PROCEDURES FOR DOSE CALIBRATION IN A 1.35 MEV VAN DE GRAAFFELECTROSTATIC ACCELERATOR

BY PAULO DA SILVA

BAEN-625 Advances in Food Engineering

Radiation Dosimetry

Dosimetry – deals with the measurement of the absorbed dose or dose rate from the interaction of ionizing radiation with matter (Attix, 1986).Radiological quantities:

Exposure, kerma, fluence, dose equivalent, energy imparted, and so on.Absorbed Dose – all other quantities can be derived from this one through physical relationships.

Absorbed Dose - Dose

Energy imparted ε by ionizing radiation to matter of mass m in a finite volume V (ICRU, 1980).

At any point P in V:

Thus the absorbed dose is the expectation value at a particular point.Common Units: 104 erg/g = 102 rad = 1 J/kg = Gy(SI).

dmdD ε

=

( ) ( ) ( ) ( ) ∑+−+−= QRRRR coutcinuoutuinε

Dosimeter

Any device that is capable of providing a reading rthat is a measure of the absorbed dose Ddeposited in its sensitive volume V by ionizing radiation (Attix, 1986).The effectiveness of ionizing radiation in food processing applications depends on proper delivery of absorbed dose. Reliable techniques for measuring dose, called dosimetry, are also crucial for ensuring the integrity of the irradiation process.

Dosimeter Types

There are four categories of dosimetry system according to their intrinsic accuracy/absoluteness and usage (ASTM, 2002) :

primary standard (ion chamber, calorimeters);reference standard (alanine, Fricke, and other chemicals);transfer standard (TLD. Traceability, stability, precision, and accuracy);routine dosimeters (process monitoring, radiochromic films) .

Ionization Chamber (IC)

(Attix, 1986)

Ionization Chamber (IC)

Assuming CPE:

where Dg is the dose in the filled gas and X is the exposure.Q is the total charge collected by the IC and m is the mass of dry airDs = Dose in sample, dT/dx = STP, f = correction factor for radiation field geometry = 1 (this case)

gg e

WmQD ⎟⎟

⎠

⎞⎜⎜⎝

⎛= .

dmdQX =

airdry in ..97.33

pieVW =

( )( ) f

dxdTdxdTDD

g

sgs ⎟

⎟⎠

⎞⎜⎜⎝

⎛=

//

Radiochromic Film (RF)

Nylon Matrix- contains the leuco dye: hexa (hydroxyethyl) paraosaniline nitrile2 films/Al pouchThickness of 42.5 Microns (± 2%, 95% C.I.)Exposure to Ionizing radiation (colorless –deep blue)Optical density (510 nm – 600 nm) – FWT-92 Radiachromic readerLinear response (0.5 – 200 kGy)Fully developed after 1 hour (long shelf life)

Radiochromic Film- Calibration

Each batch needs to be calibrated separatelyDetermine how many calibration absorbed doses need to be calibratedCalculate the specific net absorbance, k, for each dosimeter

Ao (initial absorbance), Af (post-irradiation absorbance), t (thickness of the film in mm);

tAA

k of )( −=

Radiochromic Film- Calibration

Plot the response curve versus absorbed dose (for the regression analysis use individual k values rather than average k values for a given absorbed dose);The power series form of the calibration is:

D (dose), k (specific absorbance), a and b are the calibration constants.

baDk =

Transmission Ion Chamber (TIC)

Real time monitoring the radiation delivered – needed to get the counts when irradiating real sampleLocated at the “exit beam window”Secondary Emission Foils (Aluminum)Bombarded with high energy electron the foils will emit secondary electronsCollection voltage (65 Volts)

Current integratorThe total radiation flux is measured by electronically summing the signals originating from within the foils over the time of irradiationExtensively used for Linear Accelerator

very large linear current range (as opposed to Ionization chambers –saturates above certain beam current).

Calibrating the Van De Graaff e- Accelerator

Place the IC over a specific spot (write down the coordinates) on the sample holder in front of the exit beam windowZero the IC exposure integrator and also zero the TIC current integratorStart the radiation process with a low beam current (~3 µA) at 1.35 MeV.Stop the radiation process whenever the IC experience an exposure of ~ 5000 R.

Write down the exact exposure and the counts collected over the TIC.

Repeat this process in different spots over an area of interest. Perform at least three runs per spot.

Calibrating the Van De Graaff e- Accelerator

Find the hot spot (Highest Dose collected / counts) and average it with its closest 4 neighbors. Beam Calibration Number = BCN

y)(x, coordinateat countscollected Dose average

=BCN

Transmission Ion Chamber

Calib. the VdG – TIC Mapping

#counts/same dose

hotspot

Spinach and air STP-Total

Homework

A set of IC exposure values and TIC counts over a specific area in Cartesian coordinates is given (handouts).

Find the hotspot (x,y) and the beam calibration number.A set of TIC counts and (irradiated and control) RF OD values is given (handouts).

Find the calibration constants.Data sets for irradiation of spinach bags are given (handouts).

Answer the questions.Good Luck!

References

ASTM (2002). ISO / ASTM51261 - 02 Standard Guide for Selection and Calibration of Dosimetry Systems for Radiation Processing. Annual Book of ASTM Standards, West Conshohocken, PA: ASTM International.Attix, F. H. (1986). Introduction to Radiological Physics and Radiation Dosimetry. New York: John Wiley & Sons.