On microdosimetry of neutrons of selectable energy in mixed (n,y) fields

Secondary electron emission;Particles (Nuclear physics)--Congresses;Neutrons

Saion, Elias bin. ; Watt, D. E. ; Watt, D. E.

University:University of St Andrews

Department:Physics & Astronomy (School of)

关键词: Secondary electron emission;    Particles (Nuclear physics)--Congresses;    Neutrons;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/2640/EBSaionPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

Biological damage of tissue due to intermediate energy neutrons is generally known to be very important in radiobiology and radiation protection. However, there is no suitable method to determine the quality of these neutrons in particular in the working environment of mixed (n,y) radiation fields. In this thesis, an attempt is made to develop a dosimeter based on microdosimetric principles which has the capability for such a purpose. With thisobject the basic concepts of microdosimetry are reviewed and discussed with emphasis on their application for radiation protection and in designing of the dosimeter.Microdosimetry based on low pressure tissue-equivalent proportional counters(TEPCs) is a powerful technique for determining microscopic distributions of energydeposition and quality of ionizing radiations. However the energy deposition spectra ofintermediate energy neutrons in mixed fields of fast neutrons can only be measured usingTEPC in co-axial double cylindrical form by an appropriate choice for the thickness of thecommon tissue-equivalent (TE) dividing wall separating the inner and outer counters and byappropriate use of coincidence/anti-coincidence pulse arrangements. An analytical calculationfor the response of the inner counter operating in coincidence/anti-coincidence modes withthe outer counter was developed. However there will be some events, due to fast neutrons,which will contribute to the signals from intermediate energy neutrons and which cannot beremoved by anti-coincidence. For these analytical corrections must be made. Also, theevents associated with the dividing wall inherent in the system can contribute to the responseof the inner counter and must be corrected by calculation. The calculation was possible dueto the fact that recoil particles from intermediate energy neutron interactions have effectivestopping powers and projected ranges which differ significantly from the continuousslowing down approximation (CSDA) values. By incorporating these the basic CSDAformulae for energy deposition spectra of neutrons could be extended down to intermediateenergy neutrons of about 1 keV.A prototype co-axial double cylindrical TEPC capable of separating the component ofneutrons (≤ 850 keV) in mixed (n,y) radiation fields was manufactured and tested. The thinwall dividing the inner and outer counters was fabricated from the standard A-150 TE plasticwith the thickness equivalent to the range of 850 keV protons. The operational characteristicsof the dosimeter were studied to determine its applicability for use in microdosimetry. The gas gain of the inner and outer TEPCs was measured at various simulated mean chordlengths and applied voltages. The results can be expressed according to Campion's equationwithin a given range of the electric field strength. The resolution of the inner TEPCmeasured at the operating voltages is in agreement with the theoretical prediction.A series of microdosimetric experiments were performed with mixed fields of 60Cogamma-rays and neutrons from the UTR-300 nuclear reactor and from 252Cf and241Am-Beradioactive sources. Discrimination against fast neutrons of energy > 850 keV was achievedusing an anti-coincidence unit specially designed for better efficiency of data acquisition.Discrimination against fast electrons due to photon interactions was also achieved. Spectrawith anti-coincidence are dominated by slow protons and electrons. Their mean linealenergies are higher than those of spectra without anti-coincidence. The quality factor anddose equivalent for spectra with anti-coincidence are higher than the spectra without anticoincidenceindicating the importance of intermediate energy neutrons in mixed fields. Thequality factor and the corresponding dose equivalent corrected for saturation of lineal energycorresponding to 2 nm of ionization spacing is consistently higher than those derived fromthe absorbed dose based formulae, the biophysical implications of which are discussed.Suggestion for future developments for microdosimetry of intermediate energy neutrons in mixed fields are made and discussed.