Radia:. Phys. Chem. Vol. 42, Nos 4—6, pp. 789—792,1993 0146-5724/93$6.00+ 0.00Printed in Great Britain. All rights reserved Copyright C 1993 Pergamon PressLtd

APPLICATION OF CALORIMETERS FOR 5 MeV EB ANDBREMSSTR.ABLUNG DOSIMETRY

Toshio Sato, Toru Takahashi, Toshio Saito and Masaaki TakehisaRadia Industry Co., Ltd., 168 Ooyagi, Takasaki, Gunma, 370 Japan

Arne Miller

Riso National Laboratory, DK 4000 Roskilde, Denmark

ABSTRACT

Graphite and water calorimeters, which were developed for use a 10 MeV electron beams (KB) at RisoNational Laboratory, were used for process validation and routine dosimeter calibration at a 5 MeV

EB. Water calorimeters were used for reference measurements for 5 MeV KB, the response was found to

be directly proportional to the beam current and the variation among three water calorimeters wasless than ±2 % in the range of 10 to 40 kGy. CTA, PMMA, RCD dosimeters were calibrated byirradiating the dosimeters and water calorimeters simultaii~ously. The water calorimeter was proved

to be an useful tool at 5 MeV EB. Graphite calorimeters gave reproducible readings within 3,3 %relative errors (95% confidence level) for X—ray measurement.

INTRODUCTION

Medium energy, from 4.5-5 MeV direct action accelerators, and high energy, from 10 MeV linearaccelerators, electron beams(EB) are widely used in medical device sterilization and in polymerprocessing. A dosimetry in absolute sense is required for medical device sterilization, but direct

calibration of routine dosimeters for EB is not easy work due to luck of standard radiation field

world wide. Calorimetry is in principle primary method of KB dosimetry, and is used for routinevalidation for 10 P1eV KB sterilization in northern Europe (Miller etal., 1989). This paper describes

trial for applicability of the semi-adiabatic water calorimeters for dosimetry of medium energy KB

and calibration of routine dosimeters.We also investigated the reproducibility of graphite calorimeter (Milleretal.,1990) response to

X-ray converted from 5 HeY KB.

THE WATERCALORIMETERAND RESPONSEFOR 5 P1eV EB

A detail of the Riso’ s water calorimeter was reported elsewhere (Miller etal., 1990) for 10 MeV KB.

The calorimeter was calibrated in standard KB field of National Physical Laboratory (NPL)in UnitedKingdom. The water calorimeter has thickness of 1.9 g/cm

2 and can be used for S P1eV EB which has

2. 5 g/cm2 range.

Response for 5 P1eV KB

The dose response variation of 3 calorimeters for 5 P1eV KB is shown in Table 1. The KB

characteristics are 10 mA ,140 cm in scan width ,80 cm between scan window and calorimeter, and

conveyer speed of 178,266,355 em/mm. Fig 1 shows the calorimeter response vs. beam current providing

with the remaining KB characteristics constant at 5 P1eV, 140 cm scan width, 500 cm/mm, and 80 cmbelow the window.

789

790 TosHlo SATO et a!.

Table 1. Dose responseof 3 Riso water calorimeters

Calorimeter Dose Average Dose Variation 30

Number kGy kGy

____________________________________________________________ 25

379 34,49 + 0.1

380 34. 41 34.44 — 0. 1 E 20

381 34.41 - 0.1 15

379 22.31 - 1.8 10

22.87 22.72 0. 7 : ________________________________

380 16.98 16.87 ~ 0.7 0 10 20 30 40

381 16.81 — 0.4 CalorimeterResponse kGy________ ______________ __________- Fig. 1 Calorimeter Responsevs. Beam Current

Table 1 shows that deviation of dose response among calorimeters are within ±2 %. and the value

is similar to 10 MeV KB (Milleretal., 1990). Fig.1 also demonstrate that the calorimeter response

is directly proportional to the beam current according to a recurrence analysis.It can be concluded that the water calorimeter can be used at the similar precision at 5 MeV KB as 10

MeV KB, even if 5 P1eV KB has larger dose gradients than 10 P1eV EB.These results indicate that the water calorimeter can be used for validation of 5 MeV KB processing.

CALIBRATION OF ROUTINE DOSIMETERS TRACEABLETO THE CALORIMETER

A couple of water filled wedges, shown in Fig.2, were used for depth-dose measurement corresponding to

the water calorimeter body. A strip of CTA dosimeter, which is known to increase the optical density

at 280 nm proportional to the dose (Tanakaetal.,1982), was sandwiched at the center of the wedges

and irradiated on the conveyer tray together with the water calorimeters. The absorbed dose measuredwith the water calorimeter is the average dose of the volume of the calorimeter body, and ought to be

equal to the average dose of water filled wedge for 0 - 1.9g/cm2 depth which corresponds to the

thickness of the water calorimeter body. The optical density change (AOD) of CTA was read by an NHV

FDR-01 dose reader at 2mm interval of the CTA strip thereby providing the depth-dose curve from 0 to1.9 g/cm5 depth, and the average value of the CTA depth-dose curve was calculated. This value was

divided by average dose value as measured by the water calorimeter, thereby providing the k value inequation 1 for the CTA dosimeter. This value for 5 11eV EB irradiation was calculated to be0.0057 (kGy)’ in terms of absorbed dose in water. In case of absorbed dose to CTA, the Ic value is

0.0062 (kGyYt which is similar to the value of 0.0063(kGy)_L (Tanakaetal.,1982).

D,,~or =(1xOD/K)~O.125/t Eq. 1

CTA dosimeters were irradiated on top of a PMMA plate (thickness 6 mm) and in term of dose in waterwas determined by the CTA dosimeter by use of equation 1 with Ic=O. 0057. Based on the dose reading ofwater calorimeter and of CTA dosimeter a relation between the two doses was determined to be 1.031.

Other dosimeters may now be calibrated in position top of PMMAplate and using the relation of 1.031to determine dose to the dosimeter.

Fig 3 shows the relation between calorimeter response and surface dose for film dosimeter measured by

CTA.

8th InternationalMeeting on Radiation Processing 791

(1) 10 Proportional Coeff. 1.031

0 ______________Fig.2 Schematic Sketch of Water Filled Wedges 0 10 20 30 40 50Made of 0. 5 mm PMMASheet Calorimeter Response kGy

Fig.3 Calorimeter Response and Surface dose

To calibrate routine dosimeters ( CTA ,PMMA, RCD) with water calorimeter, every 4 pieces of routine

dosimeters and 3 water calorimeters are irradiated at 5 P1eV EB for about 10, 20, 30, 40, 50 kGy. The

routine dosimeters were put on top of PMMAplate, therefore they ought to indicate surface doseOptical density of PMMAand RCD were read out by Hitachi spectrophotometer U-1200 and Radiachromic

Reader (FWT-92) respectively. These calibration curves are shown in Figs. 4- 6.

__ 10.3/1’/

WaveLength280 nm 0 __________________________________

1.0 3.0 hr. after Irr. 0 10 20 30 40 50

0 . AbsorbedDose(H20) kGy0 10 20 30 40 50 Fig.5 Calibration Curve of PMMAat 5 MeV EB

AbsorbedDose (H20) kGy

Fig.4 Calibration Curve of CTA at 5 P1eV KB (1) CTA film dosimeter (Fuji FTR 125)From the result of calibration, the absorbed dose

in terms of dose in water is given by equation 2.

0.5 D,..~.,.= (ti0D/0.0055)~0.125/t Eq. 2

0.4 ~ The k value of eq. 2 is very similar to eq.1.

FWTCataIogue,,~..—~ K value of CTA dose is 0. 0063 (Tanaka et al.. 1982).~0.3 —

In this experiment, k value is in terms of absorbed

0.2 dose in water.(2) PMMA (Radix)

01 The A OD change by KB irradiation tended to levelWaveLength 5lOnm off than gamma irradiation.

0 (3) RCD (FWT—60-0O)

0 10 20 30 40 50 The stopping power ratio of water and RCD (Sw/SRCD)AbsorbedDose(HsO) kG)’ is 1.01. The result of this calibration curve gave

somewhatlower tIOD than FWT catalogue.Fig.6 Calibration Curve of RCD at 5 P1eV KB

792 TOSHIO SAT0 et a!.

GRAPHITE CALORIMETERFOR MONITORINGX-RAY CONVERTED5 P1eV KB

The graphite calorimeter was used to monitor X-ray irradiation. This experiment was aiming use the

graphite calorimeter to routine monitoring for X-ray irradiation processing. The calorimeter was

irradiated by X-ray converted from 5 P1eV, 30 mA KB, scanning at 120 cm width, conveyer speed of

10 cm/mm, and 40 cm under the Ta target. The calorimeter was covered by a box made of 6 mm thickPMMA plate to prevent scattered electrons from reaching the calorimeter. The temperature rise in

graphite calorimeter was measuredin situ to avoid the temperaturechangeafter irradiation. Fig. 7shows the temperature in graphite disk read out in real time. Table 2 shows the reproducibility of

the calorimeter response, electron beam parameters and their deviations ( 2u ,95% confidence level).

Calorimeter No.483 T~

P Dg (T,—T,—k,)Cc-k~

40 5, Correction tar heating from enveronment(In this case

ks Correction for cooling between irr. and0 readout (In this case 1.00)

cc : Specific heat of graphiteCc—644.2+2.86~(T,+Ti)’~(J.kg’ -

35

.—~- ~-

O 2 4 6 8 10 12 14 16 18 20 22 24

Time mm.

Fig.7 Calorimeter temperature as a function of time

Table 2 Reproducibility of graphite calorimeter response

Cal.No. Dose response Deviation Energy Current Scan width Con, speed

kGy 2 a (kGy) P1eV mA cm cm/mm.

478 8.89 6±0.006 30±0.10 120±0.96 10±0.0006

480 8.72 5±0.006 30±0.06 120±0.96 10±0481 8.97 0.298 5±0.006 30±0.06 120±0.96 10±0.0006

482 9.12 5±0.008 30±0.08 120±1.01 10±0

483 8.97 5±0.010 30±0.10 120±1.01 10±0.0006

It was found that 5 graphite calorimetersgave reproducible readings within 3.3 X (2a) relative

errors. Standard deviation of electron beam parameters during irradiation is much less than that ofcalorimeter reading, and the randomerror of dosemeasurementis satisfactory. We are studying the

Systematicerrors and the results will be reported later.

REFERENCES

McLaughlin W.L. ,A.W. Boyed, K.H. Chadwick,J.C. McDonald and A. Miller(1989). Dosimetry for Radiation

Processing 113—141 Taylor & Francis, LondonMiller A. and K.H. Chadwick (1989). Dosimetry for the Approval of Food Irradiation Processes. Radiat

.

Phys.Chem. 34 999-1004Miller A. and A. Kovacs (1990). Application of Calorimeter for Routine and Reference Dosimetry at

4-10 P1eV Industrial Electron Accelerators. Radiat. Phys.Chem. 33 774—778.Tanaka R.,S.Mitomo,H.Sunaga,K.Matuda,andY.Tamura (1982). CTA Dosimeter Manual. JAERI N 82-033

(in Japanese)