NBSIR 76-1023

Direct Comparisons of the NBS

Absorbed Dose Calorimeters

Irradiated with 20 and 50 MeV

Electrons

S. R. Domen and P. J. Lamperti

Center for Radiation Research

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

T. Mikado

Electrotechnical Laboratory

Tanashi

Tokyo, Japan

July 1976

Final

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 76-1023

DIRECT COMPARISONS OF THE NBS

ABSORBED DOSE CALORIMETERS

IRRADIATED WITH 20 AND 50 MeV

ELECTRONS

S. R. Domen and P. J. Lamperti

Center for Radiation Research

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

T. Mikado

Electrotechnical Laboratory

Tanashi

Tokyo, Japan

July 1976

Final

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

DIRECT COMPARISONS OF THE NBS ABSORBED DOSE CALORIMETERS

IRRADIATED WITH 20 AND 50 MeV ELECTRONS

Steve R. Domen, Tomohlsa Mikado, and Paul J. Lamperti

Dosimetry Section

Center for Radiation Research

National Bureau of Standards

Washington D.C. 20234

ABSTRACT

Two NBS absorbed dose calorimeters were compared in 20 and 50 MeV

electron beams that were scattered with lead foils of various thickness.

The internal structures of the calorimeters are identical. The nonportable model is enclosed in a 40 cm x 40 cm x 30 cm thick graphite

medium while a 30 cm diameter x 15 cm thick graphite medium was used to

enclose the portable model. Measured results indicate that the three

internal bodies of the calorimeters and their measuring circuits were

constructed with sufficient care to produce essentially identical

calorimetric responses to about 0.1%.

1
DIRECT COMPARISONS OF THE NBS ABSORBED DOSE CALORIMETERS

IRRADIATED WITH 20 AND 50 MeV ELECTRONS

t * t

Steve R. Domen , Tomohisa Mikado , and Paul J. Lamperti

I. Introduction

Two graphite calorimeters [1], one non-portable and one portable,

were constructed at the NBS for use as standards for the measurement of

absorbed dose produced by radiation beams. Calorimeters provide the

most direct and fundamental means for measuring absorbed dose, that is,

measurement of a heating effect produced in a small volume element of

known mass. Therefore, they are the most accurate devices for measurement of absorbed dose, particularly when it is produced as a result of a

complex and unknown radiation spectrum at the position of measurement.

The absolute accuracy of these instruments is a primary concern. In

general, it would appear possible to achieve good accuracy by careful

design and construction. However, there would always be a possibility

of some unknown constructional defects contributing to significant and

measureable systematic errors. Therefore, a calorimeter cannot be

considered a trustworthy standard until it has been compared directly or

indirectly with at least one other instrument or method of measuring

absorbed dose. Direct comparisons of calorimeters of different design

and measurement procedure would provide a critical and valuable test of

these instruments.

National Bureau of Standards, Washington, D.C., 20234, U.S.A.

*Electrotechnical Laboratory, Tanashi, Tokyo, Japan.

2
Direct intercomparison of the NBS calorimeters with others has not

been achieved. However, they have been compared indirectly with other

calorimeters using ionization chambers as intermediary Instruments and

with theoretical calculations. Measurements of the quotient of absorbed

dose to specific charge in air were made with the non-portable calorimeter. The results are in agreement (to within the limits of the uncertainties) with the experimental and theoretical results of other investigators [2-4]. The portable calorimeter was used to measure the absorbed

dose in a graphite phantom and these results correspond well with the

absorbed dose calculated from exposure measurements made with a cavity

ionization chamber [5]. The portable calorimeter was also used to

measure the product of the average energy expended per ion pair (in air)

and the ratio of the average mass stopping power for carbon to that for

air. A preliminary analysis shows that the results are in agreement, to

within the limits of the uncertainties, with the results of other investigators [6].

Although the above results indicate that the NBS calorimeters are

accurate, there is merit for a direct intercomparison of the calorimeters

even though their operational and internal structures are identical.

Their direct comparison would, therefore, be a sensitive test to determine if some unknown differences in constructional details are large

enough to produce significant differences in their responses. This

report describes high precision measurements which indicate that their

responses are identical to about 0.1%.

3
II. Experimental Procedure

Figures 1 and 2 show the experimental setup which is the same as

that described in reference 2 (except for the portable calorimeter). An

aluminum vacuum window (not shown in fig. 1), a lead foil scatterer, and

air all scatter an electron beam which is monitored by two offset

ionization ~ chambers that remained at a fixed position. The uncollimated

beam passed through one of ten graphite absorbers mounted on a large

wheel, and then through one of three detectors whose central planes were

in a common plane indicated by A. The distance from the scatterer to

plane A remained at a fixed distance of 190 cm. The three detectors

were a parallel plate ionization chamber (PP) , the non-portable calorimeter (NPC) , and the portable calorimeter (PC) . Details of the detectors

and their alignment are shown in fig. 2. The detectors were mounted on

a table that was moveable perpendicular to the beam so that the axis of

each detector could be made to coincide with the beam axis. All motions

were remotely controlled. The lead barriers, (a) and (b) , remained at a

fixed position.

In order to minimize the small effects of possible beam instabilities

on the precision of the measurements, the ion chamber and the two calorimeters were moved into the beam in rapid succession. A set of measurements began with a comparison of the parallel-plate ionization chamber to

the monitor response (PP/M) , while heating of the two calorimeters by

scattered radiation was avoided by shielding them with the lead barriers

(a) and (b) . Immediately following the ionization chamber measurements,

the non-portable calorimeter was placed on the beam axis. A measurement

of this calorimeter response to the monitor (NPC/M) was made. As soon as

A
this was completed, the portable calorimeter was moved from behind

lead barrier (a) and was placed on the beam axis for a measurement of

its response to that of the monitor (PC/M) . To complete the series,

another measurement of the ion chamber response to that of the monitor

(PP/M) was made. In the meantime (as soon as the PC/M measurement was

completed), a small amount of air, at a pressure of < 0.13 kPa (< 1 mm Hg)

,

was injected into the common vacuum system of both calorimeters so that

temperature recycling could take place during the final series of PP/M

measurements. The difference between the PP/M ratios before and after

the calorimeter runs was of the order of a few tenths of a percent,

indicating good beam stability during the important short interval of

time between the two calorimeter comparisons. The time between the two

sets of PP/M ratios was about 20 minutes.

A Wheatstone bridge circuit was used to determine the response of

each calorimeter, expressed as a fractional resistance change (AR/R) of a

balancing arm which is the same as the fractional resistance change of a

thermistor embedded in a calorimetric core. The ratio of the fractional

resistance change per unit monitor response of the NPC to that of the PC

is

This ratio will indicate a difference from unity if the sensitivities

of the embedded thermistors are different. To cancel out the effect of

any difference in their sensitivities, the radiation runs must be related

to electrical calibration runs so that the response of each calorimeter

(1)

5
can be expressed in terms of absorbed dose per unit monitor response.

Hence, the proper expression (X) to be used in comparing the response of

the non-portable calorimeter to that of the portable calorimeter is:

where the first expression is derived from electrical calibration runs in

which (E/m)^^ is the electrical energy divided by the known mass of the

core in the non-portable calorimeter, and (E/m)p^ is the electrical energy

divided by the known mass of the core in the portable calorimeter. An

inspection of the above expression shows that differences in thermistor

sensitivities are cancelled.

6
III. Graphite Media Enclosing Detectors

The parallel plate ionization chamber and non-portable calorimeter

were enclosed by graphite media of identical dimensions, 40 cm x 40 cm x

30 cm thick. These differed from the graphite medium enclosing the portable calorimeter which was 30 cm in diameter and 15 cm thick.

Indications are that the difference in thickness of the surrounding

media produced a negligible effect (< 0.01%). This conclusion was derived

from comparisons of depth dose measurements in graphite irradiated with

electrons and cobalt-60 gamma rays. To illustrate, refer to fig. 3 which

shows two normalized dose curves. The curve for 20 MeV initial energy

electrons was determined with the setup shown in fig. 1. The parallel

plate ionization chamber remained at a fixed distance of 190 cm while

graphite absorbers of various thicknesses were positioned in the beam in

front of the chamber. The curve for cobalt-60 was also determined with

the detector at a fixed distance (1 m) from the source while absorbers of

various thicknesses were positioned in the beam in front of the detector

embedded in the 30 cm diameter medium. The detectors for the cobalt-60

dose measurements were the same parallel plate ionization chamber as

used in the electron beam measurements and a small spherical graphite

ionization chamber. Both chambers produced the same normalized dose

2 results. The position A' in fig. 3 is at 1.5 g/cm , the minimum thickness

in front of the spherical ionization chamber for the cobalt-60 measurements. Position A indicates the depth of the detectors during the 20 MeV

electron beam measurements. The position B (in figs. 1 and 3) corresponds

to the back surface plane of the portable calorimeter (at a depth of

2 17.3 g/cm ) where the cobalt-60 dose rate fell to 60% from the peak near

7
the surface. The cobalt-60 measurements showed that when an additional

2 thickness of graphite (BC, 4.3 g/cm ) was placed against the back surface

(plane B) , the response of the ionization chamber at A' (which corresponds

to the response of the calorimetric core at A') increased about 0.1%. An

additional increase of the same thickness (CD) showed no further change.

Hence, a comparison of the two dose curves show that an increase in thickness beyond that depth during the 20 MeV electron beam measurements would

show much less of a back-scattering effect. Position B is beyond the

2 range of the 20 MeV electron (11.3 g/cm ) and into the bremsstrahlung tail

where the dose rate was found to be 1.8% of the peak, lower by a factor of

30 from the dose rate produced by cobalt-60 at position B. Therefore,

during the 20 MeV runs, an increase of plate thicknesses against the rear

surface (B) of the calorimeter would have increased the response of the

detector at A by approximately 1/30 of 0.1% or < 0.01%. The effect would

have been even further attenuated since the back surface of the medium

2 was at D, a depth of 26.8 g/cm for the 20 MeV measurements. At 50 MeV,

2 the rear surface was at D', a depth of 31.3 g/cm , beyond the electron

2 range of 28.3 g/cm .

The differences in the lateral dimensions of the media appear to be

of greater concern than the differences of thickness described above. An

estimate of their relative importance can be obtained by considering the

dose rates at the boundaries of the medium (enclosing the portable calorimeter) compared to the dose rates at the calorimeter core. The radius

and thickness of the graphite medium enclosing the portable calorimeter

are both 15 cm, and it has been shown above that the thickness of 15 cm,

2 25 g/cm , where the dose rate for 20 MeV initial electrons was less than

8
2% of that occurring at the core, was essentially an infinite thickness.

At the radial boundary, at the distance of 15 cm from the core and in the

vicinity of plane A, the minimum dose rate was estimated to be at least

50% of that at the core. This radius is still of infinite width even for

50 MeV electrons scattered at that or at any greater radii, since its

distance from the core exceeds the range of those scattered electrons.

However, that radius is not infinite in width for bremsstrahlung generated

at radii greater than 15 cm, and at small depths into the medium. This

again may be a small effect, but it must be considered. Another thing

t

I that must be taken into consideration is that bremsstrahlung production is

more efficient at the higher energies than at the lower energies. Therefore, if the additional width of material around the non-portable calorimeter had any effect, it would have contributed more to the response of the

calorimeter at higher beam energies than at lower beam energies. Increasing the degree of scattering in this additional material, by interposition

of greater thicknesses of lead foil, will produce an increased NPC/PC

response that may be detectable. These considerations led to the following bases under which the irradiation measurements were made: a 0.13 mm

and a 0.38 mm Pb foil were used for scattering 20 MeV electrons, and a

0.25 mm and a 0.76 mm foil were used for scattering 50 MeV electrons,

thus tripling the mean-square scattering angle at each energy. The depth

at which the measurements were made were just before the peak of the dose

2 2 curve, 2 g/cm for 20 MeV electrons and 6 g/cm for 50 MeV electrons.

9
IV. Results

Table I shows the results of the irradiation runs. The irradiation

conditions are shown in columns 2 and 3, and the measured ratios are shown

in column 5. Columns 6 lists the standard deviation, and the standard

deviation of the mean, respectively, of the number of runs shown in column 4. About five runs at each irradiation condition were sufficient to

produce good precision of measurement, from 0.04% to 0.25% standard deviation of the mean. This good precision of measurement was the result of a

number of conditions, mainly the good stability of the linac beam, the

short interval of time between the two calorimeter comparisons, a large

delivered dose ("^-1700 rad), and the high sensitivity of the calorimeters.

Columns 7 and 8 list two small corrections. The front wall correction

was determined from the depth dose curves at 20 and 50 MeV to correct for

2 a small difference (36 mg/cm ) in the front wall thicknesses of the two

calorimeters. The detector positioning correction was a result of the

central planes of each calorimetric core being slightly displaced from

each other by 0.18 mm out of a scatterer to detector distance of 190 cm

and applying an inverse square correction. The corrected measured ratios

are listed in the last column, derived by multiplying columns 5, 7, and 8.

Table II shows the results of the electrical calibration of the

calorimeters. Column 3 lists the measured values. The large number of

runs shown in column 4 were made in preparation for efficient and rapid

operation of the calorimeters during the irradiation measurements. The

statistical fluctuations of the measurements as shown in the last two

columns indicate a high degree of measurement precision. The measurement

of electrical calibrating power in each calorimetric core required the

10
measurement of two potentials: across an accurately known fixed resistor

in series with the core heater, and across the heater. It can be shown

that an accurate determination of expression (2) can be made without knowing the absolute accuracy of the potential measuring instruments, but

depends on an accurate comparison of those instruments in measuring the

same potential sources and making a correction for any difference. This

was done and a small correction was applied, 0.12% ± 0.04% (standard

deviation), as a result of 21 comparisons.

Table III shows the desired ratios of the calorimetric responses.

The initial beam energies are listed in column 1 and the lead beam

scatterers are listed in column 2. The ratios of the calorimetric

responses, NPC/PC, were derived from expression (2) into which was substituted the appropriate values from tables I and II. The uncertainties were

derived by combining in quadrature appropriate uncertainties listed in the,

first two tables while ignoring other insignificant uncertainties.

The results show that the responses of the calorimeters are

essentially equal. The average of the results at 20 MeV shows no significant change when the beam scattering is increased by increasing the

scatterer thickness from 0.13 mm to 0.38 mm. Those results are essentially the same (within 0.2%) as that measured with the narrower beam at

50 MeV, with the 0.25 mm scatterer. These three irradiation conditions

produced average values of NPC/PC that are less than unity, which is opposite to what would be expected if bremsstrahlung production in the additional graphite material around the NPC was scattered to cause significant

heating in its core. The data at 50 MeV, however, suggests that with the

broader beam produced with the thicker scatterer the increase in
bremsstrahlung generated in the additional graphite at radii greater

than 15 cm may have contributed to that average being greater than unity

(by '^0.2%). If this last ratio is excluded, which would be proper if the

additional graphite produced a significant effect, then the average of the

first three differs from unity by 0.11%. That is within the experimental

uncertainties shown in table III. If all four ratios are averaged, then

the difference from unity is only 0.03%.

In conclusion, the results of these comparisons seem to indicate

that the constructional details of the bodies of the two calorimeters, and

their measuring circuits, were fabricated with sufficient care to produce

essentially equal responses of those instruments.

V. Acknowledgment

This work was supported primarily by the National Cancer Institute,

National Institutes of Health, Bethesda, Maryland. Appreciation is also

acknowledged to the Japanese Atomic Energy Bureau, Science and Technology

Agency, Tokyo, Japan for financial support of T. Mikado, guest worker at

NBS.

12
References

Domen, S. R. and Lamperti, P. J., A heat-loss-compensated

calorimeter: theory, design, and performance, J. Res. Nat. Bur.

Stand. (U.S.), 78A (Phys. and Chem.), No. 5^, 595-610 (Sept. -Oct. 197

Domen, S. R. and Lamperti, P. J., Comparisons of calorimetric and

ionometric measurements in graphite irradiated with electrons from

15 to 50 MeV, in press for publication in Med. Phys., Vol. 3, No. 5

(Sept. -Oct. 1976).

Berger, M. J., Seltzer, S. M. , Domen, S. R. , and Lamperti, P. J.,

Stopping-power ratios for electron dosimetry with ionization chamber

IAEA Symposium on Advan. in Biomed. Dosimetry, March (1975).

Berger, M. J., Seltzer, S. M. , Domen, S. R. , and Lamperti, P. J.,

Electron dosimetry with ionization chambers, NBS Symp. on Measurements for the Safe Use of Radiation, March (1976).

Loftus, T. P., Private communication.

Domen, S. R. , Report in progress.

13


Table II. Electrical Calibration Summary

Percent

Calorimeter Type of Measured No. of Uncertainty

Model Mpp <?n"rpnipn t Value runs o ^1

Non-portable Electrical energy 21.967 48 0.20 0.03

per unit mass

per fractional

Portable resistance 22.060 32 0.11 0.02

change
Table III. Summary of Calorimeter Response Ratios

Electron Pb Scatterer Percent

Energy Thickness Ratio Dif f erence Uncertainty

MeV Tnm Percent (combined)

0

20 0.13 0. 9992 -0.08 0.60 0.25

20 0.38 0.9998 -0.02 0.38 0.14

50 0.25 0.9978 -0.22 0.35 0.11

50 0.76 1.0020 +0.20 0.25 0.05
Figure Legends

Fig. 1. Experimental setup for direct comparisons of the NBS absorbed

dose calorimeters.

Fig. 2. Alignment and details of the calorimeter and parallel plate

ionization chamber.

Fig. 3. Normalized dose curves for incident irradiation with cobalt-60

gamma rays and with 20 MeV electrons.

BEAM DIRECTION

EXIT PLANE OF ABSORBERS, 0-50 g/cm'

FRONT PLATE

^900 mg/cm^

± 1500 V )

0-mm DIA. x 2.75-mnn THICK

ION-COLLECTING VOLUME

a CALORIMETER CORE)

EPOXY RESIN

(~2-mm THICK )

0.07-mm GAP

50-mm I.D. PLASTIC RING

TO ELECTROMETER

5-mmAIRGAP ^MEDIUM

40x 40x 30 cm

CALORIMETER CORE

JACKET

SHIELD:

VACUUM CHANNEL

-0.13-mm MYLAR WINDOW

2 -900 mg/cm

j—CENTRAL PLANE OF DETECTORS
3soa a3znviAidON
NBS-1 MA (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUfUJCATlON OR REPORT NO.

NBSIR 76-1023

No.

3. Recipient's Accession No.

4. TITl.H AND SUBTITLE

DIRECT COMPARISONS OF THE NBS ABSORBED DOSE CALORIMETERS

IRRADIATED WITH 20 AND 50 MeV ELECTRONS

5. Publication Date July 1976

6. Performing Organization ( ode

7. AUTHOR(S)

S. R. Domen, T. Mikado, P. J. Lamperti

8. Performing Organ. Report No.

9. HI- OR MING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. f^ontract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as No, 9

13. Type of Report & Period Covered

Final

14» Sponsoring Agency (-ode*

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 2 00- word or less factual summary ot moat si^ilicant information. If document includes a significant

bibliography or literature survey, mention it here.)

Two NBS absorbed dose calorimeters were compared in 20 and 50 MeV electron beams

that were scattered with lead foils of various thickness. The internal structures

of the calorimeters are identical. The non-portable model is enclosed in a 40 cm X 40 cm X 30 cm thick graphite medium while a 30 cm diameter x 15 cm thick

graphite medium was used to enclose the portable model. Measured results indicate

that the three internal bodies of the calorimeters and their measuring circuits were constructed with sufficient care to produce essentially identical calorimetric

responses to about 0.1%.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Absorbed dose; calorimeter; calorimetric comparisons; electron beams

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