose values
for dosimeters irradiated with this unit is given in Section 5.

Significant radiation heating of dosimeters can occur during irradiation in

this unit, so an active cooling system consisting of a flow of chilled dry
compressed air is provided to maintain the sample temperature at 24°C.

3.1.3 The vertical-beam ®°Co source consists of a single capsule containing
about 4 kCi, housed in a shielding head with a shutter and collimator assembly,
as shown in Figure 4. The dosimeters to be irradiated are placed at a measured
distance from the source, the shutter assembly is opened, and the gamma rays
emerge through the collimator unit into the shielded room. The irradiation
time is controlled by an automatic preset timer on the shutter assembly.

The absorbed dose rate available from this unit varies as the inverse square of
the distance from the source. At a distance of 1.5 meters, for May 1986, the
absorbed dose rate in water was 0.17 Gy-min’+. This absorbed dose rate has
been determined with an NBS standard graphite ionization chamber and is
rechecked on an annual basis. The estimated overall uncertainty in this rate
is about +1.3% at a 95% confidence level. See Section 5 for detailed

discussion of uncertainties in absorbed dose values for dosimeters irradiated
with this unit.


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15


Test 536/ 111111-86
NBS DB 100/100
XRG-322

December 26, 1985
Page 2 of 2 pages

The attached page describes uncertainties and related factors in high-dose
calibrations, such as the one covered by this Report of Calibration, Any questions
regarding environmental conditions, different types of materials used, and
systematic uncertainties associated with these calibrations can be answered by this
attachment. If there are further questions, they should be directed to the first

individual named below,

Measurements were made by

Dee Edeckens

D. G. Hocken, Calibrations Technician
(301)921-2201

Approved by .

!
Hi

J. W. Motz, Leader

X-Ray Physics Group

For the Director
by -

“\- Tenet
« S. Caswell, Chief —~
Ionizing Radiation Division

Center for Radiation Research

APPENDIX A
A-2


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 250-11), NBS Measurement Services: Dosimetry for
High-Dose Applications, by J. C. Humphreys, D. Hocken, and W. L.
McLaughlin, is the eleventh to be published in this new series of
special publications. It covers the dosimetry services available to
users of intense radiation fields, in particular large gamma-ray
sources and electron accelerators up to about 10 MeV. These are:
dosimeter irradiation services (see test numbers 49010S, 49040S, and
49041S in the SP 250 Users Guide); dosimeter calibrations (49020S and
49030S); and measurements of dosimeter characteristics (49050S).
Inquiries concerning the technical content of this document or the
specifications for these services should be directed to the authors or
one of the technical contacts cited in SP 250.

The Center for Radiation Research (CRR) is in the process of publishing
21 documents in this SP 250 series, covering all of the calibration
services offered by CRR. A complete listing of these documents can he
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 .

Joe D. Simmons Chris E. Kuyatt
Acting Chief Director
Measurement Services Center for Radiation Research

iii


Table 6

UNCERTAINTIES IN ABSORBED DOSE VALUES

EMPLOYING TRANSFER STANDARD DOSIMETERS

Source of Uncertainty Type A (%) Type B (%)
(random) other

Combined (in quadrature) from
Table 3 0.34 1.05

Transfer standard dosimeter
response 1.00

Dosimeter response temperature
correction 0.50

Time dependence of dosimeter
response 0.50

Effects due to differences in photon
energy spectra of sources 0.50

Type A, Type B combined (in quadrature),

separately 1.06 1.36
Combined (in quadrature) 1.72
Total: 3X Combined 5.2

26


4.1.8 Before the calibration irradiations, the dosimeter films are stored at
room temperature (~ 24°C) in a controlled humidity environment (within the
range of 40 to 50% R.H.) for at least 24 hours [7]. After this period, the
dosimeters are packaged in appropriate equilibrium build-up material. For the
absorbed dose to be specified in water (or tissue), the equilibrium material
used is polystyrene; for absorbed dose in silicon, the equilibrium material
used is aluminum. The equilibrium material is in the form of 25-mm square
blocks, which are 5 mm thick for polystyrene and 2.2 mm thick for aluminum.
There is a 12-mm square cavity, 1 mm deep, in one block of each pair of blocks.
A group of five dosimeters is stacked and placed in the cavity between the two
blocks of the appropriate material then heat sealed in two pouches of 100ym
thick polyethylene; an inner black layer for light protection, then an outer
clear layer. After packaging, the dosimeters are ready for irradiation.

4.1.9 Most calibration irradiations are performed in the NBS ®°Go pool source.
Each dosimeter package is placed in a polystyrene foam holder fitted into the
stainless steel can so as to locate it at the geometrical center of the source
array. Irradiations are done by manually lowering the water-tight irradiation
can (attached to a long handling pole) into the source array, noting the clock
time, then removing the can (manually) at the end of the calculated irradiation
time. The irradiation times for given absorbed doses are calculated before
irradiations are started, based on previous verifications of the source
absorbed dose rate and corrected for the exponential radioactive decay for ®°Co
(based on a half-life of 5.27 years). Decay corrections are done every two

days to provide a current absorbed dose rate. The decay correction factor is
given by the equation:

where absorbed dose rate on the desired date,
absorbed dose rate on December 31 of the previous year,
6°Co decay constant, and

number of days from December 31 of the previous year to the desired

date.

Now,

0.693147

\" (years) » 365.25 days/year

where T, = half life for ®°Go = 5.2714 years.
Thus A = 3.60006 x 10°“ /day.

The decay correction factor = Cy = e**, Values for Cy for various elapsed
times are given in Table 1.

To use Table 1, determine the number of days since the previous December 31
(for which the absorbed dose rate was stated). Then multiply the correction
factors from Table 1 that correspond to the total number of elapsed days. For
example, if the absorbed dose rate on April 23 was desired, the number of
elapsed days (for a non-leap year) would be 113 days. To determine the decay
correction factor, look up the factor where the horizontal row for "100"

12


pool source) during irradiation. This container may be in the form of square
blocks for FWT-60-00 thin dye films as used in the internal NBS calibration, or
in the form of a cylindrical cup for Red Perspex or similar type dosimeters.
These containers supply the material needed to provide electron equilibrium
conditions during irradiation as previously discussed in 2.7. The Red Perspex
dosimeters, for example, are about 3-mm thick and a correction for self
attenuation (of about 3%) is necessary in order to specify the correct average
absorbed dose rate in the dosimeters during calibration.

4.2.3. Upon completion of the irradiations requested by the customer, the
dosimeters are returned promptly to him. In the case of fragile or unstable
dosimeters, it is sometimes necessary to specify special shipping conditions.
For example, because of instabilities following irradiation, Red Perspex
dosimeters should be sent by express overnight mail service and should be
protected from temperature extremes during that period. Aqueous chemical
liquid dosimeter ampoules also require such precautions, as well as protection
against breakage by careful packaging. After double checking the irradiation
times and calculations of the absorbed-dose rates for that day, a Report of
Calibration is prepared and sent to the customer specifying the absorbed doses
given to his dosimeters (see Appendix A). The customer then formulates his own
calibration curve based on the NBS absorbed-dose values and readout of the
dosimeters with his own instrumentation.

4.3 Application of Transfer Dosimeters

4.3.1 When a customer requests dosimeters to be supplied for irradiation in
his facility, the procedures of 4.1.4 to 4.1.8 are followed, using unirradiated
dosimeters from a calibrated batch, i.e., reading initial absorbances, humidity
conditioning, and then packaging the dosimeters

4.3.2 The requested number of packaged unirradiated dosimeters (plus some
controls) are sent to the customer, along with instructions on proper handling
(e.g., do not open the packages), the proper absorbed dose level to be given (1
to 50 kGy), as well as reporting any special environmental conditions (e.g.,
elevated temperature during irradiation) (see Appendix B). If the temperature
of the dosimeter package is expected to exceed 30°C during irradiation in the
customer's facility, then the customer is requested to measure that temperature
during the irradiation and provide NBS with those data (preferably as a
function of time) in order that NBS can apply the proper correction factors to
the dosimeter response (see Fig. 8).

4.3.3 When the irradiations are completed by the customer, he sends the
dosimeter packages back to NBS. The packages are opened at NBS and the
dosimeters read as soon as possible following the procedures of 4.1.11 to
4.1.15. The results are corrected for elevated irradiation Cemperature, if
required, and the results are compared to the nominal absorbed dose the
customer reportedly gave the packages. Results are then supplied to the
customer in the form of a Report of Calibration (see Appendix C for an
Example).

4.4 Analysis of Dosimeters

4.4.1 Reading of customers’ dosimeters is done in a manner similar to that
used for the internal calibration procedures detailed in 4.1. For FWT-60-00

18


intersects the vertical "10" column corresponding to 110 days. This factor is
0.9611. The factor for "3", the remaining number of days, in the first row is
0.9989. Multiply these two factors to give the correction for 113 days:
0.9600. Multiply this factor by the absorbed dose rate for the previous
December 31 to give the dose rate for April 23.

4.1.10 A typical calibration of a new batch would cover an absorbed-dose range
of 1 kGy to 50 kGy with absorbed dose values of 2, 5, 10, 15, 20, 30, 40 and
50 kGy.

4.1.11 After irradiation, the dosimeters are stored for approximately 24 hours
to allow for complete development of the dye [7]. After this period, the final
spectrophotometer readings (designated A,) are made, taking care that the same
orientation of each film is used as for the initial A, readings.

4.1.12 Finally, the thickness of each dosimeter is read at three points across
the film corresponding to where the spectrophotometer light beam passes through

the dosimeter film. The mean value of these measurements is used as the film
thickness.

4.1.13 All values of initial absorbance, A,, final absorbance after
irradiation, A,, and thickness in mm, t, for each dosimeter are recorded in a
data book. The response of the dosimeter is given by, R,

4.1.14 The mean R, for each group of five dosimeters irradiated together is
plotted against the absorbed dose in the specified material. A curve is fitted
through the data by means of a least squares regression method such as a
polynomial fit. In general, the curve will be nonlinear, at least over certain
portions of the absorbed dose range. Data for both wavelengths of analysis,
605 nm and 510 nm, are used to generate separate calibration curves. See

Figs. 6 and 7 for examples.

4.1.15 Dosimeters from the calibrated batch irradiated to some unknown
absorbed dose are read according to the procedures of this section and the
resulting absorbed dose is obtained from the calibration curves. The 605 nm
calibration curve is used primarily to determine the absorbed dose value; the
510 nm results are used generally for redundancy to check on possible erroneous
interpretation of the 605 nm results.

4.2 Irradiation of Customer Dosimeters

4.2.1 Various types of dosimeters may be submitted by customers for
irradiation to various absorbed dose levels. Table 2 gives a brief listing of
some typical dosimeter types and the applicable absorbed dose range for which
they may be utilized; other types may also be processed.

4.2.2 The irradiation geometry used for these dosimeters depends on their size
and shape. In general, if the requested absorbed dose is in water, then a
container made of polystyrene with a 5-mm wall thickness is used to hold the
dosimeters in the proper location (i.e., the center of the source array for the

14


6. Internal Quality Control

6.1 Measurement assurance program (MAP) type techniques are used to ensure
that calibration irradiations are performed correctly and the process is in a
state of statistical control [16].

6.2 A number, n, of check standards, in the form of calibrated radiochromic
dye film dosimeters, are placed immediately adjacent to the test dosimeters
during each irradiation. After irradiation, the check standard dosimeters are
analyzed to produce a sequence of absorbed dose readings c,,...,c,.

Thus, for a given absorbed dose level, the accepted value of the check standard
is the mean of those values:

is

The number of degrees of freedom, v, required to estimate the initial value for
the actual standard deviation can be determined by choosing how close the
estimate should be to the true value at a given confidence level. For example,
if the estimate is to be within 20% of the true standard deviation at a 95%
confidence level, then the degrees of freedom, v = 46 = n-1, or n = 47
measurements [see sec. 2.4 of reference 14]. As the measurement assurance
program progresses, additional data are obtained on the variability of the
response of the check standard dosimeters, and a more reliable value for the
estimated standard deviation may be computed. This is done by "pooling" the
standard deviations obtained on separate occasions for the same process. If

individual standard deviations are s,,....,s, with degrees of freedom
Vayeceee »¥,, respectively, then the pooled standard deviation is
2 2
VyS17+.. ty Sy?) 5
s - | TST
Vit... HYy
The degrees of freedom associated with s, is v = v,4....4%

27


3.2 Transfer Standard Dosimeters

These dosimeters are radiochromic dye films, purchased in large batches from
Far West Technology, that have been carefully and thoroughly characterized by
NBS as to their response reproducibility, stability, absorbed dose-rate
dependence, temperature and humidity dependence, and thickness. The dosimeters
are type FWT-60-00, consisting of a nylon matrix incorporating an organic dye
(leucocyanide of hexa-hydroxyethyl pararosaniline) in the form of a thin solid
film, 1.0 cm square and 50 pm thick [6]. A new batch is calibrated before
being utilized as transfer dosimeters as described in the Operating Procedures.

3.3 Spectrophotometer

The primary instrument employed for the analysis (reading) of the radiochromic
transfer dosimeters is a high performance UV/VIS spectrophotometer. The
instrument presently used is a Varian/Cary 219. This instrument has very low
stray light characteristics so as to permit it to measure optical absorbance
values of 3 or less with an uncertainty of less than 1%. The absorbance scale
of the instrument is checked annually with a set of neutral density filters
(SRM 930D). The wavelength scale is checked annually by means of emission
lines of well known wavelength from a mercury discharge lamp. The wavelength
scale must be within 0.2 nm of the specified value for the entire wavelength
range (200 to 800 nm).

3.4 Thickness Gauge

The radiochromic transfer dosimeters are normally 50 wm thick and are measured
with a thickness gauge to a resolution of 0.1 wm. The gauge is a Mitutoyo
electronic Model DGS-E with a linear displacement transducer that has a special
NBS developed digital display (not provided by the manufacturer). The
calibration of the thickness scales is performed annually by use of a set of
Brown & Sharpe (Super-A Jo Blocks) metric gauge blocks.

4. Operating Procedures

4.1 Internal Dosimeter Calibration

The radiochromic dye film dosimeters described under 3.2 are calibrated by
using the following procedures.

4.1.1 Upon receiving a new batch of dosimeters from the manufacturer, initial
visual inspection is performed to detect any obvious flaws (fingerprints, water
marks, etc.) that might make the batch unsuitable.

4.1.2 These dosimeters are sensitive to ultraviolet (UV) light and thus must
be protected at all times from sunlight (even through windows), fluorescent
lamps, or other sources. One effective method to achieve this is to filter
laboratory fluorescent lamps with wrap-around filters made of a special UV-
absorbing Mylar (trade named Llumar). Even with such filters, the dosimeters
are only exposed to light during essential handling procedures, otherwise they
are stored in the dark.


NATIONAL BUREAU OF STANDARDS
MEASUREMENT SERVICES:
DOSIMETRY FOR HIGH-DOSE APPLICATIONS

1. Description of Service

1.1 The National Bureau of Standards, Ionizing Radiation Division, provides
dosimetry services for individual users of intense radiation fields, in
particular, large gamma-ray sources and electron accelerators up to
approximately 10 MeV. These services include the administering of known
absorbed doses of ionizing photons to customer-supplied dosimeters (such as
solid radiochromic or liquid chemical types) or test samples that are sent to
NBS (schedule 49010S, previously 8.6A), [1]" where they are packaged in
appropriate conditions of electron equilibrium and are then irradiated in the
NBS standard ©°Co calibration facility to specific agreed-upon absorbed dose
values in the nominal "high-dose" ranges of 10-108 grays (10°-108 rads). The
dosimeters may either be spectrophotometrically read and evaluated by NBS
(schedule 49040S and 490418, previously 8.6D and 8.6E) or sent back to the
customer for analysis and evaluation.

1.2 Another service (schedules 49020S and 49030S, previously 8.6B and 8.6C)
consists of supplying to customers calibrated transfer dosimeters (radiochromic
dye type), packaged in appropriate equilibrium material, such as polystyrene or
aluminum, for irradiation and subsequent readout and absorbed dose
interpretation. These dosimeters are provided in sealed packages and sent to
the customer for irradiation to nominal agreed-upon absorbed dose levels in a
prescribed geometrical arrangement. The unopened packaged dosimeters are then
returned to NBS to be read and evaluated. The absorbed dose range that is
suitable for the radiochromic dosimeters is 1 to 50 kGy (0.1 to 5 Mrad) in
water, silicon, aluminum, graphite, or certain plastics.

1.3 Finally, NBS offers special measurement services (schedule 490508,
previously 8.6F) such as the determination of temperature dependence, absorbed
dose-rate dependence or reproducibility of dosimeter response, and measurement
of detailed absorbed dose distributions in specific irradiation geometries and
in selected absorbing materials. The absorbed dose distribution measurements
can include absorbed dose profiles in heterogeneous absorbers and at surfaces
and interfaces of different substances. Such information is important in
research leading to the commissioning of a radiation process, and in
measurement assurance that provides quality control of a given radiation
treatment.

“Numbers in square brackets indicate references, to be found at the end of the
text,


Table 4

UNCERTAINTIES IN ABSORBED DOSE VALUES

FOR IRRADIATIONS WITH GAMMACELL 220

Type A (%) Type B (8)
Source of Uncertainty (random) (other)

NPL dichromate reference 1.50
dosimeter measurement of dose rate

Source decay, half life 0.01

Timing of irradiations: 0.08
transit dose

Conversion of dose rate: 0.30
water to silicon, etc.

Attenuation corrections in 0.03
equilibrium material

Effect of dosimeter positioning 0.25
in irradiation chamber

Geometrical effects of equilibrium 1.00
material on dose rate

Source decay correction 0.02
for given date

Type A, Type B combined (in 0.26 1.83
quadrature), separately

Combined (in quadrature) 1.85

TOTAL: 3X Combined 5.5


Table 3

UNCERTAINTIES IN ABSORBED DOSE VALUES

FOR _IRRADIATIONS WITH POOL SOURCE

Source of Uncertainty

Graphite calorimeter/ion chamber
measurement of pool source dose rate

Source decay, half life
Timing of irradiations

Conversion of dose rate: graphite to
water, silicon, etc.

Attenuation corrections in equilibrium
material

Effect of dosimeter positioning in
irradiation can

Geometrical effects of equilibrium
material on dose rate

Source decay correction for given date

Type A, Type B combined (in quadrature),
separately

Combined (in quadrature)

TOTAL: 3X Combined

al

Type A (%) Type B (%)
(random) (other)

0.14
0.01
0.23
0.30
0.03
0.25
1.00
0.02
0.34 1.05
1.11
3.3


TABLE OF CONTENTS

1. Description of Service

2. Design Philosophy and Theory

3. Description of System .
3.1 ®°Co Irradiation Facilities
3.2 Transfer Standard Dosimeters
3.3 Spectrophotometer
3.4 Thickness Gauge

4. Operating Procedures
4.1 Internal Dosimeter Calibration .
4.2 Irradiation of Customer Dosimeters
4.3 Application of Transfer Dosimeters
4.4 Analysis of Dosimeters

5. Assessment of Uncertainties.

6. Internal Quality Control

7. Safety

References .

Appendix A:

Appendix B:

Appendix C:

Measurements

A Report of Calibration of Results of Transfer Dosmeter

Page

14
20
20
27
29

30

A Report of Calibration of Irradiation of Customer's Dosimeters

Instructions to Customer for Irradiation of Transfer Dosimeters


ABSTRACT

This document describes calibration services available at the National Bureau
of Standards tor the standardization ot high absorbed dose measurements ot
ionizing radiation. The areas of application of such measurements include
medical product sterilization, electronic device radiation hardness testing and
food processing. Detailed descriptions of the NBS dosimetry procedures and

uncertainty assessments are given. Measurement assurance program techniques
are discussed.

Key words: Dosimeter calibration, ionizing radiation, irradiation facilities,
measurement assurance program, uncertainties.


4.1.3 Random dosimeter samples are taken from the batch to be calibrated.
These samples are irradiated to specific absorbed dose levels and their
responses are assumed to be indicative of the expected responses of the
unirradiated dosimeters in the rest of that batch. The validity of this
assumption has been verified experimentally for periods of 6 to 12 months.
After 12 months a new calibration of the batch should be performed.

4.1.4 Handling of the individual dosimeters is done only with fine-tipped
tweezers, touching only the outer 2 mm from the edges of the 1 cm square films,
and, thus, avoiding scratches, fingerprints, etc., in the center where the
analyzing light beam passes through. Small amounts of dust may be removed by
gently wiping with lens tissue; however, any films that show obvious flaws such
as fingerprints, dirt, optical imperfections, etc., are discarded.

4.1.5 The films are marked in a corner with a permanent ink pen so that
consistent orientation may be maintained in all subsequent analysis procedures.
The film is placed in a special black-anodized aluminum holder that masks the
analyzing spectrophotometer light beam to a 6-mm square while holding the film
perpendicular to the sample light beam in a reproducible position. A matching
empty holder is placed in the reference light beam. With no films in either
holder, the spectrophotometer may be properly balanced.

4.1.6 All films are irradiated to a low absorbed dose level of 1.0 kGy before
any optical analysis is performed. The slight coloration developed in the
films as a result of this uniform low absorbed dose is proportional to the dye
concentration in the films. Thus, optical absorbance readings of the films
after this initial "pre-calibration" irradiation are made in order to eliminate
the effects of variations in dye concentration and, therefore improve the
precision of the response of the system. These "pre-calibration" irradiations
are performed with the films packaged ten at a time under equilibrium
conditions as described in 4.1.8.

4.1.7 Initially, each film is analyzed by a wavelength scan of the
spectrophotometer from 5 nm above the primary absorption peak (at 605 nm) to 5
nm below on an expanded absorbance scale (0.1A full scale, where A represents
optical absorbance units). This scan will sometimes show optical interference
fringes in the form of oscillations (see Fig. 5). If the peak-to-peak
oscillations are greater than some specified value (0.001A), then the results
are considered to be unsatisfactory. The holder and film are removed from the
spectrophotometer and the film is rotated 90° and then read again. If the
oscillations are less than the specified value, then the film is considered
acceptable. If all possible orientations are tried and the oscillations are
still greater than the specified limit, then that film is rejected and another
is selected to be tested in the same fashion. If the film reading is
acceptable, then it is marked with permanent ink in the lower left corner with
a unique code number. These initial readings are designated A,. All
subsequent readings of the film are done with this orientation. The
wavelengths of analysis are 605 nm and 510 nm, using a Varian/Cary 219
spectrophotometer with a spectral bandwidth of 3.5 nm.

10


LIST OF FIGURES

Page
Photograph of model of NBS ®° Co Pool Source 4
Schematic drawing of Gammacell 220 self-shielded ®° Co irradiator
in the chamber load position . 6
Schematic Drawing of Gammacell 220 irradiator in the irradiate
position . . 7
Schematic drawing of teletherapy type shielded-head ®° Co
irradiator. . 2... ee ee ee ee ee 8
Examples of optical spectral scans of unirradiated radiochromic
dye film dosimeter . : 11
Typical 605nm calibration curve for radiochromic dye film
dosimeters 15
Typical 510nm calibration curve for radiochromic dye film
dosimeters . 16
Dependence of response of radiochromic dye film dosimeters on
temperature during irradiation . aa 19

vii


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16


NBS Measurement Services:

Dosimetry for
High Dose
Applications

NBS
Special
Publication
250-11

Jimmy C. Humphreys
Dene Hocken
William L. McLaughlin

U.S. Department of Commerce
National Bureau of Standards


Test 536/222222-86
NBS DB 100/001
XRG- 321

October 10, 1985
Page 4 of 5 pages

Analysis of the dosimeter response consists of determining the mean 4A/mm for each
group of five dosimeters irradiated together and the standard deviation of each

group as shown below.

Dosimeter Mean AA/mm % Std. Dev., n-1
No. (605 nm) (510 nm) (605 nm) (510 nm)
696-700 --Controls-- -- -.
701-705 27.39 3.321 3.24% 2.70%
706-710 14,71 1,748 2.942 2.42%
711-715 24.31 2.949 2.52% 2.51%
716-720 17.93 2.150 2.65% 1.17%
721-725 28.76 3.465 2.20% 2.54%
726-730 27.69 3.338 2.59% 1.31%
731-735 12.25 1.470 1.73% 1.56%
736-740 33.09 4.036 3.13% 3.07%
741-745 26.81 3.244 3.33% 3.05%
746-750 28.69 3.465 2.20% 2.00%
APPENDIX C

c-4


2.5 The absorbed-dose rates encountered in radiation processing can cover an
extremely wide range. For ®°Co sources, the rates may be as low as a few grays
per minute; for electron beams, rates as high as 10!° grays per second are not
uncommon. Therefore, practical routine dosimeters must either be calibrated at
approximately the same absorbed-dose rate employed in processing or their
response must be known to be essentially absorbed dose-rate independent.

2.6 The principal dosimetry need in radiation processing is the provision for
calibration of the response of routine dosimeters (i.e., a physical change such
as optical absorbance) as a function of absorbed dose in water over the energy
and absorbed dose-rate range of interest. However, if for the specific
radiation type (i.e., photons or electron beams), the energy and absorbed dose
rate requirements cannot be met by the calibration facility, then the response
of the routine dosimeters must be shown to be independent of those qualities
over the range of interest.

2.7 An important requirement for performing proper calibration measurements is
that "electron equilibrium" conditions should exist during irradiation [2]

Such conditions exist when the number and energy of secondary electrons
(generated by the primary photons) entering the volume of interest are equal to
the number and energy of secondary electrons leaving the volume. Electron
equilibrium may be established by surrounding the dosimeter with an appropriate
thickness of material. For example, 5 mm of polystyrene is commonly used for
®Co irradiations and the absorbed dose in polystyrene is usually converted to
absorbed dose in water. If the absorbed dose is to be specified in silicon,
then an appropriate material thickness for surrounding the dosimeter would be
2.2 mm of aluminum.

3. Description of System

3.1 Co irradiation facilities. NBS employs three ©°Co gamma-ray irradiators
for its high-dose calibration service.

3.1.1 The pool-type source consists of a stationary annular array of twelve
source rods of about 5 kiloCurie (kCi) total activity. It is situated at the
bottom of a twelve-foot deep tank filled with water for shielding and personnel
protection. The source array is shown in Figure 1. Irradiation of dosimeters
is done manually by inserting into the source array a watertight stainless-
steel can with an inside diameter of 8 cm. The volume of uniform absorbed dose
rate is about 4 cm in diameter by 4 cm high, centered at 6.5 cm above the
inside bottom of the can on the central axis. The absorbed dose rate of this
source has been previously determined by an adiabatic graphite spherical
calorimeter and a graphite ionization chamber of identical dimensions to an
overall uncertainty estimated to be about +0.4% at a 99% confidence level [3].
Periodic verification checks of the absorbed dose rate is done annually by
means of reference transfer dosimeters in collaboration with the National
Physical Laboratory (NPL) of the U.K. The absorbed dose rate in water in May
1986 was 0.076 kGysminTM!. A complete discussion is given in reference [3] of
the sources of the uncertainties in the absorbed dose rate of this ®°Co
facility. The effect of these uncertainties on absorbed dose values assigned
to irradiations performed in this facility is discussed in Section 5.


2. Design Philosophy and Theory

2.1 There is a need for the standardization of high-absorbed-dose measurements
used in the radiation processing industry in order to provide measurement
assurance and traceability to national standards. Some areas of applications
of radiation processing include medical product sterilization, polymer
modification, hardness testing of electronic devices, and processing of food or
food packaging materials.

2.2 The principal sources of radiation employed in processing are ®°Co gamma
rays and electron beams from accelerators. Other photon sources used to a much
more limited extent are 1°7Cs gamma rays and high-energy bremsstrahlung x rays.
The absorbed-dose range of interest is from about 100 Gy (10 krad) to 50 kGy (5
Mrad), depending on the application. The upper limit of electron beam energy
is assumed to be about 10 MeV to avoid the possibility of nuclear activation.
The lower practical energy limit for electron beams ie determined by product
thickness and may be as low as 0.1 MeV for the curing of thin coatings.

2.3 The physical quantity of interest is absorbed dose in a material.
Absorbed dose is defined as energy absorbed per unit mass of a specific
material; the units are

1 gray (Gy) = 1 joule per kilogram (J kg +) = 100 rad.

Generally, the materials of interest in radiation processing are made up of low
atomic-number elements such as carbon, hydrogen, oxygen, and nitrogen. Such
materials are quite well approximated by water. Thus, water is usually used as
the reference material in which the absorbed dose measurement is specified. An
exception is for electronic device hardness testing in which the reference
material generally is silicon.

2.4 The primary standard employed in this service is a radiation-absorption
calorimeter. When a material is irradiated by a photon or electron source, the
energy absorbed results in a temperature increase. The absorbed dose, D, in
the material is given by

D = E/m = cAT

where E = energy absorbed in the material,
m = mass of the material,
c = specific heat of the material,
AT = temperature increase of the material.

The absorbed dose rate from a radiation source may be determined in a similar
manner by measuring the rate of temperature increase in an irradiated material
For a radionuclide source, the absorbed dose rate at some other time (different
from the actual measurement time) may be calculated by using the well-known
exponential decay rate (i.e., the half life).


£ XW | unitep states OE PARTMENT OF COMMERCE
: * | National Bureau of Standards
4 @ | Gaithersburg, Maryland 20899

"Sears ot

June 27, 1986
FEDERAL EXPRESS PRIORITY 1

Mr. John Doe

Senior Engineer

Gamma Radiations, Inc.
123 University Avenue
Chicago, IL 60606

Dear Mr. Doe:
Reference: Your P.O. No. 12345

Enclosed are the packaged dosimeters which you requested for irradiation in
your facility. There are fourteen packages: thirteen for irradiation plus
one control package which should not be irradiated. Please do not open the
packages at any time; they provide a controlled environment for the dosi-
meters. If the temperature of the packaged dosimeters is expected to

increase more than 10°C above room temperature during irradiation, please
provide information on the temperature as a function of time for the longest
irradiation period. Other irradiation data, such as date and time of irra-
diation, length of the irradiation period, and any other pertinent information
should be included with the returned dosimeters. The nominal absorbed doses
received by these dosimeters should be within the range of 1 kGy (0.1 Mrad)
and 30 kGy (3.0 Mrad). Please return the dosimeters to us as soon as the
irradiations are completed so that we may get the results back to you promptly.

If you have any questions, please call me or Jim Humphreys at (301)921-2201.
Sincerely,

Aint Mea ki

Dene G. Hocken

Calibrations Technician

X-Ray Physics Group

Center for Radiation Research

Enclosures

APPENDIX B
B-1


3% at a 95% confidence level for use of these dosimeters. The uncertainty
corresponding to one standard deviation would be 1.5%. Other dosimetry
methods (such as calorimetry) are being developed to improve the accuracy of
the determination of this absorbed dose rate in the future.

(2) The other uncertainty factor that is different from the pool source is
the timing of irradiations. There is a significant transit time when the
dosimeters are entering and leaving the radiation field that is not
accounted for by the irradiation timer. This transit time has been
determined to effectively add 4.1 seconds to the irradiation time set on the
automatic timer. This transit time was determined with an estimated
uncertainty of one second at a 95% confidence level, or 0.5s corresponding
to one standard deviation. To minimize the effect of the transit time
uncertainty, irradiation times of at least 10 minutes are used. Thus, the
uncertainty in irradiation time caused by this transit time uncertainty
would be 0.08% for a 10 minute irradiation

5.4 For irradiations done with the vertical beam source, the uncertainties in
absorbed dose values are given in Table 5. Some of the uncertainties have the
same values as for the pool source discussed in 5.2. Those that are different
are:

(1) Calibration of the exposure rate at a specific distance from the

source by a standard graphite cavity ionization chamber to an estimated
uncertainty of + 0.7% at a 95% confidence level [14]. Thus, the uncertainty
corresponding to one standard deviation would be 0.35%.

(2) Conversion of the exposure rate to absorbed dose rate in water
contributes an additional uncertainty of about 1.1% [15] due to factors such
as the uncertainties in the values for W (average energy required to produce
an ion pair in air), the displacement correction factor, the ratio of energy
transfer and energy absorption coefficients, and the ratio of absorbed dose
to kerma. The uncertainty value is for a 95% confidence level. The value
corresponding to one standard deviation would be 0.553.

(3) The uncertainty value given in Section 3.1.3 (41.3%) is obtained by
combining the uncertainties from (1) and (2) in quadrature.

(4) The accuracy of timing of irradiations is determined by the uncertainty
of the shutter transit time (~0.6s) and is estimated to be about 0.25 at a
95% confidence level. The value corresponding to one standard deviation
would be 0.1s. The minimum irradiation time that would generally be used

is about 1000s, so the uncertainty contribution would be about 0.018.

(5) The error in positioning the dosimeters at a location different from
that of the calibrating ion chamber was estimated to be 1.0 mm at a 95%
confidence level for a source distance of 1000 mm. The value corresponding
to one standard deviation would be 0.5 mm and would contribute a random
uncertainty of about 0.05%.

5.5 The uncertainties in absorbed dose values assigned to the results when NRS
supplies transfer standard dosimeters to a customer for irradiation in the
customer's irradiation facility are given in Table 6. The transfer dosimeters
are calibrated only in the pool source, so the combined uncertainties
associated with irradiations with that source are listed in the first line of
Table 6. Additional uncertainties (corresponding to one standard deviation)
associated with the response characteristics of the transfer dosimeters are
listed in the rest of Table 6. The total uncertainty is obtained in the same
manner as that discussed in 5.1.

23


Test 536/222222-86
NBS DB 100/001
XRG- 321

October 10, 1985
Page 5 of 5 pages

The attached page describes uncertainties and related factors in high-dose
calibrations, such as the one covered by this Report of Calibration. Any questions

regarding environmental conditions, different types'of materials used, and
systematic uncertainties associated with these calibrations can be answered by this
attachment. If there are further questions, they should be directed to the first

individual named below.

Measurements were made by

Neue G. Heoken '
D. G. Hocken, Calibrations Technician
(301)921-2201

Approved by
sunt

J. W. Motz, Leader
X-Ray Physics Group

For the Director

by
RS Casuel(

R. S. Caswell, Chief
Ionizing Radiation Division
Center for Radiation Research

APPENDIX C
c-5


The sources of the uncertainties are as follows:

(1) The calorimeter/ion chamber measurements of the pool source absorbed
dose rate were analyzed in reference [3] for all possible sources of error
contributions such as core mass, electrical power, attenuation, and heating
rate for the calorimeter, and air mass, current measurement, and collection
efficiency for the ion chamber. All factors were combined in quadrature
(square root of the sum of the squares) to give an overall value of 0.43% at
a 99% confidence level. The value for Table 3 would be 0.43% divided by 3
or 0.148%.

(2) The uncertainty in the half life of ®°Co was reported to be 5 parts out
of 52,714 or 0.01% [11]. The level of confidence was not reported, so it
was assumed to correspond to one standard deviation.

(3) The uncertainty in the timing of the irradiations was estimated to be a
maximum of 5s for the manual manipulation of the irradiation can into the
source array. This would correspond roughly to a 99% confidence level, so
dividing this time by 3 would give an uncertainty of 1.7s for one standard
deviation. For a 1 kGy absorbed dose at the current absorbed dose rate, the
irradiation time is about 750s: 1.7s is about 0.23% of this time.

(4) For the conversion of the absorbed dose rate of the source from one
material (e.g., graphite) to another (e.g., water), the uncertainty in the
ratio of mass energy absorption coefficients for photon spectra with
significant scattered components down to about 100 keV is estimated to be
about 0.6% at a 95% confidence level [12]. Thus, the uncertainty value for
one standard deviation would be 0.6% divided by 2 or 0.38.

(5) For the correction for attenuation in the equilibrium buildup material
surrounding the dosimeters, the uncertainty in the mass attenuation
coefficients down to 5 keV is about 2% at a 95% confidence level [12], or 13%
corresponding to one standard deviation. The correction factor is about 3%,
so the uncertainty in the factor would be about 0.03%.

(6) The effect of variation of the location of the dosimeter package
during irradiation can be estimated from isodose profiles that were
determined previously. With a location imprecision of about 0.5 cm, the
uncertainty is estimated to be 0.5% at a 95% confidence level. The
uncertainty value corresponding to one standard deviation would be 0.25%.
(7) The effects of different geometrical configurations for the equilibrium
material (i.e., a cylinder compared to a slab) has been examined previously
[13] and are estimated to contribute about a 3% uncertainty at a 99%
confidence level. The uncertainty corresponding to one standard deviation
would be 1%.

(8) The uncertainty due to the calculation of the correction for source
decay for a particular date can be estimated from the values in Table 1.
Corrections are made every two days, so the maximum uncertainty would be
about 0.07% (the difference between day 0 and day 2 values) at a 99%
confidence level. The uncertainty corresponding to one standard deviation

would be 0,028.

5.3 For irradiations done with the Gammacell 220 irradiator, the estimated
uncertainties for absorbed dose values are given in Table 4. Most of the
uncertainties are the same as for the pool source irradiations shown in Table 3
with some notable exceptions:
(1) The absorbed dose rate has been measured only with the NPL dichromate
reference transfer dosimeters [4, 5]. NPL gives an overall uncertainty of

22


Table

Table

Table

Table

Table

Table

LIST OF TABLES

®°Co Decay Corrections Factors (Cy)
Various Dosimeters Types

Uncertainties in Absorbed Dose Values
Pool Source

Uncertainties in Absorbed Dose Values
Gammacell 220... .......

Uncertainties in Absorbed Dose Values
Vertical Beam Source .

Uncertainties in Absorbed Dose Values
Standard Dosimeters . .

for Irradiations with
for Irradiations with
for Irradiations with

Employing Transfer

Page

13

17

21

24

25

26


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13


References

1. Uriano, G.A. Garner, E.L. Kirby, R.K., and Reed, W.P., Editors, NBS
Calibration Services Users Guide 1986-1988 Edition, NBS Special Publication 250
and Fee Schedule for SP 250, Office of Physical Mcasurcment Services, National
Bureau of Standards, Gaithersburg, MD 20899 (1986)

2. Greening, J. R., Fundamentals of Radiation Dosimetry, Chapters 5 and 6,
Medical Physics Handbook 6, Adams Hilger Ltd., Bristol, U.K. (1981).

3. Petree, B., and Lamperti, P., “A Comparison of Absorbed Dose Determinations
in Graphite by Cavity Ionization Measurements and by Calorimetry", J. Res.
Natl. Bur. Stds., 71C, No.1, pp. 19-27 (1967).

4. Sharpe, P.H.G., "Dichromate Dosemeter Reference Service", National Physical
Laboratory, U.K. (1984).

5. Sharpe, P.H.G., Barrett, J.H., and Berkley, A.M., "Acidic Aqueous
Dichromate Solutions as Reference Dosimeters in the 10-40 kGy Range", Int. J.
Appl. Radiat. Isot., 36, pp. 647-652 (1985).

6. McLaughlin, W. L., Jarrett, R. D., Sr., and Olejnik, T. A., "Dosimetry",
Chapter 8 in Preservation of Food by Ionizing Radiation, Vol. I,

Josephson, E. S., and Peterson, M. S., Eds., CRC Press, Boca Raton, FL, pp.
189-245 (1982).

7. Levine, H., McLaughlin, W. L., and Miller, A., "Temperature and Humidity
Effects on the Gamma-Ray Response and Stability of Plastic and Dyed Plastic
Dosimeters", Radiat. Phys. Chem., 14, pp. 551-574 (1979).

8. Kaarls, R., "Report of the BIPM Working Group on the Statement of
Uncertainties", lst meeting, 21-23 Oct. 1980.

9, Giacomo, P., "News From the BIPM", Metrologia, 17, pp. 66-74 (1981).

10. Ku, H.H., "Notes on the Use: of Propagation of Error Formulas", Precision
Measurement and Calibration, NBS Special Publication 300-Vol.1, pp. 331-341
(1969).

11. NCRP Report No. 58, 2nd Ed., "A Handbook of Radioactivity Measurements

Procedures", National Council on Radiation Protection and Measurements,
Bethesda, MD (1985).

12. Hubbell, J.H., "Photon Mass Attenuation and Energy-absorption Coefficients
from 1 keV to 20 MeV", Int. J. Appl. Radiat. Isot., 33, pp. 1269-1290 (1982).

30


13. McLaughlin, W.L., "A National Standardization Programme for High-Dose
Measurements", High-Dose Measurements in Industrial Radiation Processing, IAEA
Tech. Report No. 205, IAEA, Vienna, pp. 17-32 (1981).

14. Loftus, T.P., and Weaver, J.T., "Standardization of ®°Co and 13’Cs Gamma-
Ray Beams in Terms of Exposure", J. Res. Natl. Bur. Stds., 78A, No.4, pp. 465-
476 (1974).

15. ICRU Report 14 "Radiation Dosimetry: X Rays and Gamma Rays with Maximum
Photon Energies Between 0.6 and 50 MeV", Int. Com. on Radiat. Units and
Measurements, Bethesda, MD (1969).

16. Belanger, B. Measurement Assurance Programs Part I: General Introduction,
and Croarkin, C., Part II: Development _and Implementation, National Bureau of
Standards Special Publication 6761 and 67611 (1984).

17. Natrella, M.G., Experimental Statistics, NBS Handbook 91 (1963).

31


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19


Library of Congress Catalog Card Number: 88-600506

National Bureau of Standards Special Publication 250-11
Natl. Bur. Stand. (U.S.), Spec. Publ. 250-11, 49 pages (Mar. 1988)
CODEN: XNBSAV

Certain commercial equipment, instruments, or materials are identified in this paper in order to ade-
quately specify the experimental procedure. Such identification does not imply recommendation or
endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment
identified are necessarily the best available for the purpose.

U.S, GOVERNMENT PRINTING OFFICE
WASHINGTON: 1988

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


Test 536/222222-86
NBS DB 100/001
XRG- 321

October 10, 1985
Page 3 of 5 payes

Date,
Dosimeter Time

No. Thickness

of A A. BA/mm
(605 an}{s10 nm) Reading (605 nm) {510 fm) __ (mm) (605 nm)(510 nm)

721 0.1645 0.0768 9/9/85 1.6103 0.2525 0.052 27.80 3.379
722 0.1629 0.0787 15:10 1.5869 0.2474 0.050 28.48 3.374
723 0.1506 0.0599 . 1.6251 0.2367 0.051 28.91 3.467
724 0.1449 0.0596 . 1.6124 0.2376 0.050 29.35 3.560
725 0.1551 0.0603 . 1.6466 0.2411 0.051 29.24 3.545

726 0.1560 0.0640 9/9/85 1.5462 0.2343 0.052 26.73 3.275
727 0.1521 0.0660 15:15 1.5674 0.2401 0.052 27.22 = 3.348
728 0.1456 0.0628 . 1.5400 0.2312 0.050 27.89 3.368

729 0.1487 0.0619 . 1.6037 0.2309 0.051 28.53 3.314
730 0.1456 0.0613 “ 1.5786 0.2339 0.051 28.10 3.384

731 0.1531 0.0603 9/9/85 0.7674 0.1338 0.051 12.04 1.441
732 0.1536 0.0627 15:20 0.8066 0.1405 0.052 12.56 1.496
733 0.1467 0.0622 . 0.7493 0.1348 0.050 12.05 1.452
734 0.1533 0.0626 . 0.7802 0.1384 0.051 12.29 1.486
735 0.1526 0.0607 . 0.7690 0.1344 0.050 12.33 1.474

736 0.1512 0.0620 9/9/85 1.8718 0.2748 0.051 33.74 4.173
737 0.1516 0.0728 15:30 1.7489 0.2688 0.051 31.32 3.843
738 0.1477 0.0592 . 1.8657 0.2635 0.051 33.69 4.006
739 0.1508 0.0671 . 1.8354 0.2701 0.050 33.69 4.060
740 0.1469 0.0599 . 1.7632 0.2608 0.049 32.99 4.100

741 0.1614 0.0790 9/9/85 1.4965 0.2416 0.052 25.68 3.127
742 0.1516 0.0666 15:40 1.5044 0.2305 0.052 26.02 3.152
743 0.1581 0.0620 “ 1.5585 0.2317 0.051 27.46 93.327
744 0.1471 0.0608 . 1.5296 0.2278 0.050 27.65 3.340
745 0.1464 0.0602 . 1.5344 0.2273 0.051 27.22 3.276

746 0.1501 0.0629 9/9/85 1.6048 0.2418 0.051 28.52 3.508
747 0.1518 0.0604 15:45 1.6099 0.2355 0.050 29.16 3.502
748 0.1538 0.0644 . 1.6830 0.2481 0.052 29.41 3.533
749 0.1488 0.0680 . 1.5938 0.2435 0.052 27.79 = 3.375
750 0.1467 0.0642 . 1.6026 0.2380 0.051 28.55 3.408

APPENDIX C
C-3


U.8. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MARYLAND 20899 Test 536/222222-86

NBS DB 100/001
XRG-321
October 10, 1985

REPORT OF CALIBRATION

of
Gammacel1 220 Irradiator

for

Gammaray, Inc.
999 Cobalt Street
Miami, FL 32111

Attention: James Doe
Ref. P.O. Now: 32123

Transfer dosimetry packets, consisting of. five calibrated FWT-60-00 radiochromic
dosimeters (NBS Batch 144), held between 5,0-mm thick blocks of polystyrene and
sealed inside two thin polyethylene pouches (R.H. = 43% at time of sealing), were
supplied to Gammaray for irradiation in their Gammacel] 220 irradiator facility.
Upon their return to NBS,.the dosimeters were analyzed on 9 September 1985, at two
wavelengths (605 nm and 510 nm) using a Cary Model 219 spectrophotometer (3.5 am
S.B.W.). Absorbed dose interpretations were made from a calibration of this batch
of radiochromic dosimeters performed in April 1985. The results are summarized in
the following table:

NBS Absorbed
Dosimeter Dose in H,0, kGy*

No. (605 nm
696-700 (Controls) 0
701-705 18.55
706-710 9.00
711-715 16.25
716-720 11.40
721-725 19.70
726-730 18.90
731-735 7.50
736-740 23.20
741-745 18.30
746-750 19.60

“1 key = 0.1 Mrad APPENDIX C

C-1


;COVER — LOCKS SECURELY IN PLACE
— EASILY REMOVED

ENTIRE IRRADIATION CHAMBER
ACCESSIBLE

ACCESS OLE 1018
with ig 12 THe
aT Tor END DNUY

HINGED LEAD COLLAR
SWINGS CLEAR OF IRRADIATION
CHAMBER

COBALT 69
SOURCE ASSEMBLY

FRONT VIEW SHOWING DRAWER UP

Figure 2 - Schematic drawing of Gammacel] 220 self-shielded 6%
irradiator in the chamber load position

6


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NBS MEASUREMENT SERVICES:
DOSIMETRY FOR HIGH DOSE
APPLICATIONS

Jimmy C. Humphreys
Dene Hocken

William L. McLaughlin

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

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued March 1988


check all calibration reports. The condition of each group of test dosimeters
to be irradiated is evaluated when they are received to ensure they are of high
quality. Any unusual condition or appearance is checked by consulting with the
requesting customer before proceeding. Checks of the performance of the
various analytical instruments used are done on a periodic basis as described
in Section 3. Plans are underway to further automate the data handling, such
as automatic timing of the irradiation calibration.

7. Safety

7.1 Operation of these calibration services involves use of high-intensity
ionizing radiation sources that have the potential to expose staff personnel to
hazardous levels of radiation. NBS health physics rules and Nuclear Regulatory
Commission regulations require that personnel using these sources have formal
training in the correct operation and safe use of them. In some cases, tests
are given after the training to verify that the personnel have understood the
most important aspects of that training.

7.2 Access to all high-intensity source areas is limited to those staff with a

need to use them. Doors to these areas are kept locked at all times with
control maintained over the minimum number of keys available to the areas.

29


Table 2

VARIOUS DOSIMETER TYPES

Dosimeter Type Absorbed Dose Range
kGy in H,0

FWT-60-00 dye film* 1-50

Harwell Red Perspex® 5-50

Harwell Amber Perspex? 1-20

Harwell HX Perspex and Radix
clear PMMA” 3-100

Ceric-cerous sulfate solutions 0.3-100

Potassium and silver dichromate

solutions 1-50
Fricke (ferrous sulfate) solution 0.01-0.4
Thermoluminescence dosimeters 0.01-1

(LiF, CaF,, etc.)

FWI Opti-chromic 0.02-10
(liquid in optical waveguide)

®The material of this film is primarily "Elvamide"

nylon from DuPont. The radiochromic sensor is hexa-hydroxyethyl
pararosaniline cyanide.

>"Perspex" and "Radix" are trade names for polymethyl methacrylate
(PMMA) .

7


Center for Radiation Research

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

Bureau of Standards,

The Center provides the Nation with standards and measurement services for ionizing

radiation and for ultraviolet, visible, and infrared radiation; coordinates and furnishes essential support
to the National Measurement Support System-for ionizing radiation; conducts research in radiation related
fields to develop improved radiation measurement methodology; and generates, compiles, and critically

evaluates data to meet major national needs.

Atomic and Plasma Radiation Division

The Center consists of five Divisions and one Group.

Carries out basic theoretical and experimental research into the
spectroscopic and radiative properties of atoms and highly ionized
species; develops well-defined atomic radiation sources as radiometric
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

Atomic Spectroscopy
Atomic Radiation Data
Plasma Radiation

Provides the central national basis for the measurement of far ultra-
violet, soft x-ray, and electron radiation; develops and disseminates
radiation standards, measurement services, and data for for these radia-
tions; conducts theoretical and experimental research with electron,
laser, ultraviolet, and soft x-ray radiation for measurement applica
tions; determines fundamental mechanisms of electron and photon inter-
actions with matter; and develops advanced electron- and photon-based
measurement techiques. The Division consists of the following Groups:

Radiometric Physics Division

Far UV Physics
Electron Physics
Photon Physics

Provides national measurement standards and support services for ultra-

violet, visible, and infrared radiation; provides standards dissemination

and measurement quality assurance services; conducts research in optical
radiation, pyrometry, photometry, and quantum radiometry; and develops
spectroradiometric and spectrophotometric standards and calibration
procedures. The Division consists of the following Groups:

Radiation Source and Instrumentation Division

Spectral Radiometry
Spectrophotometry
Radiometric Measurement Services

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 instrumentation. The
Division consists of the following Groups:

lonizing Radiation Division

Accelerator Research
Linac Operations
Electronic Instrumentation
Mechanical Instrumentation

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:

Nuclear Physics Group

Office of Radiation Measurement

Radiation Theory

Radiation Chemistry and Chemical
Dosimetry

Neutron Measurements and Research

Neutron Dosimetry

Radioactivity

X-Ray Physics

Dosimetry

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.


UNCERTAINTIES AND RELATED FACTORS IN HIGH-DOSE CALIBRATIONS

Standard High-Dose Gamma-Ray Irradiations at NBS
(Estimated uncertainty: + 3.3% at a 99% confidence level)

The high-dose calibrations at NBS (schedule 49010S) involve the administration
of ®°Co gamma radiation absorbed doses in certain materials (e.g., water,
carbon, aluminum, silicon, polystyrene, etc.), under environmentally controlled
conditions. The dose values are based on standard graphite calorimetry
measurements, which are corrected by certain modifying factors (the estimated
photon spectrum, tabulated weighing factors, such as radiation energy
absorption and attenuation factors, and source decay factors). The
irradiations are usually carried out using the NBS ®%Co gamma-ray pool source.
The uncertainty values cited above are for the absorbed doses given typical
thin dosimeters (5 or less irradiated at a time) held under layers
approximating electron equilibrium in a given material, in the absorbed dose
region of 1 KGy and upward.

Absorbed Dose Evaluations Based on Traceability through
Mailed Chemical Transter Dosimeters
(Estimated uncertainty: + 5.2% at a 99% confidence level)

It must be recognized by calibration users that routinely applied chemical
dosimeters consisting of liquid solutions, solid films, ceramics, crystalline
materials, plastics, etc., are subject to imprecision and systematic error due
to environmental effects during field use, production, irradiation, and
readout. Therefore, if these factors are not sufficiently known and
understood, absorbed dose interpretations using such systems have
reproducibility and accuracy values that are difficult to establish. Although,
as indicated above, NBS in its high-dose calibrations (e.g. schedule 49010S)
administers to within prescribed uncertainty limits standard absorbed doses of
gamma radiation under controlled environmental conditions, the routine
dosimeters for field use are subject to uneven quality control of manufacture
and are occasionally susceptible to anomalous batch-to-batch or intrabatch
sensitivity differences, poor reproducibility of readout results, and adverse
environmental effects.

In the case of absorbed dose evaluation based on mailed transfer NBS-
controlled radiochromic film dosimeters (nominal dose range 1 x 10° to 5 x 104
Gy) that are traceable to ®°Co gamma-ray calibrations at NBS, the uncertainty
values cited above (+ 5.2%) may be assumed as long as suitable care against
environmental and other sources of systematic error is exercised.

A detailed list of the various sources of uncertainty that make up the overall
uncertainties given above may be obtained by requesting such information from
NBS.

Appendix C

C-6


Table 5

UNCERTAINTIES IN ABSORBED DOSE VALUES

FOR _IRRADIATIONS WITH VERTICAL BEAM SOURCE

Type A (%) Type B (%)
Source of Uncertaint {xandom) —(other)
Graphite ion chamber 0.35
measurement of exposure rate
Conversion of exposure rate 0.55
to absorbed dose rate in water
Source decay, half life 0.01
Timing of irradiations: 0.01
shutter transit time
Conversion of dose rate: 0.30
water to silicon, etc.

Attenuation corrections in 0.03
equilibrium material

Effect of dosimeter positioning: 0.05

distance from source

Source decay correction 0.02

for given date

Type A, Type B combined 0.05 0.72
(in quadrature), separately

Combined (in quadrature) 0.72

TOTAL: 3X combined 2.2

25


UNCERTAINTIES AND RELATED FACTORS IN HIGH-DOSE CALIBRATIONS

Standard High-Dose Gamma-Ray Irradiations at NBS
(Estimated uncertainty: + 3.3% at a 99% confidence level)

The high-dose calibrations at NBS (schedule 490108) involve the
administration of ®°Co gamma radiation absorbed doses in certain materials
(e.g., water, carbon, aluminum, silicon, polystyrene, etc.), under
environmentally controlled conditions. The dose values are based on
standard graphite calorimetry measurements, which are corrected by certain
modifying factors (the estimated photon spectrum, tabulated weighing
factors, such as radiation energy absorption and attenuation factors, and
source decay factors). The irradiations are usually carried out using the
NBS calibrated ®°Co water-well type irradiator. The uncertainties cited
above are pertinent to absorbed values specified in a given material, with
the dosimeters irradiated under approximate equilibrium conditions, in the
absorbed dose region of 1 KGy and upward.

Absorbed Dose Evaluations Based on Traceability through
Mailed Chemical Transfer Dosimeters

(Estimated uncertainty: + 5.2% at a 99% confidence level)

It must be recognized by calibration users that routinely applied chemical
dosimeters consisting of liquid solutions, solid films, ceramics,
crystalline materials, plastics, etc., are subject to imprecision and
systematic error due to environmental effects during field use, production,
irradiation, and readout. Therefore, if these factors are not sufficiently
known and understood, absorbed dose interpretations using such systems have
reproducibility and accuracy values that are difficult to establish.
Although, as indicated above, NBS in its high-dose calibrations (e.g.
schedule 49010S) administers to within prescribed uncertainty limits
standard absorbed doses of gamma radiation under controlled environmental
conditions, the routine dosimeters for field use are subject to uneven
quality control of manufacture and are occasionally susceptible to
anomalous batch-to-batch or intrabatch sensitivity differences, poor
reproducibility of readout results, and adverse environmental effects.

In the case of absorbed dose evaluation based on mailed transfer NBS-
controlled radiochromic film dosimeters (nominal dose range 1 x 10° to
5 x 104 Gy) that are traceable to ®°Co gamma-ray calibrations at NBS, the
uncertainty values cited above (+ 5.2%) may be assumed as long as suitable

care against environmental and other sources of systematic error is
exercised.

A detailed. list of the various sources of uncertainty that make up the

overall uncertainties given above may be obtained by requesting such
information from NBS.

Appendix A

A-3


dye film dosimeters, the procedures are identical to those discussed in 4.1.

For other types.of dosimeters, the spectrophotometric analysis is tailored to
meet their particular requirements. For example, Red Perspex dosimeters are

read at only one wavelength (640 nm) and require a different holder than the

FWT-type film.

5. Assessment of Uncertainties

5.1 The estimates of uncertainties for the absorbed dose values assigned to a
given group of dosimeters irradiated together in the ®°Co pool source are given
in Table 3. The designation of the types of uncertainties (A and B) follows
the recommendations of the BIPM Working Group on the Statement of Uncertainties
[8, 9]. Type A is the random component and is stated as an estimate of a
sample standard deviation. These can be determined by standard (objective)
statistical methods. Type B uncertainties are determined by “other methods",
thus implying some element of subjective evaluation. Type B uncertainties may,
in some cases, correspond roughly to "systematic" contributions, but this may
not always be true. In line with the BIPM recommendations, the term
"systematic" is avoided. Those recommendations also treat both types of
uncertainties equally; that is, as being equivalent to "standard deviations".
Thus, each type is combined separately in quadrature (i.e., the square root of
the sum of the squares). An overall combined uncertainty is obtained by
combining the two types once again in quadrature. Finally, a total uncertainty
is obtained by multiplying a "safety" factor (such as 2 or 3); in this case the
factor chosen was 3. For a normal population distribution this factor would
correspond approximately to the statistical "Student's t-factor" at a 99%
confidence level and a sample size of about 14 (13 degrees of freedom),

assuming that the combined uncertainties can actually be treated as a standard
deviation.

5.2 The values of the uncertainties given in Table 3 for irradiations done
with the pool source were determined from published literature or from
experimental estimates. The published values usually are given for a 95% or
99% confidence level, so the values given in Table 3 are obtained by dividing
the published values by two or three respectively so as to correspond to a
"standard deviation" of one. The experimental estimates are usually the
"maximum" expected range which would correspond roughly to a 99% confidence
level. Thus, values for Table 3 were obtained by dividing these estimates by
three. The values of uncertainties listed are stated in terms of percentages
of the sample mean of the mesured quantity. This corresponds to a percentage
of an estimated coefficient of variation [10] given by:

100 100, s,
x A
where oz = estimated standard deviation of the sample mean,
x = mean of the sample size n,
and Sy = computed sample standard deviation

20


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Figure 3 - Schematic Drawing of Gammacell 220 irradiator in the
irradiate position