Z. Kohout

Highly sensitive silicon detectors of thermal neutrons

Planar semiconductor diodes supplemented with a layer of an appropriate neutron converter such as 6LiF can be used for thermal neutron counting or imaging. Neutrons interacting in the converter generate alphas and tritons which enter the semiconductor and are detected there. However, simple planar devices suffer from limited detection efficiency which cannot reach more than about 5%. The limit in detection efficiency can be overcome by etching a 3D microstructure of trenches, pores or columns in the detector and filling it with the neutron converter. The overall neutron detection efficiency of such structure with pores was simulated. The results indicate an increase in the detection efficiency by factor of 6 in comparison with a standard planar neutron detector. Samples with different silicon column sizes were fabricated to study the electrical properties of 3D structures. The charge collection efficiency in silicon columns from 10 mum to 800 mum wide and 80 mum high was measured. Single pad detectors with pores were also fabricated and tested for thermal neutron detection. The samples have square pores of 20 mum wide, ~60 mum deep. The pore pitch is 70 mum. 6LiF was used as the neutron converter in all cases. Pulse height spectra of the filled samples irradiated by thermal neutrons were measured. The measurement proved functionality of such detectors and its usability for thermal neutron detection.

Spatially correlated and coincidence detection of fission fragments with the pixel detector Timepix

Charged-particle coincidence correlated measurements such as angular correlations between rare and main fission fragments measured with conventional detectors provide only partial and limited information (energy cutoff, narrow range of studied ion Z numbers). Many of these drawbacks arise from the standard solid state detectors used so far which can be solved simultaneously by usage of highly segmented single-quantum counting pixel detectors. The Timepix pixel device, which is equipped with energy and time sensitivity capability per pixel, provides high granularity, wide dynamic range and per pixel threshold. This detector operated with integrated USB-readout interfaces such as the USB 1.0 and FITPix devices and the data acquisition software tool Pixelman, both developed for the pixel detectors of the Medipix-family, enables a variety of instrumental configurations, visualization, real-time event-by-event selection as well as vacuum and portability of operation for flexible measurements on different targets and setups. These features combined with event track analysis provide enhanced signal to noise ratio with a high suppression of background and unwanted events. The detector provides multi-parameter information (position, energy and time) for basically all types of ionizing particles in a wide dynamic range of energy (pixel energy threshold ≈ 4 keV), interaction/arrival time (timepix clock step ≥ 100 ns) and position (pixel size = 55 μm). High selectivity is achieved by spatial and time correlation in the same sensor. In addition, several detectors can be run in coincidence. The open and close exposition (shutter) time as well as the readout DAQ can be fully synchronized. For this purpose, we have assembled a modular multi-parameter, tunable and extendable coincidence detector array system based on two and more Timepix devices which can be coupled with supplementary detectors (solid state ΔE detectors and/or ionization chambers) for enhanced ion selectivity. We describe the individual configurations and techniques together with the experiments carried out at several neutron beam/source facilities. We summarize the results and capabilities of application.

Study of the characteristics of silicon MESA radiation detectors

Abstract The MESA process for building silicon diodes is described. I – V and C – V features of MESA detectors are given. Results of pulse-height spectra measurements with α particles incident on the front and back sides of a MESA diode establish the energy resolution of these detectors, show the evolution of their response as a function of applied bias voltage, and bring information about the influence of MESA structure on charge collection. The characteristics of MESA detectors as a function of fluence are investigated in view of their possible use in high particle fluence environment. Charge collection data obtained from the measurements of the current-pulse response induced by β and α particles are presented as a function of applied bias voltage and particle fluence. Some electrical characteristics of detector material, namely the effective impurity or dopant concentrations ( N eff ), the electron ( μ e ) and hole ( μ h ) mobilities, are studied as a function of fluence using a charge transport model. A comparison is made with the features of standard planar (SP) silicon detectors.

Silicon sensors with pyramidal structures for neutron imaging

Neutron detection is a valuable tool in nuclear science research, homeland security, quality assurance in nuclear plants and medical applications. Recent developments and near future instrumentations in neutron imaging have a need for sensors with high spatial resolution, dynamic range, sensitivity and background discrimination. Silicon based neutron detectors can potentially fulfil these requirements. In this work, pad and pixel detectors with pyramidal micro-structures have been successfully fabricated that should have an improved detection efficiency when compared to conventional planar devices. Titanium di-boride (TiB2) and lithium fluoride (LiF) were deposited as the neutron converters. Excellent electrical performances were measured on both simple pad and pixel detectors. A selection of pad detectors was examined by alpha spectroscopy. Measurement with thermal neutrons from a 241Am-Be source shows an improvement in relative efficiency of up to 38% when compared to conventional planar devices.

Characterization and calibration of novel semiconductor detectors of thermal neutrons for ESA space applications

For the study and detection of neutrons in space environments such as planetary and earth orbiting missions semiconductor silicon diode detectors have been characterized and calibrated at various thermal neutron sources. Two types of diodes adapted for thermal neutron detection were investigated: silicon MESA planar detectors equipped with thin 6LiF layers and silicon heterodiodes with a layer of natural boron or enriched 10B. The response and absolute detection efficiency have been measured. The influence of bias voltage and converter layer thickness were studied. As neutron sources we used a homogenous isotropic thermal neutron field by a set of PuBe radionuclide sources placed in a graphite pile as well as a parallel thermal neutron beam with high Cd ratio (105) and suppressed gamma background. Depending on the converter layer thickness and/or boron layer thickness as well as the choice of the threshold level, efficiencies of approximately 1% are obtained for both the silicon diodes with thin 6LiF and the boron rich silicon detectors. These values guarantee optimal stability of operation in remote and different environments as well as maximum signal-to-noise ratio by enhanced suppression of unwanted signals and gamma background.

Position-sensitive coincidence detection of nuclear reaction products at the Prague Van-de-Graaff accelerator

In low-energy nuclear reactions of astrophysical interest or fusion studies the spatial- and time-correlated detection of two and more reaction products can be a valuable tool in studies of reaction mechanisms, resolving reaction channels and measuring angular distributions of reaction products. For this purpose we constructed a configurable array of position-sensitive detectors based on the hybrid semiconductor pixel detector Timepix. Additional analog-signal electronics provide self-trigger together with extended multi-device control and synchronized readout electronics by a customized control and coincidence unit. The instrumentation, developed and used for detection of fission fragments in spontaneous and neutron induced fission as well as in charged particle detection in neutron induced reactions, is being implemented for low-energy light-ion induced nuclear reactions. Application and demonstration of the technique with two Timepix detectors on p+p elastic scattering at the Van-de-Graaff (VdG) accele...

Development of neutron image sensor technology for AD-BANG project

A demonstrator of the portable position sensitive neutron detecting device for neutron imaging in a mixed radiation field is presented. A detector of the device is based on multilayer perpendicularly crossed strip sensors interlaced with neutron converter. A 6LiF converter producing energetic alpha particles and tritons is used. The converter permits precise determination of a position of the neutron interaction even in a radiation fields with a high gamma background. The neutron detection efficiency is increased with a 3D structured surface of the multilayer silicon strip sensor. The demonstrator is comprised of the neutron sensitive multilayer silicon strip detector, a 128 channel general purpose charge sensitive amplifier ASICs IDEAS VATAGP8 with nominal dynamic range of +/− 250 fC, a 13 bit analogue to digital conversion and an FPGA control. Basic characteristics of the amplifier like amplifier linearity probed with an external charge generator is shown. Moreover, results of experimental tests of the demonstrator with alpha, beta, gamma sources and neutrons is demonstrated. The device may be used for neutron imaging via position sensitive detection of particular neutrons on the basis of coincidence detection of neutron conversion products or on the basis of response comparison of individual strips. An effect of charge sharing between individual strips on position resolution will be elaborately discussed.