Solid-state Ionization Chamber Research Articles

Characterization of Diamond and Silicon Carbide Detectors With Fission Fragments

Experimental fission studies for reaction physics or nuclear spectroscopy can profit from fast, efficient, and radiation-resistant fission fragment (FF) detectors. When such experiments are performed in-beam in intense thermal neutron beams, additional constraints arise in terms of target-detector interface, beam-induced background, etc. Therefore, wide gap semi-conductor detectors were tested with the aim of developing innovative instrumentation for such applications. The detector characterization was performed with mass- and energy-separated fission fragment beams at the ILL (Institut Laue Langevin) LOHENGRIN spectrometer. Two single crystal diamonds, three polycrystalline and one diamond-on-iridium as well as a silicon carbide detector were characterized as solid state ionization chamber for FF detection. Timing measurements were performed with a 500-µm thick single crystal diamond detector read out by a broadband amplifier. A timing resolution of ∼10.2 ps RMS was obtained for FF with mass A = 98 at 90 MeV kinetic energy. Using a spectroscopic preamplifier developed at INFN-Milano, the energy resolution measured for the same FF was found to be slightly better for a ∼50-µm thin single crystal diamond detector (∼1.4% RMS) than for the 500-µm thick one (∼1.6% RMS), while a value of 3.4% RMS was obtained with the 400-µm silicon carbide detector. The Pulse Height Defect (PHD), which is significant in silicon detectors, was also investigated with the two single crystal diamond detectors. The comparison with results from α and triton measurements enabled us to conclude that PHD leads to ∼50% loss of the initial generated charge carriers for FF. In view of these results, a possible detector configuration and integration for in-beam experiments has been discussed.

Photoconductive gain in single crystal diamond detectors

Diamond crystals equipped with two metal electrodes can be operated as solid state ionization chambers for the detection of energetic radiation. Under irradiation with single α particles, the generated free electrons and holes are collected with a maximum efficiency close to 100%. When the same detectors are used for dosimetry in high intensity and high energy photon or particle beams, photoconductive gain G with values up to ≈106 is frequently observed as described in the literature. In this work, we studied theoretically the irradiation induced conductivity of perfect diamond single crystals with ohmic contacts containing nitrogen and boron with concentrations NN and NB, respectively, as only chemical impurities. Based on four rate equations, two considering the charge states of N and B and two the concentrations of free carriers n and p, and, additionally, the neutrality condition, we could derive analytical solutions for the gain G as a function of impurity concentrations, crystal thickness, and excitation density. It turned out that G varies systematically with the compensation ratio R=(NN−NB)/NB over five orders of magnitude. For R≈103, the gain G is close to unity. With decreasing R, the gain increases ∝1/R until saturation is reached for R≪1 and G≈104–105. Our theoretical data yield plausible explanations for the major trends that have been found experimentally in previous studies. They provide a valuable guideline for the future synthesis of diamond crystals to be used for manufacturing UV and radiation detectors.

Pulse-resolved intensity measurements at a hard X-ray FEL using semi-transparent diamond detectors.

Solid-state ionization chambers are presented based on thin diamond crystals that allow pulse-resolved intensity measurements at a hard X-ray free-electron laser (FEL), up to the 4.5 MHz repetition rate that will become available at the European XFEL. Due to the small X-ray absorption of diamond the thin detectors are semi-transparent which eases their use as non-invasive monitoring devices in the beam. FELs are characterized by strong pulse-to-pulse intensity fluctuations due to the self-amplified spontaneous emission (SASE) process and in many experiments it is mandatory to monitor the intensity of each individual pulse. Two diamond detectors with different electrode materials, beryllium and graphite, were tested as intensity monitors at the XCS endstation of the Linac Coherent Light Source (LCLS) using the pink SASE beam at 9 keV. The performance is compared with LCLS standard monitors that detect X-rays backscattered from thin SiN foils placed in the beam. The graphite detector can also be used as a beam position monitor although with rather coarse resolution.

Test study of boron nitride as a new detector material for dosimetry in high-energy photon beams

The aim of this test study is to check whether boron nitride (BN) might be applied as a detector material in high-energy photon-beam dosimetry. Boron nitride exists in various crystalline forms. Hexagonal boron nitride (h-BN) possesses high mobility of the electrons and holes as well as a high volume resistivity, so that ionizing radiation in the clinical range of the dose rate can be expected to produce a measurable electrical current at low background current. Due to the low atomic numbers of its constituents, its density (2.0 g cm−3) similar to silicon and its commercial availability, h-BN appears as possibly suitable for the dosimetry of ionizing radiation. Five h-BN plates were contacted to triaxial cables, and the detector current was measured in a solid-state ionization chamber circuit at an applied voltage of 50 V. Basic dosimetric properties such as formation by pre-irradiation, sensitivity, reproducibility, linearity and temporal resolution were measured with 6 MV photon irradiation. Depth dose curves at quadratic field sizes of 10 cm and 40 cm were measured and compared to ionization chamber measurements. After a pre-irradiation with 6 Gy, the devices show a stable current signal at a given dose rate. The current–voltage characteristic up to 400 V shows an increase in the collection efficiency with the voltage. The time-resolved detector current behavior during beam interrupts is comparable to diamond material, and the background current is negligible. The measured percentage depth dose curves at 10 cm × 10 cm field size agreed with the results of ionization chamber measurements within ±2%. This is a first study of boron nitride as a detector material for high-energy photon radiation. By current measurements on solid ionization chambers made from boron nitride chips we could demonstrate that boron nitride is in principle suitable as a detector material for high-energy photon-beam dosimetry.

Measurement of submicrometric intrinsic spatial resolution for active pixel sensors

To measure the intrinsic spatial resolution of silicon pixel sensor is usually a non-trivial task, particularly for small pixel sizes where the multiple scattering may be the limiting factor. In this work, we present a new measurement technique to obtain the intrinsic spatial resolution of silicon active pixel sensors. The method relies on the capability of the device to record the passage of a charged particle, incoming at a grazing angle, over several tens or hundreds of pixels, acting as a solid state ionization chamber and thus defining a track. The track will then be fitted by a line and the intrinsic spatial resolution will be obtained using two methods: i) extracted by the σ of the fit; ii) defining a telescope-on-chip configuration to find a residual distribution.Comparison with a more traditional measurement (telescope configuration) and a discussion on the limit of this technique, when the pixel size shrinks, will also be presented.

IBIC characterization of an ion-beam-micromachined multi-electrode diamond detector

Deep Ion Beam Lithography (DIBL) has been used for the direct writing of buried graphitic regions in monocrystalline diamond with micrometric resolution. As part of the development and the characterization of a fully ion-beam-micromachined solid-state ionization chamber, a device with interdigitated electrodes was fabricated by using a 1.8MeV He+ ion microbeam, which scanned a 40μm thick homoepitaxial detector grade diamond sample grown by chemical vapor deposition (CVD). In order to evaluate the ionizing-radiation-detection performance of the device, charge collection efficiency (CCE) maps were extracted from Ion Beam Induced Charge (IBIC) measurements carried out by probing different arrangements of buried micro-electrodes.The analysis of the CCE maps allowed an exhaustive evaluation of the detector features, in particular the individuation of the different role played by electrons and holes in the formation of the induced charge pulses.Finally, a comparison of the performances of the detector with buried graphitic electrodes with those relevant to conventional metallic surface electrodes evidenced the formation of a dead layer overlying the buried electrodes as a result of the fabrication process.

Single crystal CVD diamond as an X-ray beam monitor

Synchrotron X-ray beams may now be focussed to < 1 μm and devices with similar spatial precision are required to monitor the beam position or provide real-time information for position control. Over the X-ray energy range 5–25 keV, diamond is an outstanding material for the fabrication of ‘semitransparent’ beam monitors. Tests were made at the ESRF with ‘electronic grade’, single crystal diamond samples which were patterned with metal contacts and operated as solid-state ionization chambers. A uniform spatial response flat within 0.2% was seen when the devices were mapped with a sub-micron synchrotron X-ray beam, and the photoelectric current generated was linear with the beam intensity and showed no time lag effects. The beam induced currents were measured both at DC and at the RF frequency of the synchrotron. A sensitivity of < 13 nm to beam movement was obtained.

Radiological X-ray dosimetry with single crystal CVD diamond detectors

Pulsed X-ray measurements have been carried out by using new single crystal epitaxial CVD diamond samples about 100 μm thick grown by microwave CVD in Roma Tor Vergata Laboratories on a 300 μm thick, low cost, HPHT diamond substrate. Both sandwich arranged and surface interdigitated contacts were used for the detectors in order to compare their performances. A standard X-ray radiological apparatus was used with maximum X-ray energies from 50 to 120 keV. Delivered doses were in the range between 0 and 50 mGy, with pulse duration times between 0.01 and 0.5 s were obtained either by using different time-integrated mAs products or, at a fixed value of mAs, by using different values of the bias voltage of X-ray generator. The linearity of the dose signal Q given by the integrated current with respect to dose recorded by ionization chamber Δ was checked by evaluating dose linearity index, i.e. the exponent in the equation Q = AD Δ. The values of Δ were very close to 1 (between 1.02 and 1.07) in the case of a fixed mAs product, while it was around 0.8 at fixed voltage in the range 50–120 kV. The response time of the detector as deduced from the tail of the pulse was between 15 and 20 ms. The reproducibility was very good among different series of measurements, with no memory or priming effect. These results suggest a strong and actually real possibility for diamond detectors to be used as solid state ionization chambers in radiology and for diagnostic purposes.

Signal formation and decay in CdTe x-ray detectors under intense irradiation.

The response of Cd(Zn)Te Schottky and resistive detectors to intense x-rays is investigated in a commercial computed tomography (CT) system to assess their potential for medical diagnostics. To describe their signal height, responsivity, signal-to-noise ratio (SNR), and detective quantum efficiency the devices are modeled as solid-state ionization chambers with spatially varying electric field and charge collection efficiency. The thicknesses and pixel areas of the discrete detector elements are 0.5-2 mm and a few mm2, respectively. The incident spectrum extends from 26 to 120 keV and comprises 10(10) quanta/s cm2. It photogenerates a carrier concentration in the semiconductor that is two to three orders of magnitude above the intrinsic concentration, but remains to a similar extent below the charge densities on the device electrodes. Stable linear operation is achieved with the Schottky-type devices under high bias. Their behavior can be modeled well if negatively charged near-midgap bulk defects with a concentration of 10(11)-10(13) cm-3 are assumed. The bulk defects explain the amount and time constant (about 100 ms) of the detrapping current measured after x-ray pulses (afterglow). To avoid screening by the trapped space charge the bias voltage should exceed 100(V) x [detector thickness/mm]2. Dark currents are of the order of the generation-recombination current, i.e., 300 pA/mm3 detector volume. With proper device design the signal height approaches the theoretical maximum of 0.2 A/W. This high responsivity, however, is not exploited in CT since the SNR is determined here by the incident quantum noise. As a consequence of the detrapping current, the response speed does not meet CT requirements. A medium-term effort for crystal growth appears necessary to achieve the required reduction of the trap density by an order of magnitude. Scintillation based detectors are, therefore, still preferred in fast operating medical diagnostic systems.

A simplified instrument for solid-state high-gamma dosimetry

The characteristics of the radiation-resistant γ-dose-rate meter used as a convenient solid-state ionization chamber, are described. The instrument comprises a probe for exposure-rate measurement and a simple circuit with an I.C. operational amplifier. The temperature coefficient (<0.3%/°C) is constant between 0–50°C at a certain dose rate, and the effects of the integral dose and the dose rate on the temperature coefficient are small. The directional dependence can be neglected by minimizing the volume of the electrode material or by compensation for electron backscattering. The practical usefulness of the dosimeter is discussed in comparison with that of commercial exposure-rate meters.

Silicon Detector System for X-Ray Pulse Calorimetry of Laser-Produced Plasmas*

AbstractThe instrumentation for measuring x-ray yields from laser produced plasma is described. This new type of calorimeter is composed of a silicon detector, a charge-sensitive preamplifier and an analog-to-digital readout scheme for multiplexing up to ten detector outputs.X-rays interacting with the detector produce hole-electron pairs in proportion to the total energy lost in the detector (∼1012 eV). In this application the detector can be characterized as a solid-state ionization chamber. The detector signal is coupled to a charge-sensitive preamplifier which generates a voltage pulse proportional to the x-ray energy absorbed. In this way the x-ray energy is measured by “direct conversion” rather than measuring the temperature rise due to an energy flux.

A solid-state ionization chamber for the high-dose-rate measurement of gamma-rays

A radiation damage-resistant gamma-dose-rate meter in the form of an N/ P-type silicon solar cell was investigated as a solid-state analogue of a gaseous ionization chamber. The ionization current was found to be a linear function of exposure rate in the range of 10 2 ∾ 10 6 R/hr. The sensitivity of the Cu-doped cell, with a bulk resistivity of 200 Ω-cm, is the most stable, and the sensitivity degradation rate is only about 0·1 per cent per 10 6 R. The sensitivity can also be stabilized to some degree by pre-irradiation. The temperature dependence in various cells was found to be less than 0·4 per cent/°C between 5° ∾ 60°C. The non-directional dependence of the dosimeter probe was established to an accuracy of about ±1 per cent.

Energy balance method for pulse shape determination in solid-state ionization chamber

There has been some discussion in the literature about the expression to be used for the determination of pulse shape from a solid-state ionization chamber, particularly from a p-n junction detector. In these detectors a fixed space-charge exists in the depletion region even in the absence of charge pairs produced by the incident radiation. This leads to a non-linear rate of collection of charge. A correct expression for the rate of collection of charge in such a field is obtained from the energy balance between the rate at which the charge acquires energy, the rate at which energy is stored in the electrostatic field, and the rate at which the battery supplies it. The discrepancies in the various expressions have been attributed to the fact that in these derivations the contribution of displacement current is not taken into account properly.

Energy Dependence of Proton Irradiation Damage in Silicon

The energy dependence of radiation damage in silicon for proton energies in the range 1.35 to 130 MeV has been measured by observing the degradation of the bulk minority carrier diffusion length in silicon solar cells. Variability in proton flux determination at four different accelerators was minimized by employing prebombarded solar cells with known minority carrier diffusion lengths as calibrated solid-state ionization chambers. Where beam intensity measurement comparisons with Faraday cups could be made, agreement to better than 5% was obtained. The quantity characterizing the damage rate is the rate of change of the inverse square diffusion length with flux K≡d(1/L2)/dΦ. The 1-Ω-cm p-type silicon degraded, on the average at a rate six times less rapid than 1-Ω-cm n type, independent of energy. Room temperature annealing gave 30% to 50% decrease in K whenever the diffusion length was measured during and after irradiation. The energy variation of K agrees with the variation predicted by Rutherford scattering below 8 MeV, but decreases less rapidly at higher energies. The measured diffusion lengths increased with excess carrier density n from 2% per decade at n=109cm−3 to 20% per decade at n=1014cm−3. The reported results, obtained at low excess carrier density, can be used to predict solar cell degradation under conditions of outer space illumination if the appropriate excess carrier density is used. Failure to take into account the diffusion length variation will result in an underestimate of the solar cell output of less than 7%.

Application of surface barrier solid state ionization chambers to measurements on fission

Abstract Gold-silicon surface barrier detectors ∼ 3 4 ″ in diameter have been made and used in several experiments on the fission process: (1) A pair of detectors, with detecting surfaces parallel and facing each other at a distance comparable with their diameters and with U235 deposited on one detector, has been used to measure the mass distribution in fission with thermal neutrons for several values of the total kinetic energy of the fragments. Values as high as 500 to one for the ratio of asymmetric to symmetric fission were obtained. (2) A similar arrangement, but including a third detector, was used for the measurement of the mass distribution in ternary fission. (3) The ratio of ternary to binary fission for U235 fission with thermal neutrons was determined. (4) The variation of the fission cross section of U233 with energy was measured for neutron energies in the range 6 eV to 19 eV. The usefulness of solid state detectors and their superiority over most other types of known detectors for these measurements is clearly evident.