Ion-implanted Silicon Detectors Research Articles

Advanced Direct Digital Synthesis Generator Design for Transuranic Nuclide Alpha Spectrometry Pulses

Alpha energy spectrum measurement has been employed in the nuclear waste disposal of transuranic nuclides (such as 239Pu and 241Am), supervision, and disposal process. The alpha spectrum is made up of alpha particles, which have a fast-moving helium nucleus and an energy of 4–8 MeV with weak penetration ability. Removing alpha particles from radioactive nuclides is an important scientific issue. In this study, a transuranic nuclide alpha particle pulse generator that produces simulated alpha particle pulses similar to real particles was designed. Field programmable gate array (FPGA) was adopted as its core chip and we obtained the digital pulse waveform using software tracing points while simulating real alpha particles by random numbers. Accordingly, the alpha energy spectrum of a radioactive source 241Am was obtained using a passivated ion-implanted planar silicon (PIPS) detector. Afterward, the alpha particle was extracted from the energy spectrum and was then compared to the alpha particle pulse of the two methods, deriving a result. Here, both groupings of particle pulse waveforms were found to be very similar, and the periodic error of the particle was observed to be less than 1%. Furthermore, the amplitude and time interval of the particle were apparently similar to the actual spectrometry pulse.

Alpha-particle emission probabilities of 231Pa derived from first semiconductor spectrometric measurements

The nuclide 231Pa is a member of the 235U decay chain. It is a complex alpha emitter with 25 identified alpha emissions. Formerly published alpha-particle emission probabilities were derived from measurements taken with magnetic spectrometers. This work presents the first measurements made with semiconductor detectors. High-resolution alpha-particle spectrometry was carried out at CIEMAT and JRC using ion-implanted planar silicon detectors. Alpha-particle emission probabilities of 23 transitions were derived from deconvolutions of the spectra. For the major lines, uncertainties are lower than 1%, a significant improvement to existing data. The new data set will allow a more accurate evaluation of the decay scheme of 231Pa.

EBT3 film calibration in the Bragg peak region for proton beams below 5 MeV

This work presents the study of the optical density as function of absorbed dose for radiochromic films (model EBT3 Gafchromic) irradiated with protons of energies below 5 MeV. Once the calibration curves are obtained, the study of the optical density as function of the linear energy transfer (LET) is presented. Such studies are useful in proton dosimetry applied to pre-clinical and in vitro radiobiological research.Measurements were performed at the National Centre of Accelerators (CNA) in Seville with a proton beam produced by a 3 MV Tandem accelerator. The incident energy onto the films was measured with an ion implanted silicon detector, and the particle fluence was monitored using a parallel plate ionization chamber. With this proton beam it is possible to investigate the interaction of high LET protons with EBT3 films with narrow energy spreads, as compared with therapeutic proton beams in the Bragg peak region. The data of the beam energy at the position of the film were used to simulate the energy deposition in the active layer of the film, by means of Monte Carlo codes. The measurements were performed for dose values up to 30 Gy and for five values of incident energy, obtained with a passive degradation of the beam by interposing mylar foils of different thickness upstream the EBT3 film.For each energy and corresponding value of proton LET, curves of the optical density as function of the dose are provided for the three color channels red, green and blue (RGB). The dose has been calculated considering two scenarios: dose to water (considering the film composition made by water) and dose to lucite (assuming the real composition of the film). The first corresponds to the case of clinical interest and the second to the real situation of the measurements. The comparison shows a non-negligible shift of the calculated doses even if density corrections are included. In the plateau region of the Bragg curve the film response can be well described by a simple polynomial function (at low doses up to about 4 MeV the response is almost linear). In regions covering the Bragg maximum the film response suffers a quenching and the optical density values were found to be 20–30 % lower than expected.

Dose estimates to the public due to 210Po ingestion via cocoa powder from Lolodorf high background radiation area, Cameroon

210Po activity concentrations have been measured in Lolodorf high background radiation area in cocoa beans which are hand-processed into cocoa powder for breakfast purposes to estimate radiological dose to human. 210Po has been also measured in cocoa leaves and compared to the cocoa beans 210Po content. The analysis has been carried out by CANBERRA alpha spectrometry using ion-implanted silicon detectors. 210Po activity concentrations in cocoa beans varied from 2.31 ± 0.23 to 8.09 ± 0.56 Bq.kg−1, while these values varied from 21.7 ± 0.87 to 66.67 ± 1.58 Bq.kg−1 in cocoa leaves. The corresponding mean values are 4.96 ± 1.86 and 42.54 ± 16 Bq.kg−1 on a dry weight basis respectively. The obtained values confirm the fact that 210Po activity concentrations in cocoa leaves are high compared to the cocoa beans due to the deposition of 222Rn daughters in the atmosphere. The mean radiological doses to human were founded to be 0.227, 0.134, 0.083 and 0.062 mSv/year for children 2- to 7-year-olds, 7- to 12-year-olds, 12- to 17-year-olds and for adult respectively. Ingestion of cocoa powder by the most exposed group ages (children) might not exceed the recommended dose limit for members of the public, which is 1 mSv/year.

The AMS measurement of 236U at CIRCE

Accelerator mass spectrometry (AMS) is an ultrasensitive technique for measuring long-lived actinides, e.g., 236U, 237Np and Pu isotopes. In order to improve the detection limit for actinides abundance, and to increase the detection efficiency in actinides AMS measurement, a 16-strip silicon detector was used to identify actinides at the Center for Isotopic Research on Cultural and Environmental heritage in Caserta, Italy. The sensitivity of 236U/238U was 1 × 10−11 by special resolution and 5.0 × 10−12 by time resolution. The pulse height defect of 236U in an ion-implanted silicon detector in the low-energy range with 17.26 MeV is presented.

Typical uncertainties in alpha-particle spectrometry

Alpha-particle spectrometry is routinely performed with the aim of measuring absolute activities, activity ratios between different alpha-emitting nuclides or decay data such as branching factors, alpha emission probabilities and relative half-lives. It is most commonly performed with ion-implanted silicon detectors. Strong features of the technique are the low background levels that can be achieved due to low sensitivity to other types of radiation, the intrinsic efficiency close to 1 which reduces the efficiency calculations to a geometrical problem and the uniqueness of the energy spectra for each α-decaying nuclide. The main challenge is the limitation to the attainable energy resolution, even with thin and homogenous sources, which causes alpha energy peaks to be partially unresolved due to their width and low-energy tailing. The spectral deconvolution often requires fitting of analytical functions to each peak in the alpha spectrum. True coincidence effects between alpha particles and subsequently emitted conversion electrons cause distortions of the alpha spectra which lead to significant changes in the apparent peak area ratios. Optimum energy resolution can only be achieved on very thin sources, which puts constraints on the source preparation techniques. Radiochemical separations may be needed to extract the alpha emitters from voluminous matrices and efficiency tracing is performed by adding in another isotope by known amounts. Typical uncertainty components are discussed by means of some hypothetical examples.

Low-temperature technique of thin silicon ion implanted epitaxial detectors

A new technique of large-area thin ion implanted silicon detectors has been developed within the R&D performed by the FAZIA Collaboration. The essence of the technique is the application of a low-temperature baking process instead of high-temperature annealing. This thermal treatment is performed after B+ ion implantation and Al evaporation of detector contacts, made by using a single adjusted Al mask. Extremely thin silicon pads can be therefore obtained. The thickness distribution along the X and Y directions was measured for a prototype chip by the energy loss of α-particles from 241Am (〈E α 〉 = 5.5 MeV). Preliminary tests on the first thin detector (area ≈ 20 × 20 mm2) were performed at the INFN-LNS cyclotron in Catania (Italy) using products emitted in the heavy-ion reaction 84Kr (E = 35 A MeV) + 112Sn. The ΔE − E ion identification plot was obtained using a telescope consisting of our thin ΔE detector (21 μm thick) followed by a typical FAZIA 510 μm E detector of the same active area. The charge distribution of measured ions is presented together with a quantitative evaluation of the quality of the Z resolution. The threshold is lower than 2 A MeV depending on the ion charge.

Determination of difficult-to-measure actinides in evaporator concentrate

Radiochemical separation of americium, neptunium and plutonium was tested on model solution of evaporator concentrate sample for isolation of each of them for alpha-spectrometry analysis. A good chemical separation of radionuclides of interest from the matrix and interfering radionuclides is required, once the relatively small difference in alpha-particle energy makes it difficult to spectrometrically separate the peaks. This paper describes use of the molecular recognition technology product AnaLig®Pu-01 gel from IBC Advanced Technologies, Inc. to effectively and selectively pre-concentrate, separate and recover americium, neptunium and plutonium from model solution of evaporator concentrate samples from Nuclear Power Plant which belong to the most difficult matrices to analyse. The method is suitable for analysing highly contaminated samples of radioactive waste in a relatively short time. For counting the alpha activity of 241Am, 239,240Pu, 238Pu and 237Np ORTEC 576A alpha-spectrometer equipped with ULTRATM ion implanted silicon detectors (600 mm2 active area) was used. The spectra were processed by using the Alpha-visionTM 32-bit emulation software from the EG&G ORTEC company.

Development of a large-area silicon α-particle detector

Circular ion-implanted silicon detector of α-particles with a large, 5-cm2, sensitive area has been developed. An advantage of the detector is that the detector surface is easily cleanable with chemicals. The hardened surface of the detector shows no signs of deterioration of the spectroscopic and electrical characteristics upon repeated cleaning. The energy resolution along the diameters of the detector was (1.0±0.1)% for the 5.486-MeV α-particles. Detailed tests of the charge collection efficiency and uniformity of the detector entrance window were also performed with a 5.5-MeV He2+ microbeam.

Alpha-particle emission probabilities of 236U obtained by alpha spectrometry

High-resolution alpha-particle spectrometry was performed with an ion-implanted silicon detector in vacuum on a homogeneously electrodeposited 236U source. The source was measured at different solid angles subtended by the detector, varying between 0.8% and 2.4% of 4π sr, to assess the influence of coincidental detection of alpha-particles and conversion electrons on the measured alpha-particle emission probabilities. Additional measurements were performed using a bending magnet to eliminate conversion electrons, the results of which coincide with normal measurements extrapolated to an infinitely small solid angle. The measured alpha emission probabilities for the three main peaks – 74.20 (5)%, 25.68 (5)% and 0.123 (5)%, respectively – are consistent with literature data, but their precision has been improved by at least one order of magnitude in this work.

Optimization of parameters of alpha spectrometry with silicon detector for low level measurements of actinides in environmental samples

Determination of actinides in environmental and biological samples is an important activity of radiation protection program at nuclear energy facilities. High resolution alpha spectrometry with passivated ion implanted Silicon detectors is widely used for the determination of actinides concentration. Low levels of activity concentrations in these samples often require long counting duration of a few days to obtain accurate and statistically significant data for further impact assessment. In alpha spectrometry, the chamber in which Si detector operated is a critical component and maintained at a desired vacuum for minimizing the alpha particle attenuation. Experimental evaluation of variations in energy resolution and tailing of alpha spectra was investigated under different chamber air pressures from about 6.7Pa to more than 2700Pa under the chamber hold mode and pump electrically switched off conditions. As part of validation, data collected on an IAEA inter-comparison exercise sample are presented under short and long counting durations with pump operating and switched off conditions respectively. It has been observed that the FWHM values do not significantly degrade, to impact the low and medium level concentration alpha spectra, for variations in vacuum chamber pressures from about 6.7Pa to 2700Pa.

Measurement of the 226Th and 222Ra half-lives

The half-lives of 226Th and 222Ra were measured by counting alpha-particle emissions from sources as a function of time. The 226Th sources were prepared from an open 230U source, capturing recoil atoms after alpha-particle decay on glass disks or even directly onto a detector. Similarly, the 222Ra sources were obtained by self-transfer of recoil atoms from 226Th sources. The activity measurements were performed in (nearly-)2π geometry with an ion-implanted silicon detector. The decay curves were analysed in different ways, incl. moment analysis, trying to avoid the pitfalls of bias of least-squares fits to Poisson distributed data. The observed half-life values are T1/2(226Th)=30.70 (3) min and T1/2(222Ra)=33.6 (4) s. Literature values show some inconsistency.

Measurement of the 230U half-life

The 230U half-life was determined by measuring the decay curve of 230U sources by various nuclear detection techniques: α-particle counting at a defined small solid angle; 4πα+β counting with a windowless CsI sandwich spectrometer, a liquid scintillation counter and a pressurised proportional counter; gamma-ray spectrometry with a HPGe detector and nearly-2π α-particle counting with an ion-implanted silicon detector. Depending on the technique, the decay was followed for 100–200d, which is 5–10 times the 230U half-life. The measurement results of the various techniques were in good mutual agreement. The mean value, T1/2(230U)=20.23 (2)d, is lower than the literature value which is based on one measurement in 1948 and resulted in a half-life value of 20.8d without statement of uncertainty. A correction for the ingrowth of the long-lived 210Pb and its daughter products may have been overlooked in the past.

High-resolution alpha-particle spectrometry of the 230U decay series

High-resolution alpha-particle spectrometry was performed on the 230U decay series. A 230U source was prepared on a stainless steel disc by electrodeposition in an ammonium nitrate solution. Spectrometry of the alpha-particle energy spectrum was performed with ion-implanted planar silicon detectors in vacuum. A set of alpha emission probabilities is presented for 230U and 226Th. The measured peak intensities were corrected mathematically for coincidental detection of alpha-particles and conversion electrons emitted in the same decay. A good agreement with literature data was observed. The uncertainty budget and the correlation matrix are presented. The validity of the alpha-particle energies was tested and could be confirmed for most peaks within a few keV, but discrepancies were found for the 2nd peak of 226Th and the main peak of 218Rn.

Measurements of the half-life of 214Po and 218Rn using digital electronics

The half-lives of 214Po and 218Rn have been measured. The radionuclides were produced in the decay of a 230U source and the emitted alpha-particles were measured in nearly-2π geometry with an ion-implanted planar silicon detector. The data acquisition was performed with a digitiser operated in list mode, saving the energy and time of detection (10ns precision timestamp) of each event. The half-lives were deduced from the time differences between the alpha-decays populating the nuclide of interest and those corresponding to its decay. Different methods were applied, based on delayed coincidence counting and time-interval distribution analysis. The resulting half-lives are 33.75 (15)ms for 218Rn and 164.2 (6)μs for 214Po, both in agreement with some of the literature values, and obtained with higher precision in this work.

Sub-nanosecond time-of-flight for segmented silicon detectors

Development of a multichannel time-of-flight system for readout of a segmented, ion-passivated, ion-implanted silicon detector is described. This system provides sub-nanosecond resolution ( δ t ≈ 370 ps ) even for low energy α particles which deposit E ≤ 7.687 MeV in the detector.

A high-sensitivity large volume cryogenic detector for radon in gas

A new detector for 222Rn measurements in gases is described. It is based on the low temperature collection of radon on a stainless steel plate and subsequent counting of emitted alpha-particles using an ion-implanted silicon detector. A very low background of 25 counts/day for the 222Rn energy window was achieved. High detection efficiency of 32% (2π geometry) and the large 65 liter volume of the detector allow radon measurements at the level of 12 mBq/m3 with 30% accuracy (relative error). Behavior of the alpha-emitting radon daughters born on the cooled stainless steel plate, 218Po and 214Po, was also analyzed.

Development of a time-of-flight spectrometer at the Ruder Bošković Institute in Zagreb

A new TOF telescope has been constructed for thin film and surface analysis. The timing system consists of two electrostatic mirror type detectors of Busch design. The detection efficiency of timing stations for very light ions was significantly improved using DLC (diamond like carbon) foils coated with LiF instead of the conventionally used carbon foils. Ion energy is measured by a 300mm2 ULTRA ion-implanted silicon detector. For the ERDA measurements with heavy and energetic ion beams, a time-of-flight (TOF) spectrometer is positioned at 37.5°. Spectrometer can be easily moved to 120° backward angle for time-of-flight RBS analysis with low energy beam of light ions. Positioning and fine adjustments of sample orientation are performed with a motorized sample stage. The same spectrometer can be also installed at the ion microprobe scattering chamber for 3D elemental imaging.

Absolute activity measurement of radon gas at IRA-METAS

This paper describes the system of the Swiss national metrological institute (IRA-METAS) for the absolute standardisation of radon gas. This method relies on condensing radon under vacuum conditions within a specified cold area using a cryogenerator, and detecting its alpha particles with an ion-implanted silicon detector, through a very accurately defined solid angle. The accuracy of this defined solid angle standardisation technique was corroborated by another primary measurement method involving 4 πγ NaI(Tl) integral counting and Monte Carlo efficiency calculations. The 222Rn standard submitted by IRA-METAS to the Système International de Référence (SIR) at the Bureau International des Poids et Mesures (BIPM) has also been found to be consistent with an analogous standard submitted by the German national metrological institute (PTB). IRA-METAS is able to deliver radon standards, with activities ranging from a few kBq to 350 kBq, in NIST-Type ampoules, and glass or steel containers usable for calibrating radon-measuring instruments.

An irradiation chamber for experimental studies on SSNTD response to alpha particles

An irradiation apparatus was designed in our laboratory to study the detection efficiency dependence of SSNTDs (CR-39, LR115, Makrofol) on alpha particles of different energy and incident angle. The system was designed to irradiate up to nine detectors in a short time, allowing to obtain reproducible conditions on many detectors and therefore to reduce random variations of the results. The system is composed of a pressure-controlled stainless-steel chamber containing a 241 Am source and a circular rotating table with 10 detector holders, one of which is devoted to an ion-implanted silicon detector for irradiation energy monitoring. The table rotation is controlled electronically via a photodiode-based system, so that the position of the detectors under the source is known with a maximum uncertainty of 0.5 mm. The detector holders allow to change the detector (both passive and active) to source inclination angle continuously and with an uncertainty better than 1 ∘ . The source-to-detector distance is controlled electronically and can be varied from 5 to 30 cm with an uncertainty of about 0.1 mm. Some simulations, obtained using the transport code TRIM, to project the irradiation chamber and its main characteristics are reported and discussed.