Tissue equivalent proportional counters (TEPCs) are commonly used for radiation monitoring in areas where a mixture of neutron and photon radiations may be present, such as those commonly encountered in nuclear power plants. In such radiation fields, the dose rate of each component can vary drastically from extremely low to very high. Among these possible combinations of radiation fields with very different dose rates, a mixed field of an intense photon and a weak neutron dose component is the more commonly encountered. This study describes the measurement of lineal energy spectra carried out with a 5.1 cm (2 inch) diameter spherical TEPC simulating a 2 μm diameter tissue site in low energy (33 – 330 keV neutrons) mixed photon-neutron fields with varying dose rates generated by the McMaster University 1.25 MV double stage Tandetron accelerator. The Tandetron accelerator facility was employed to produce neutrons using thick 7Li targets via the 7Li(p, n)7Be reaction. A continuous spectrum of neutrons is generated at any selected proton beam energy which is very narrow at beam energies very close to the threshold of the reaction 1.88 MeV and becomes wider as the proton beam energy moves further away from the threshold energy of the reaction. Dose rates which resulted in dead times as high as 75% for the data acquisition system were employed to study the effect of dose rate on the measured quality factors, microdosimetric averages (y ̅_f and y ̅_D)absorbed dose and dose equivalent. The dose rate at a given beam energy was varied by changing the accelerator beam current. A variety of mixed neutron gamma fields was generated using neutron beams with mean energies extending approximately from 33 keV to 330 keV with the 7Li target using proton beam energies ranging from 1.89 to 2.5 MeV. In direct beams, 478 keV photons which are produced in the 7Li target via inelastic scattering interaction 7Li(p, p'γ)7Li dominate the low LET component of the mixed field of radiation. When a 2 cm thick polyethylene moderator was inserted between the neutron producing target and the counter, the low LET component of the mixed radiation field also contained 2.20 MeV gamma rays originating from 1H(n, γ)2H capture interactions in the moderator. We have observed that high dose rates due to both photons and neutrons in a mixed field of radiation result in pile up of pulses and distort the lineal energy spectrum measured under these conditions. The pile up effect and hence the distortion in the lineal energy spectrum becomes prominent with dose rates which result in dead times larger than 25% for the high LET radiation component. In intense neutron fields, which may amount to 75% dead time, a 50% or even larger increase in values for the measured microsdosimetric averages and the neutron quality factor was observed. This study demonstrates that moderate dose rates which do not result in dead times of more than 20-25% due to either of the component radiations or due to both components of mixed field radiation generate results which are acceptable for operational health physics mixed neutron-gamma radiation monitoring using tissue equivalent proportional counters. / UOIT

This project was designed to determine the feasibility of constructing a multichamber proportional counter. A multi-chamber detector is designed to increase the total surface area which will increase the number of radiation interactions that occur per unit dose. Surface area can be changed without changing the detector volume by subdividing the active volume into several smaller volumes that can then be used as mini detectors whose data can be summed and used to determine the absorbed dose. This will allow the total surface area to remain the same as that of the more common 12.5 cm (5 in.) spherical detector and a decreased total volume resulting in a more compact detector design. However, subdividing those volumes causes problems with electric field fringing at the ends of the mini detectors. In order to correct this, guard ring and field tube designs which operate at a lower voltage than the detector cathode were tested. Results from this study showed that the field tube design provided the best overall resolution but it only outperformed the other designs by a maximum of 5%. However the field tube design doubles the length of the detector which would result in a larger overall detector package. The performance of the single and double ring configurations was suitable for radiation monitoring applications. These findings show that it is feasible to use an array of subdivided detector volumes instead of a spherical detector.

The Sudbury Neutrino Observatory (SNO), a heavy water Cherenkov experiment, was designed to detect solar Boron-8 neutrinos via their elastic scattering interactions on electrons, or charged current and neutral current (NC) interactions on deuterium. In the third phase of SNO, an array of Helium-3 proportional counters was deployed to detect neutrons produced in NC interactions. A simulation of the current pulses and energy spectra of the main kinds of ionization events inside these Neutral Current Detectors (NCDs) was developed. To achieve this, electron drift times in NCDs were evaluated with a Monte Carlo method, and constrained by using wire alpha activity inside the counters. The pulse calculation algorithm applies to any ionization event, and takes into account processes such as straggling, electron diffusion, and propagation through the NCD hardware. A space charge model was developed to fully explain the energy spectra of neutron and alpha events. Comparisons with data allowed the various classes of alpha backgrounds to be identified, and gave evidence for the spatial non-uniformity of Uranium-238 and Thorium-232 chain nuclei in the counter walls. The simulation was applied to determine the fractional contents of the main types of alpha backgrounds in each NCD string. The number of neutron capture events in the array was extracted via a statistical separation, using Monte Carlo generated alpha background pulse shape parameter distributions and minimal energy information. The inferred total Boron-8 solar neutrino flux is: φ_NC< = 5.74 ± 0.77 (stat) ± 0.39 (sys) x 10⁶ cm^-2s^-1 in agreement with Standard Solar predictions and previous SNO results.

The purpose of this study was to determine how well the Monte Carlo transport code FLUKA can simulate a tissue-equivalent proportional counter (TEPC) and produce the expected delta ray events when exposed to high energy heavy ions (HZE) like in the galactic cosmic ray (GCR) environment. Accurate transport codes are desirable because of the high cost of beam time, the inability to measure the mixed field GCR on the ground and the flexibility they offer in the engineering and design process. A spherical TEPC simulating a 1 um site size was constructed in FLUKA and its response was compared to experimental data for an 56Fe beam at 360 MeV/nucleon. The response of several narrow beams at different impact parameters were used to explain the features of the response of the same detector exposed to a uniform field of radiation. Additionally, an investigation was made into the effect of the wall thickness on the response of the TEPC and the range of delta rays in the tissue-equivalent (TE) wall material. A full impact parameter test (from IP = 0 to IP = detector radius) was performed to show that FLUKA produces the expected wall effect. That is, energy deposition in the gas volume can occur even when the primary beam does not pass through the gas volume. A final comparison to experimental data was made for the simulated TEPC exposed to various broad beams in the energy range of 200 - 1000 MeV/nucleon. FLUKA overestimated energy deposition in the gas volume in all cases. The FLUKA results differed from the experimental data by an average of 25.2 % for yF and 12.4 % for yD. It is suggested that this difference can be reduced by adjusting the FLUKA default ionization potential and density correction factors.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Douglas S. McGregor / The 3He gas shortage for neutron detection has caused an increase in research efforts to develop viable alternative technologies. 3He neutron detectors cover areas ranging from 10–1000 cm2 in cylindrical form factors and are ideal for many nuclear applications due to their high intrinsic thermal neutron detection efficiency (> 80%) and gamma-ray discrimination (GRR ≤ 1 x 10-6) capabilities. Neutron monitoring systems for nuclear security applications include Radiation Portal Monitors (RPM’s), backpack, briefcase, and hand-held sensors. A viable replacement technology is presented here and compares three neutron detectors, each with different neutron absorber materials, to current 3He standards. These materials include Li and/or B silica aerogels, LiF impregnated foams, and metallic Li foils. Additionally, other neutron absorbing materials were investigated in this work and include LiF coated Mylar, B foils, BN coated carbon foam, and BN coated plastic honeycomb. From theoretical calculations, the Li foil material showed the greatest promise as a viable 3He alternative, thus a majority of the research efforts were focused on this material. The new neutron detector was a multi-wire proportional counter (MWPC) constructed using alternating banks of anode wires and 95% enriched 6Li foils sheets spaced 1.63 cm apart. In total, six anode banks and five layers of foil were used, thus an anode wire bank was positioned on each side of a suspended foils. Reaction products from the 6Li(n,α)3H reaction were able to escape both side of a foil sheet simultaneously and be measured in the surrounding gas volume concurrently. This new concept of measuring both reaction products from a single neutron absorption in a solid-form absorber material increased the intrinsic thermal neutron detection efficiency and gamma-ray discrimination compared to coated gas-filled detectors. Three different sizes of Li foil MWPC neutron detectors were constructed ranging from 25–1250 cm2 and included detectors for RPM’s, backpacks, and hand-held systems. The measured intrinsic thermal neutron detection efficiency of these devices was approximately 54%, but it is possible to exceed 80% efficiency with additional foils. The gamma-ray discrimination abilities of the detector exceeded 3He tubes by almost three orders of magnitude (GRR = 7.6 x 10-9).

Neutron imaging is a non-destructive technique with many applications in diverse fields such as industrial quality assurance, archaeology, and border security. However with the high cost of the standard fill gas 3He and the high cost of scaling conventional digital imaging systems to large areas its applications are limited. Here is presented the proof of concept for a gaseous neutron imaging system utilizing a 10B4C solid state converter and THGEM technology and 2D passive delay line readout. THGEMs used for signal amplification can be produced cost effectively and in large areas by PCB manufacturers. This combined with the reduced channel processing requirements of delay lines over individual pixel readouts results in a cost-effective and scalable system when compared to similar designs using solid state multipliers such as silicon photomultipliers. Here is presented a proof of concept of this imaging system with data acquisition accomplished by digitization and offline image reconstruction achieving mean X and Y resolution of <sigma_x> = (1.37+-0.24) mm and <sigma_y> = (1.15+-0.13) mm respectively. Studied in parallel with this system is the effectiveness of gadolinium oxide based paint as a thermal neutron shield and image contrast agent. / Thesis / Doctor of Philosophy (PhD) / In this work is presented the development process of a novel and cost-effective neutron imaging system capable of imaging soft biological and dense materials that X-rays are unable to penetrate. Such a system may be scaled to large areas for many applications including the study of large archeological objects or employed as a security measure to monitor border checkpoints for transportation of controlled radioactive materials.

Characterizing high z high energy (HZE) particles in cosmic radiation is of importance for the study of the equivalent dose to astronauts. Low pressure, tissue equivalent proportional counters (TEPC) are routinely used to evaluate radiation exposures in space. A multiple detector system composed of three TEPC of different sizes was simulated using the Monte-Carlo software toolkit GEANT4. The ability of the set of detectors to characterize HZE particles, as well as measure dose, was studied. HZE particles produce energetic secondary electrons (-rays) which carry a significant fraction of energy lost by the primary ion away from its track. The range and frequency of these delta rays depends on the velocity and charge of the primary ion. Measurements of lineal energy spectra in different size sites will differ because of these delta ray events and may provide information to characterize the incident primary particle. Monte Carlo calculations were accomplished, using GEANT4, simulating solid walled proportional detectors with unit density site diameter of 0.1, 0.5 and 2.5 µm in a uniform HZE particle field. The simulated spherical detectors have 2 mm thick tissue equivalent walls. The uniform beams of 1 GeV/n, 500 MeV/n and 100 MeV/n 56Fe, 28Si, 16O, 4He and proton particles were used to bombard the detector. The size effect of such a detector system was analyzed with the calculation results. The results show that the y vs. yf(y) spectrum differs significantly as a function of site size. From the spectra, as well as the calculated mean lineal energy, the simulated particles can be characterized. We predict that the detector system is capable of characterizing HZE particles in a complex field. This suggests that it may be practical to use such a system to measure the average particle velocity as well as the absorbed dose delivered by HZE particles in space. The parameters used in the simulation are also good references for detector construction. characterizing HZE particles in a complex field. This suggests that it may be practical to use such a system to measure the average particle velocity as well as the absorbed dose delivered by HZE particles in space. The parameters used in the simulation are also good references for detector construction.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22441号 / 工博第4702号 / 新制||工||1734(附属図書館) / 京都大学大学院工学研究科原子核工学専攻 / (主査)教授 神野 郁夫, 教授 斉藤 学, 准教授 櫻井 良憲 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Cílem této práce je seznámení se s problematikou detekce ionizujícího záření a jeho detekcí zejména pomocí proporcionálních detektorů a navrhnutí algoritmu pro jejich klasifikaci a jeho následná implementace do FPGA. V první části práce je obecně popsáno, jak jednotlivé typy plynem plněných detektorů fungují. V druhé části je věnován prostor algoritmům pro klasifikaci pulzů, které se objevují v literatuře. Následuje návrh vlastního algoritmu, jeho rozbor a rozebrání výsledků. Ve třetí části následuje popis samotné realizace navrhnutého řešení na platformě RedPitaya. Je zde rozebrána celková architektura navrhnutého systému, detailně popsány jak jednotlivé bloky v logice FPGA, tak i ostatní skripty zajišťující zpracování naměřených výsledků a jejich vizualizaci.

The effective dose equivalent, H (or the effective dose, E ) to an individual is the primary limiting quantity in radiation protection. However, techniques for measuring H for neutrons have not been fully developed. In this regard a new tissue equivalent proportional counter (TEPC) based on a gas electron multiplier (GEM) for measuring H*(10), which is a conservative estimate of H, for neutrons was designed and constructed. The deposited energy distribution for two different neutron sources (a Cf-252 source and a AmBe source) was measured using the new TEPC. The measurements were performed using two different proportional gases: P-10 gas and a propane-based tissue equivalent gas at various pressures. A computer simulation of the new TEPC, based on the Monte Carlo method, was performed in order to obtain the pulse height distributions for the two neutron sources. The simulated results and the measured results were compared. Results show that the experimental results agree with the computational results within 20% of accuracy for both Cf-252 and AmBe neutron sources. A new model GEM-based TEPC was developed for use in obtaining H*(10). The value of H*(10) for the Cf-252 source and for the AmBe source using experimental measurements was obtained. These results are presented in this study. The study shows that the GEM-based TEPC can successfully estimate H*(10). With these results and some refinements, this GEM-based TEPC can directly be used as a neutron rem meter.