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STOCHASTIC VARIABLES IN THE ENERGY DEPOSIT AND THEIR MEANING IN THE HAZARD OF NEUTRONS

Objective


An experimental apparatus to measure the avalanche characteristics of a cylindrical TEPC at low pressure was assembled and measurements were performed.
THE PROPOSAL CONCERNS THE STUDY OF THE STOCHASTIC VARIABLE "Y" (LINEAL ENERGY) AT SIMULATED DIAMETERS LESS THAN 1 MICROMETER.
AT VERY SMALL SIMULATED DIAMETERS THE STRAGGLING OF THE CHARGED PARTICLES BECOMES IMPORTANT.
AS THE TARGET THICKNESS DIMINISHES, THE STRAGGLING OF THE CHARGED PARTICLES INCREASES THE ENERGY ABSORPTION. THAT PRODUCES AN INCREASING PROBABILITY OF HIGHER ENERGY EVENTS AND CAN BE CONSIDERED AS AN INCREASE OF RADIATION QUALITY OF THE BEAM WITH RELEVANT CONSEQUENCES FOR THE RADIOPROTECTION AIMS. THIS EFFECT CHANGES WITH THE ENERGY OF THE NEUTRONS AND IN GENERAL INCREASES WITH NEUTRON ENERGY, BUT THE ELASTIC SCATTERING INTERACTION SEEMS TO PRODUCE A STRAGGLING EFFECT THAT CAN NOT BE DISREGARDED EVEN AT LOW NEUTRON ENERGIES. FOR THIS REASON MEASUREMENTS ARE PLANNED FROM SOME TENS OF KEV TO SOME TENS OF MEV.

THE STUDY WILL CONCERN BOTH EXPERIMENTAL AND THEORETICAL STUDIES. THE NEUTRON BEAMS WILL BE SUPPLIED BY LEGNARO LABORATORIES AND THE GSF LABORATORY OF THE NEUHERBERG (MUENCHEN) RESEARCH CENTER.

NEUTRON ENERGIES BETWEEN ABOUT 10 KEV AND SOME TENS OF MEV WILL BE EMPLOYED. FOLLOWING THE EXPERIENCE MADE IN MANUFACTURING A TISSUE-EQUIVALENT PROPORTIONAL COUNTER (TEPC), ALREADY WORKING AT SIMULATED DIAMETERS LESS THAN 1 MICROMETER, A NEW TEPC WILL BE MADE IN ORDER TO MEASURE THE VARIABLE "Y" PROPERLY IN ALL THE NEUTRON FIELDS. THE CALCULATIONS WILL BE PERFORMED IN COOPERATION WITH GSF-MUENCHEN WHICH HAS LONG-YEARS EXPERIENCE IN THE THEORETICAL CALCULATIONS OF ENERGY DEPOSITION IN BIOLOGICALLY RELEVANT VOLUMES.

BOTH EXPERIMENTAL AND THEORETICAL AIMS NEED TO FACE AND RESOLVE SEVERAL PROBLEMS. ALTHOUGH A MICRODOSEMETER, WORKING AT SIMULATED DIAMETERS LESS THAN 1 MICROMETER, HAS BEEN ALREADY MANUFACTURED THE PHYSICAL PARAMETERS INVOLVED IN THE CHARACTERISTICS OF A GAS PROPORTIONAL COUNTER HAVE NOT BEEN DEEPLY STUDIED AS YET.

THE TEPC HAS TO HAVE HIGH GAINS AT VERY LOW PRESSURES AND A GOOD ENOUGH RESOLUTION : TWO CHARACTERISTICS WHICH ARE IN CONTRADICTION AT LOW PRESSURES. BEFORE MANUFACTURING A NEW TEPC AN EXPERIMENTAL STUDY WILL BE PERFORMED ON THE GAS GAIN CHARACTERISTICS, ANODE AND GRID STRUCTURE IN ORDER TO OPTIMIZE GAIN AND RESOLUTION.
MICRODOSIMETER SPECTRA AT SIMULATED DIAMETERS LESS THAN 1 MICROMETER ARE AFFECTED BY STRAGGLING EFFECTS, WHEREAS THE CASWELL AND COYNE'S CODE DOES NOT ALLOW FOR THESE EFFECTS.
A THEORETICAL STUDY, BASED ON A ALPHA-SOURCE IONISATION SPECTRA, WILL BE PERFORMED IN ORDER TO ALLOW THE STRAGGLING TO BE INCORPORATED INTO THE MICRODOSIMETRIC CALCULATIONS.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Istituto Nazionale di Fisica Nucleare
Address
Via Romea 4
35020 Legnaro Padova
Italy