The use of organic scintillators is an established method for the measurement of neutron spectra above several hundred keV. Fast neutrons are detected largely by proton recoils in the scintillator resulting from neutron elastic scattering with hydrogen. This leads to a smeared rectangular pulse-height distribution for monoenergetic neutrons. The recoil proton distribution ranges in energy from zero to the incident neutron energy. In addition, the pulse-height distribution is further complicated by structure due to energy deposition from alpha particle recoils from interactions with carbon as well as carbon recoils themselves. In order to reconstruct the incident neutron spectrum, the pulse-height spectrum has to be deconvoluted (unfolded) using the computed or measured response of the scintillator to monoenergetic neutrons. In addition gamma rays, which are always present when neutrons are present, lead to Compton electron recoils in the scintillator. Fortunately, for certain organic scintillators, the electron recoil events can be separated from the heavier particle recoil events in turn to distinguish gamma-ray induced events from neutron-induced events. This is accomplished by using the risetime of the pulse from the organic scintillator seen in the photomultiplier tube as a decay of light. In this work, an organic scintillator detection system was assembled which includes neutron-gamma separation capabilities to store the neutron-induced and gamma-induced recoil spectra separately. An unfolding code was implemented to deconvolute the spectra into neutron and gamma energy spectra. In order to verify the performance of the system, a measurement of two reference neutron fields will be performed with the system, unmoderated Cf-252 and heavy-water moderated Cf-252. After the detection system has been verified, measurements will be made with an AmBe neutron source.

In this thesis, manufacturing of microirradiators, electrodeposition of radioactive elements such as Ni-63, and applications of these radioactive sources are discussed. Ni-63 has a half life of 100 years and a low energy beta electron of 67 keV, ideal for low dose low linear energy transfer (LET) research. The main focus of the research is on the novel Ni-63 microirradiator. It contains a small amount of total activity of radiation but a large flux, allowing the user to safely handle the microirradiator without extensive shielding. This thesis is divided into nine chapters. Properties of microirradiators and various competing radioactive sources are compared in the introduction (chapter 1). Detailed description of manufacturing Ni-63 microirradiator using the microelectrode as the starting point is outlined in chapter 2. The microelectrode is a 25 µm in diameter Pt disk sealed in a pulled 1 mm diameter borosilicate capillary tube, as a protruding wire or recessed disk microelectrode. The electrochemically active surface area of each is verified by cyclic voltammetry. Electrodeposition of nickel with a detailed description of formulation of the electrochemical bath in a cold "non-radioactive setting" was optimized by using parameters as defined by pourbaix diagrams, radioactive electroplating of Ni-63, and incorporation of safety regulations into electrodeposition. Calibration and characterization of the Ni-63 microirradiators as protruding wire and recessed disk microirradiators is presented in chapter 3. In chapters 4 through 6, applications of the Ni-63 microirradiators and wire sources are presented. Chapter 4 provides a radiobiological application of the recessed disk microirradiator and a modified flush microirradiator with osteosarcoma cancer cells. Cells were irradiated with 2000 to 1 Bq, and real time observations of DNA double strand breaks were observed. A novel benchtop detection system for the microirradiators is presented in chapter 5. Ni-63 is most commonly measured by liquid scintillation counters, which are expensive and not easily accessible within a benchtop setting. Results show liquid scintillation measurements overestimates the amount of radiation coming from the recessed disk. A novel 10 µCi Ni-63 electrochemically deposited wire acting as an ambient chemical ionization source for pharmaceutical tablets in mass spectrometry is in chapter 6. Typically, larger radioactive sources (15 mCi) of Ni-63 have been used in an ambient ionization scenario. Additionally, this is the first application of using Ni-63 to ionize in atmosphere pharmaceutical tablets, leading to a possible field portable device. In the last chapters, chapters 7 through 8, previous microirradiator experiments and future work are summarized. Chapter 7 illustrates the prototype of the electrochemically deposited microirradiator, the Te-125 microirradiator. In conjunction with Oak Ridge National Laboratory, Te-125m is a low dose x-ray emitting element determined to be the best first prototype of an electrochemically deposited microirradiator. Manufacturing, characterization, and experiments that were not successful leading to the development of the Ni-63 microirradiator are discussed. In chapter 8, future work is entailed in continuing on with this thesis project. The work presented in the thesis is concluded in chapter 9.

Identifying the matter in the universe is one of the main challenges of modern cosmology and astrophysics. An important part of this matter seems to be made of non-baryonic particles. EDELWEISS is a direct dark matter search using cryogenic germanium bolometers in order to look for particles that interact very weakly with the ordinary matter, generically known as WIMPs. An important challenge for EDELWEISS is the radioactive background and one of the ways to identify it is to use a larger variety of target crystals. Sapphire is a light target which can be complementary to the germanium crystals already in use. Spectroscopic characterization studies have been performed using different sapphire samples in order to find the optimum doping concentration for good low temperature scintillation. Ti doped crystals with weak Ti concentrations have been used for systematic X ray excitation tests both at room temperature and down to 30 K. The tests have shown that the best Ti concentration for optimum room temperature scintillation is 100 ppm and 50 ppm at T = 45 K. All concentrations have been checked by optical absorption and fluorescence.<br />After having shown that sapphire had interesting characteristics for building heat-scintillation detectors, we have tested if using a sapphire detector was feasible within a dark matter search. During the first commissioning tests of EDELWEISS II, we have proved the compatibility between a sapphire heat-scintillation detector and the experimental setup.

The radiological dispersal device (RDD) is a weapon of great concern to those agencies responsible for protecting the public from the modern age of terrorism. In order to effectively respond to an RDD event, these agencies need to possess the capability to rapidly identify the radiological agents involved in the incident and assess the uptake of each individual victim. Since medical treatment for internal radiation poisoning is radionuclide-specific, it is critical to identify and quantify the radiological uptake of each individual victim. This dissertation describes the development of automated analytical components that could be used to determine and quantify multiple radionuclides in human urine bioassays. This is accomplished through the use of extraction chromatography that is plumbed in-line with one of a variety of detection instruments. Flow scintillation analysis is used for ⁹⁰Sr and ²¹⁰Po determination, flow gamma analysis is used assess ⁶⁰Co and ¹³⁷Cs, and inductively coupled plasma mass spectrometry is used to determine actinides. Detection limits for these analytes were determined for the appropriate technique and related to their implications for health physics.

Scintillation based gamma-ray detectors are widely used in medical imaging, high-energy physics, astronomy and national security. Scintillation gamma-ray detectors are field-tested, relatively inexpensive, and have good detection efficiency. Semi-conductor detectors are gaining popularity because of their superior capability to resolve gamma-ray energies. However, they are relatively hard to manufacture and therefore, at this time, not available in as large formats and much more expensive than scintillation gamma-ray detectors. Scintillation gamma-ray detectors consist of: a scintillator, a material that emits optical (scintillation) photons when it interacts with ionization radiation, and an optical detector that detects the emitted scintillation photons and converts them into an electrical signal. Compared to semiconductor gamma-ray detectors, scintillation gamma-ray detectors have relatively poor capability to resolve gamma-ray energies. This is in large part attributed to the "statistical limit" on the number of scintillation photons. The origin of this statistical limit is the assumption that scintillation photons are either Poisson distributed or super-Poisson distributed. This statistical limit is often defined by the Fano factor. The Fano factor of an integer-valued random process is defined as the ratio of its variance to its mean. Therefore, a Poisson process has a Fano factor of one. The classical theory of light limits the Fano factor of the number of photons to a value greater than or equal to one (Poisson case). However, the quantum theory of light allows for Fano factors to be less than one. We used two methods to look at the correlations between two detectors looking at same scintillation pulse to estimate the Fano factor of the scintillation photons. The relationship between the Fano factor and the correlation between the integral of the two signals detected was analytically derived, and the Fano factor was estimated using the measurements for SrI₂:Eu, YAP:Ce and CsI:Na. We also found an empirical relationship between the Fano factor and the covariance as a function of time between two detectors looking at the same scintillation pulse. This empirical model was used to estimate the Fano factor of LaBr₃:Ce and YAP:Ce using the experimentally measured timing-covariance. The estimates of the Fano factor from the time-covariance results were consistent with the estimates of the correlation between the integral signals. We found scintillation light from some scintillators to be sub-Poisson. For the same mean number of total scintillation photons, sub-Poisson light has lower noise. We then conducted a simulation study to investigate whether this low-noise sub-Poisson light can be used to improve spatial resolution. We calculated the Cramér-Rao bound for different detector geometries, position of interactions and Fano factors. The Cramér-Rao calculations were verified by generating simulated data and estimating the variance of the maximum likelihood estimator. We found that the Fano factor has no impact on the spatial resolution in gamma-ray imaging systems.

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M. Sc.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. [167]-170). Also available via the World Wide Web.

Thesis (MSc.-Res.)--University of Wollongong, 2008. / Typescript. Includes bibliographical references: leaf 66-69.

Thesis (M.S. in Physics)--Naval Postgraduate School, January 1964. / Thesis Advisor(s): Hawns, William W. "January 1964." Description based on title screen as viewed on June 2, 2010 DTIC Descriptor(s): (Neutron Flux), (Research Reactors, Measurement, Neutron Activation, Reactor Lattice Parameters, Reactor Control, Power, Reactor Operation, Reactor Power Density, Scintillation Counters, Foils (Materials), Gold, Gamma Rays, Nuclear Radiation Spectrometers, Indium, Monitors, Errors, Probability, Statistical Analysis. DTIC Identifier(s): AGN-201 Reactors. Includes bibliographical references (p. 20). Also available in print.