An avalanche effect yielding inherent gain can be exploited in thin, single-crystal chemical vapor deposition (scCVD) diamond. It occurs when a high enough bias is applied across the diamond thickness while avoiding breakdown. This charge multiplication effect was studied previously with alpha particles and heavy ions either by using the transient current technique or by measuring the energy spectrum. The measurements we obtained to evaluate the charge multiplication performance of a 10 μm thick scCVD diamond detector used a novel approach—we employed an electrometer to characterize the response of the detector by performing directly coupled current measurements (time-averaged charge, at 1 Hz sampling) when exposed to 14.1 MeV neutrons from deuterium-tritium fusion. We measured both the dark and irradiated currents from the detector over a range of applied displacement field values from 2 to 75 V/μm. A histogram method with central mean and standard deviation width was used to determine the current over each measurement duration typically from 100 to 300 seconds. The dark-subtracted irradiated current (i.e., contrast) was used to evaluate the gain of the detector at each applied displacement field. The contrast at an applied displacement field between 15 and 20 V/μm was higher than the expected linear increase in contrast proportional to the increased applied bias, indicating the possible presence of avalanche events in the diamond. The detector response also indicated possible polarization and charge depletion effects. These results provide an opportunity to further explore the use of thin scCVD diamond as a fast neutron current mode detector with inherent gain.
During the dynamic compression of a subcritical object that is simultaneously receiving a neutron pulse, its fission 𝛾-ray signal can be measured and the die-off in its time-response is directly correlated with peak reactivity and neutron multiplication within. Hence, the signal measured by a time-of-flight (TOF) gamma detector array operating in currentmode is a convolution of the incident neutron pulse, detector time response, and fission signal from the object. Accurate determination of the true fission 𝛾-ray emission rate from the object requires a detector that is both highly sensitive (to preserve statistics) and fast (to minimize distortion of the signal shape from the object). In collaboration with scientists at Mission Support and Test Services (MSTS) and the Nevada National Security Site (NNSS), a TOF 𝛾-ray detection system was designed at Los Alamos National Laboratory (LANL) to meet these experimental objectives. This system consists of a 3-m-diameter array of ∼ 150 hexagonal detector pixels, each operating in current mode. Each pixel consists of a large-volume, fast-plastic scintillator coupled to a 5-in photomultiplier tube via a plastic light guide that uses total internal reflection for optical transport to the photocathode. Development details of this pixel design using statistical and time response metrics, laboratory measurements of full pixels and photomultiplier tubes, and high-fidelity GEANT4 simulations, are given. In closing, fielding considerations and expected performance capabilities for the full detector array are also described.
Diamond photoconductive detectors have been shown to detect fast neutrons with high gamma insensitivity. Depending on the application and the incident neutron energy, there are many possible choices when considering how diamond elements may be sized, arranged, and instrumented. As part of our design effort, we are using Geant4 and MCNP6.2 to simulate the effects of fast neutrons impinging on diamond detectors ranging in thickness from a few microns to a few hundred microns that are 4 mm on a side with intervening materials and other physical parameters. The models may be used to compare diamond detector measurements with incident neutrons ranging from ~1 to 14.1 MeV to better understand the nuclear and atomic physics effects contributing to an electronic signal. We are investigating pulse height, signal-to-noise ratio, and timing characteristics of prototype single-crystal chemical vapor deposition diamond detectors.
Often only a single physical process or component is investigated in the simulation of radiation detector systems. The results are then considered to be representative of what is expected in the correlating physical experiment. Although singular assessments may serve as a good estimate, the overall performance of a radiation detector system depends on several physical processes and the performance of all components within the system. Our Geant4-based multiphysics simulation toolkit couples radiation transport with optical photon processes, providing simulations of radiation detector systems components from the scintillator through the photocathode of the photodetector. Work to incorporate the backend detector components, including the complete photodetector and subsequent electronics (e.g., amplifiers, digitizers), is underway. Geant4 is used to model the radiation transport and optical photon processes that occur in the front-end detector system components when exposed to a chosen source. These components include the scintillator, detector housing, optical coupling to the photodetector, and photocathode of the photodetector. Characteristics of several detector systems that have been studied include time response; pulse height spectra; number of photoelectrons per MeV; detector efficiency versus incident quanta energy; and effects on detector response due to change in geometries, materials, and reflectivity. Comparison of these characteristics by means of this toolkit enables the selection of the optimal individual components; thus, it is possible to specify the radiation detector system best suited to meet the requirements of any physical experiment.
Silicon-based photodetectors offer several benefits relative to photomultiplier tube-based scintillator systems. Solid-state
photomultipliers (SSPM) can realize the gain of a photomultiplier tube (PMT) with the quantum efficiency of silicon.
The advantages of the solid-state approach must be balanced with adverse trade-offs, for example from increased dark
current, to optimize radiation detection sensitivity. We are designing a custom SSPM that will be optimized for green
emission of thallium-doped cesium iodide (CsI(Tl)). A typical field gamma radiation detector incorporates thallium
doped sodium iodide (NaI(Tl)) and a radiation converter with a PMT. A PMT's sensitivity peaks in the blue wavelengths
and is well matched to NaI(Tl). This paper presents results of photomultiplier sensitivity relative to conventional SSPMs
and discusses model design improvements. Prototype fabrications are in progress.
Conference Committee Involvement (1)
Penetrating Radiation Systems and Applications VII
2 August 2005 | San Diego, California, United States
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