Electron-beam calorimeters for Electron-beam calorimeters for

industrial-processing dosimetryindustrial-processing dosimetry

Ganesan RamanathanGanesan Ramanathan

A typical traceability chain for A typical traceability chain for high dose dosimetryhigh dose dosimetry

Calorimetry in industrial radiation Calorimetry in industrial radiation processingprocessing. .

Primary standard calorimeters

Reference standards

Field dosimeters

Material Approx.

sensitivity(º C / kGy)

Dose- range(kGy)

Graphite 1.33 1.5 – 15

Water 0.24 5 – 50

Polystyrene 0.71 3 – 40

Water CalorimetersWater Calorimeters• Water calorimeters are not reference

dosimeters

• Calorimeter consists of polystyrene

petri dish filled with water and a glass-

encapsulated thermistor

• Temperature after irradiation increases

by 0.5% per minute for 10 minutes due

to the heat released from the

polystyrene petri dish.

• It is uncertain whether the mass and

specific heat of the Petri dish should

be fully included in the calculations

Polystyrene CalorimetersPolystyrene Calorimeters• High impact strength grade

polystyrene with density of 1.05

g.cm -3 is used.

• Temperature gradient in proportion

to dose-gradient since polystyrene

is a poor thermal conductor.

• As observed by Domen, the

specific heat capacity of

polystyrene increases

with each irradiation

Graphite CalorimetersGraphite CalorimetersNIST Graphite Calorimeter (Earlier developed)

Test irradiation of the 2-disc Test irradiation of the 2-disc calorimeter at MIRFcalorimeter at MIRF

Radiation Damage of thermistorsRadiation Damage of thermistors• Thermistors were irradiated to a cumulative dose of 0.5 Mrad. Within the experimental

error of 1 % no change in the resistance or temperature coefficient of resistance was

observed.

(from "Pulse radiation dosimetry of 4 MeV bremsstrahlung with a thermistor bridge", P

H White and R H Gobbett, Journal of Physics E: Scientific Instruments 1975 Volume 8,

684-687)

• An accumulated absorbed dose of 4.3 MGy at the rate of 25 kGy/min from 3 MeV electrons produced a resistance change of only about 0.01 percent. A change in the beta value of thermistor was also observed. (from "A sealed water calorimeter for measuring absorbed dose", Journal of Research of the National Institute of Standards and Technology, Vol.99, Number 2, March-April 1994, 121- 141).

Calibration of ThermistorsCalibration of Thermistors

R= R 0 e ß ( 1/ T – 1/T0)

ß changes by 0.7%/ ºK