Mark D. Hammig

Design considerations for thin film coated semiconductor thermal neutron detectors-I: basics regarding alpha particle emitting neutron reactive films

Semiconductor-based thermal neutron detectors provide a compact technology for neutron detection and imaging. Such devices can be produced by externally coatingsemiconductor-charg ed-particle detectors with neutron reactive films that convert free neutrons into charged-particle reaction products. Commonly used films for such devices utilize the 10 B(n,a) 7 Li reaction or the 6 Li(n,a) 3 H reaction, which are attractive due to the relatively high energies imparted to the reaction products. Unfortunately, thin film or ‘‘foil’’ type thermal neutron detectors suffer from self-absorption effects that ultimately limit neutron detection efficiency. Design considerations that maximize the efficiency and performance of such devices are discussed. Theoretical and experimental results from front coated, back coated, and ‘‘sandwich’’ designs are presented. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.W

1-D POSITION SENSITIVE SINGLE CARRIER SEMICONDUCTOR DETECTORS

Abstract A single polarity charge sensing method has been studied using coplanar electrodes on 5 mm cubes of CdZnTe γ-ray detectors. This method can ameliorate the hole trapping problem of room-temperature semiconductor detectors. Our experimental results confirm that the energy resolution is dramatically improved compared with that obtained using the conventional readout method, but is still about an order of magnitude worse than the theoretical limit. A method to obtain the γ-ray interaction depth between the cathode and the anode is presented here. This technique could be used to correct for the electron trapping as a function of distance from the coplanar electrodes. Experimental results showed that a position resolution of about 0.9 mm FWHM at 122 keV can be obtained. These results will be of interest in the design of higher performance room-temperature semiconductor γ-ray detectors.

An Investigation of Nanocrystalline Semiconductor Assemblies as a Material Basis for Ionizing-Radiation Detectors

Nanocrystalline (NC) semiconductor materials have previously been studied as a means of increasing the exciton multiplicity upon the impingement of visible light, for applications such as solar cells. If the multi-exciton states have highly uniform multiplicities across macroscopic NC samples, then one can also potentially quench the statistical counting noise associated with charge-carrier creation in the bulk. We thus assess the viability of using NC semiconductor materials for the detection of ionizing radiation. Using CdTe and PbSe NC films, we report on the colloidal synthesis and deposition procedures for spherical NC particles with dimensions less than 10 nm. In particular, using a thioglycolic-acid (TGA)-stabilized CdTe NC solution adsorbed to polycations, CdTe nanocrystalline films were deposited on glass and metallic substrates using either the layer-by-layer (LBL) method or by using drop-casting techniques. A scanning electron microscope was used to study the surface morphology and impurity concentrations of the CdTe and PbSe-films, and by sandwiching the films between evaporated metallic electrodes, the junction properties of the material were studied, and rectifying characteristics were observed. The resulting depletion region and the materials response to alpha particle impingement were studied.

Nuclear radiation detection via the deflection of pliable microstructures

Abstract The feasibility of detecting single quanta of radiation via mechanical deflection is examined. When a particle strikes an object, momentum is transferred to the impacted body. The resulting body motion can be correlated to the energy of the incident particle. Microelectronic fabrication techniques are used to build the sensing microstructures. Typical levers which have been fabricated have dimensions of width =1 μ m, length =5 μ m, and thickness =0.05 μ m. Linear beam deflection theory is used to model the vibration of the levers. Theoretical pulse height distributions demonstrate that the signals from heavy-ion impacts are measurable, but those from light-ion collisions are swamped by the thermal noise contribution. Thus, if light ions are to be detected, then the effects of thermal noise must be reduced.

Delay-line electrode partitioning as a means of simplified position-sensing

We have investigated various means by which charge-cloud mapping can be accomplished via simplified detector architectures, in order to enable ubiquitous and economical detector deployment. After investigating, via models and measurements, both amplitude and time-based methods, we established that amplitude-based methods; that is, methods that employ pulse-shape analysis, require relatively complicated detector structures in order to achieve fine position sensing. A structurally simple time-based scheme, in which a delay-line electrode structure is used to isolate the position of charge collection, however can be implemented to provide fine lateral position-sensing. As discussed in this paper, we completed the fabrication of a proof-of-concept device and used it to demonstrate the validity of the technique.

The design and construction of a mechanical radiation detector

A method for constructing pliable bodies that respond to radiation impact is discussed. Microelectronic fabrication techniques are used to build the deflecting microstructures. The body motion is detected by measuring the capacitance change between the lever and a ground pad. This measurement technique requires levers that have low intrinsic stress. The stresses for polysilicon and silicon nitride thin films are measured. Stress balance is demonstrated between compressive polysilicon and tensile silicon nitride layers. The stress of 45.9 nm of doped polysilicon is reduced from -2140.08 MPa to -106.40 MPa by the addition of a 43.0 nm silicon nitride layer.

The degree of enhancement in a gamma-ray image gleaned from recoil-electron tracking

In order to further enable the deployment of nuclear radiation imaging instruments, one can include the directional information of the products of a radiation’s interaction. In this paper, we quantify the degree of improvement that can be achieved for recoil-electrons in silicon, and further, how that enhancement depends on the electron energy as well as the detector’s lateral spatial resolution. The simulated results show that for detectors of even moderate spatial resolution, the gamma-ray direction can be highly localized. Specifically, if the detector has 100 micrometer spatial resolution, then the direction of the recoil-electron can be confined to 10° or better, for recoil energies greater than 300 keV, compared with a simple back-projection onto a 360° cone for Compton-scatter events. This same performance can be achieved with a 10 micrometer design down to a few 10’s of keV recoil-electron energy; that is, over the entire energy range of interest. I. INTRODUCTION

Fabrication and signal readout of the Si-based delay-line radiation detector

In order to image the radiation field, instruments measure the position of interaction of the incident quanta, a task typically accomplished by partitioning the detector or its readout. We previously validated a structurally simple time-based measurement scheme, in which a delay-line electrode structure is used to isolate the position of charge collection [1]. If the transmission-line electrode structure is properly designed, then the time difference between the pulses at the readout ends can be used to measure the position at which the charge was collected. Furthermore, if the timing uncertainty is significantly less than the transit time along a single strip in that design, then the position can be isolated to one strip of the pattern or better. In this paper, we describe the fabrication techniques used to overcome the two greatest limitations of the previous detectors; namely, high leakage currents and high propagation losses. In the following, we describe, through both models and measurements, the means by which: a) one can achieve impedance matching- by conversion of the electrode into a 50 Ω asymmetric strip line, and b) one can minimize the propagation losses- by the use of laminated and/or thicker electrodes and the use of low-loss dielectrics as a part of asymmetric strip line design. To accomplish low leakage designs, we describe the fabrication of both delay-line PIN devices as well as detectors with metal-semiconductor junctions. Finally, we discuss the trade-offs when employing current-sensing for semiconductor-based radiation detectors and demonstrate the means by which the current profiles are translated into maps at which the charge is initially deposited.

Optimization of the position resolution in semiconductor detectors

In our effort to image the cloud of mobile charge- carriers induced by a radiation interaction in a solid-state material, we have performed a fundamental investigation of the electrode design and pulse shape analysis routines required to optimize the detectors position resolution, when limited by both temporal and carrier uncertainty. The interactions of ions and gamma-rays with silicon are modeled using Monte Carlo techniques so that the currents induced on the measurement electrodes can be predicted. Those predictions are then compared to the measured results derived from a high-resistivity silicon detector, on which metal-semiconductor junctions are used to form the rectifying and ohmic contacts.

Suppression of interface-induced noise by the control of electron-phonon interactions

We study the influence of various types of contacting media and contact area on the current-fluctuation level in semiconductors, testing the supposition that the electronic noise is governed, in part, by phonon-leaking dynamics to the environment. Using passivated and gettered silicon PIN diodes as experimental test-beds, the presented data lends credence to the prediction that the phonon-refraction characteristics of the semiconductor-metal interface substantially impacts the current fluctuations in the solid. Specifically, if one implements metallic contacts with lower phonon-reflecting characteristics, such as those composed of silver or palladium, or if one increases the area through which phonons can leak to the surrounding environment, then the leakage current decreases.