V. Linhart

Efficiency of composite boron nitride neutron detectors in comparison with helium-3 detectors

It has been clearly demonstrated that the composite boron nitride (BN) semiconductor polycrystalline bulk detectors with BN grains embedded in a polymer matrix operate as an effective detector of thermal neutrons even if they contain natural boron only. A reasonable signal to noise ratio has been achieved with detector thickness of about 1mm. A Monte Carlo simulation of neutron reactions in the BN detector was done to estimate the thermal neutron detection efficiency. The result was compared with widely used He3 based detectors to prove advantages of BN detectors, which are especially promising for neutron imaging and for large area sensors.

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.

Scanning of irradiated silicon detectors using alpha particles and low-energy protons

Abstract In a spectroscopic study of non-irradiated and proton-irradiated silicon diodes, the detectors were illuminated from the front side and from the rear side by various alpha particle sources (mainly ThC’) and by monoenergetic protons with energies from 1.0 to 2.5 MeV. Their response characteristics have been studied as a function of the incoming particle energy and the applied bias voltage. The charge collection efficiency was determined as a function of fluence.

Charge sharing studies with a Medipix1 pixel device

Abstract The Medipix1 chip is a prototype (digital) CMOS imaging chip that emerged from particle detection in high energy physics experiments. It was designed at CERN following specifications from the Medipix1 Collaboration. This contribution aims at demonstrating the applicability of Medipix1 Si and GaAs pixel devices for alpha and X-ray particle detection. A study of “charge sharing” between pixels and test of “spatial resolution” using narrow beam and “edge” image contrast was performed. For photons with energy

Polycrystalline BN and LiF Based Semiconductor Alpha Particle and Neutron Detectors

Composite, polycrystalline, semiconductor, hexagonal BN alone or mixed with B₄C or BMg₂ and cubic LiF were embedded in an insulating matrix which acts as a binder and tested as alpha particle and neutron detectors. The boron containing semiconductors have the natural abundance of ¹⁰B which is ~20%. In the case of LiF is the natural content of ⁶Li only 7.4%. The matrix binder further reduces the contents of ¹⁰B and ⁶Li to about one half. For alpha particles from a ²⁴¹Am alpha source of ~10⁵ cm^-2s^-1, 5.5 MeV gives the detector a wide peak in the spectrum. In the case of the thermal neutron detection, if the sources are very weak of only ~100 cm^-2sec^-1, the neutron counting is possible by subtracting the total signal from the noise. The resulting a signal to noise (S/N) ratio is of only ~2 and one needs a larger collection time of hours for a larger number of pulses to be collected. With a source of a higher neutron flux such as a nuclear reactor, which can offer flux of 10⁷ cm^-2s^-1, is the S/N ratio of ~33. LiF composites with only 3.7% of ⁶Li could also detect very weak neutron sources, but the very low amount of ⁶Li produced a S/N ratio of ~1.4. Compared with widely used ³He detectors is the thermal neutron detection efficiency of the BN and LiF based detectors significantly higher for the same sizes of detectors. Even at a pressure of 3 atmospheres of an inch in diameter ³He detector is the detection efficiency 64% compared with ~72% of a 1 mm thick composite BN detector. Results of enriched ⁶LiF and mixed BN composite detector measurements with stronger neutron sources will be also reported.

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.

STUDY OF GAAS DETECTOR RESPONSE WITH LOW-ENERGY PROTONS

Abstract Semi-insulating Czochralski-grown GaAs detectors were studied by monoenergetic protons with energies from 0.45 to 2.40 MeV. The response characteristics of the detector irradiated from the front side and the back side are shown. An attempt to account for the splitting of the observed peaks in terms of large scale potential fluctuations expected in this material, is presented.

Integrated USB based readout interface for silicon strip detectors of the ATLAS SCT module

An integrated portable USB based readout interface for the ATLAS semiconductor trackers (SCT) has been built. The ATLAS SCT modules are large area silicon strip detectors designed for tracking of high-energy charged particles resulting in collisions on Large Hadron Collider (LHC) in CERN. These modules can be also used on small accelerators for medical or industry applications where a compact and configurable readout interface would be useful. A complete custom made PC-host software tool was written for Windows platform for control and DAQ with build-in online visualization. The new constructed interface provides integrated power, control and DAQ and configurable communication between the detector module and the controlling PC. The interface is based on the Field Programmable Gate Array (FPGA) and the high speed USB 2.0 standard. This design permits to operate the modules under high particle fluence while minimizing the dead time of the whole detection system. Utilization of the programmable device simplifies the operation and permits future expansion of the functionality without any hardware changes. The device includes the high voltage source for detector bias up to 500 V and it is equipped with number of devices for monitoring the operation and conditions of measurement (temperature, humidity, voltage). These features are particularly useful as the strip detector must be operated in a well controlled environment. The operation of the interface will be demonstrated on data measured with different particles from radiation sources.

Dental implant imaging with pixel detectors

We make use of the state-of-the-art semiconductor detector Medipix2 in order to obtain high resolution low-dose images of samples with dental implants in real time. The goal is to develop a method of direct observation and monitoring of the interface between dental implants and the surrounding material and tissue.

Silicon planar MESA diodes as radiation detectors

Abstract Silicon diodes built according to the planar MESA process are investigated for their possible use as radiation detectors. Electrical characteristics of planar MESA (PM) detectors are studied as a function of applied bias voltage and 10 MeV proton fluence in view of possible application in high-radiation environment. A comparison is made between the features of PM and silicon standard planar (SP) detectors.