Energy Response Of Detector Research Articles

Analysis of computational problems expressing the overall uncertainties: photons, neutrons and electrons

In the frame of the EU Coordination Action CONRAD (coordinated network for radiation dosimetry), WP4 was dedicated to work on computational dosimetry with an action entitled 'Uncertainty assessment in computational dosimetry: an intercomparison of approaches'. Participants attempted one or more of eight problems. This paper presents the results from problems 4-8-dealing with the overall uncertainty budget estimate; a short overview of each problem is presented together with a discussion of the most significant results and conclusions. The scope of the problems discussed here are: the study of a (137)Cs calibration irradiator; the manganese bath technique; the iron sphere experiment using neutron time-of-flight technique; the energy response of a RADFET detector and finally the sensitivity and uncertainty analysis for the recoil-proton telescope discussed in the companion paper.

Determining the thermal diffusivity in microcalorimeter absorbers and its effect on detector response

An x-ray microcalorimeter consists of an absorber and a thermometer connected to each other, and to a heat sink, via well defined thermal conductances. The standard theoretical derivation of energy resolution treats the absorber and thermometer as point elements that are internally isothermal. In reality, the finite size and internal diffusivity of the absorber and thermometer prevents these elements from instantly achieving a uniform temperature, leading to a variation in observed pulse shapes as a function of the interaction’s position within the absorber. These variations result in a distortion of the detector response and a subsequent degradation of the energy resolution. This paper presents diffusivity measurements for x-ray microcalorimeters fabricated at the NASA/GSFC. Using a diffusion model we developed, we show quantitatively how a 2eV Gaussian response is distorted into a non-Gaussian profile roughly 12eV wide at an energy of 6keV for an absorber diffusivity of 104μm2∕μs. We then present a method for eliminating the effect of pulse shape variation on the detector energy response with a modified optimal filter approach.

Influences and their corrections for the new [formula omitted] TL area dosemeter Seibersdorf

A new area dosimeter for environmental and workplace monitoring based on LiF:Mg,Ti (TLD-100) thermoluminescence detectors was developed at the Austrian Research Centers in Seibersdorf for measuring ambient dose equivalent, H * ( 10 ) , in photon radiation fields. After a prototype design phase using a novel combination method of experimental and Monte Carlo simulation techniques the dosemeter is now in its test and optimization phase. Two versions of the Seibersdorf H * ( 10 ) area dosemeter with one and two TL cards each with four TLD-100 chips are investigated. Dosimetric and practical properties are evaluated according to standard (ÖNORM, DIN, IEC) recommendations and correction methods for various influence quantities (individual detector sensitivity, energy and angular response, coefficient of variation, linearity) are developed. Additionally, uncertainty estimates of those influence quantities are presented and discussed based on a dosimetric model containing average TL readings up to 1 Sv.

Mean energy of response to galactic cosmic ray spectrum: IMP 8 and Climax neutron monitor

Solar modulations of galactic cosmic ray (GCR) intensity contain a wealth of information about their transport in the heliosphere. To extract this information from the data one studies the dependence of the observed modulations on the mean energy of response of detectors providing data for the analyses. There is a great deal of confusion about the detector energy response to GCR spectrum in the literature. We present a preliminary report on the computations of the mean energy of response for the Climax neutron monitor (CL/NM) and IMP 8 cosmic ray nuclear composition instrument to GCR protons for 1973–1998, covering the solar cycles 21 and 22. We find that for penetrating proton channel on IMP 8 the mean energy changes by a factor of over two whereas for the neutron monitor the change is only 21%. However, the corresponding change for the computed modulation function is a factor of about 3.5.

Energy and spatial response of silicon strip detectors to X-rays

This paper describes an experimental investigation of the energy and spatial response of silicon strip detectors used for X-ray measurements. The measurements of single strip amplitude distributions have been performed for a p +–n silicon strip detector irradiated with X-rays for different detector bias voltages and for two measurements geometries (with the detector irradiated from either the strip side or from the ohmic contact side). The measured amplitude distributions have been compared with those obtained from simulations using the developed simulation package. The spatial response of the detector has been measured by scanning an edge across the strips and measuring the corresponding strip count rate. The measured spatial response has been compared with that obtained from simulations.

Development of double-sided silicon strip detectors (DSSD) for a Compton telescope

The low noise double-sided silicon strip detector (DSSD) technology is used to construct a next generation Compton telescope which is required to have both high-energy resolution and high-Compton reconstruction efficiency. In this paper, we present the result of a newly designed stacked DSSD module with high-energy resolution in highly packed mechanical structure. The system is designed to obtain good P-side and N-side noise performance by means of DC-coupled read-out. Since there are no decoupling capacitors in front-end electronics before the read-out ASICs, a high density stacked module with a pitch of 2 mm can be constructed. By using a prototype with four-layer of DSSDs with an area of 2.56 cm × 2.56 cm , we have succeeded to operate the system. The energy resolution at 59.5 keV is measured to be 1.6 keV (FWHM) for the P-side and 2.8 keV (FWHM) for the N-side, respectively. In addition to the DSSD used in the prototype, a 4 cm wide DSSD with a thickness of 300 μ m is also developed. With this device, an energy resolution of 1.5 keV (FWHM) was obtained. A method to model the detector energy response to properly handle split events is also discussed.

Measurement of output factors for small photon beams

A variety of detectors and procedures for the measurement of small field output factors are discussed in the current literature. Different detectors with or without corrections are recommended. Correction factors are often derived by Monte Carlo methods, where the bias due to approximations in the model is difficult to judge. Over that, results appear to be contradictory in some cases. In this work, output factors were measured for field sizes from 4 mm up to 180 mm side length with different detectors. A simple linear correction for the energy response of solid state detectors is proposed. This led to identical values down to 8 mm field size, as long as the size of the detector is small against the field size. The correction was of the order of a few percent. For a shielded silicon diode it was well below 1%. A physically meaningful function is proposed in order to calculate output factors for arbitrary field sizes with high accuracy.

TH‐C‐VaIA‐03: Accurate Dosimetry in Narrow Stereotactic Photon Fields

Dosimetric measurements for stereotactic radiosurgery or radiotherapy fields typically include determination of relative output factors, tissue maximum ratios and off‐axis beam profiles. Accurate measurements of each of these quantities are made more difficult when the measured field sizes are either comparable to the detector size, or less than the distance required for lateral electronic equilibrium. These difficulties are present when measuring either single, static‐field cone or micro‐MLC linac‐based delivery systems, or simultaneous multiple‐field Gamma Knife delivery systems.Several detector characteristics should be considered when choosing a measurement system. The detector size in the radial direction will determine the extent of the beam profile integrated in the reading. Typically solid‐state detectors or film (radiographic or radiochromic) have been used to reduce this volume‐averaging effect. Several other detection systems have also been used, including pin‐point ionization chambers, thermoluminescence dosimetry, and diamond and scintillation detectors. Techniques exist to correct for finite detector volume, including the extrapolation of measured results to zero‐volume, and deconvolution of measured data with the detector response function.The detector energy response also should be considered due to the variation in the number of low energy scattered photons with field size and depth. Additionally, for field sizes below lateral electronic equilibrium, variations in the stopping power ratios affect the conversion of ionization to dose, particularly for higher‐energy measurements. Both of these effects increase with beam energy and depth.This lecture will provide an overview of the measurement techniques used to determine dosimetric quantities for stereotactic fields. Recommendations for measurement systems will be made, and limitations to each system will be discussed. Estimations of error will be provided, both for individual measurements and in the overall planned dose. When available, comparisons will be made with results predicted by theoretical Monte Carlo calculations.Educational Objectives:1. Understand the basic principles and practical aspects of clinical dosimetry for stereotactic radiosurgery.2. Understand the limitations and applicability of various detector types and sizes for small field measurements.3. Understand current techniques available to correct data measurements in small fields.

F23 Optimization of the peak-to-background ratio and the low energy response of silicon drift detectors for high resolution X-ray spectroscopy

F23 Optimization of the peak-to-background ratio and the low energy response of silicon drift detectors for high resolution X-ray spectroscopy

TU‐E‐330D‐03: Evaluation of An OSL Dosimetry System for CT Quality Assurance and Dose Optimization

Purpose: Recent development of wide‐beam multidetector CT and conebeam CT demands alternative methodology to CTDI. In this study, the Optically Stimulated Luminescence (OSL) technique was evaluated for CT quality assurance and dose optimization. Method and Materials: CT scans were performed on 38‐mm2 thin Luxel™ dosimeters oriented in axial plane of a GE LightSpeed Ultra and at the center of a CT body dosimetry phantom. A 5‐mm beam collimation was selected to determine the energy response of the OSL detector at 80, 120, and 140 kVp stations. mA response of the OSL detector was evaluated from 60 to 350 mA. Helical scans of varying length coverage were also performed with the detector placed at the isocenter. The exposed OSL discs were measured using a Riso TL/OSL‐DA‐15 reader. Each OSL measurement was followed by a standard beta source irradiation and subsequent OSL measurement to normalize for the differing mass of the disks. The normalized OSL signal reading was used in the data analysis. Results: Good mA linearity was observed at all 3 kVp stations. There are discontinuities seen at ∼ 250 mA and it is known to be caused by tube focal spot change. The 120 and 140 kVp data show good correlation with ionization chamber reading. For 80 kVp, OSL signals to ion chamber readings are all higher, particularly at low mA range, indicating higher sensitivity of the OSL detector over the ion chamber. The helical scans show an increased OSL signal with scan length and a leveling of the signal at large scan coverage. Conclusion: The OSL detectors are of small size and responded well to CT exposure. It provides a practical technique for quantifying the dose at any location of a phantom for quality assurance and hence exhibits a potential for estimating patient organ dose.

PET energy-based scatter estimation and image reconstruction with energy-dependent corrections

In this paper we propose a comprehensive energy-based scatter correction approach for positron emission tomography (PET). We take advantage of the marked difference between the energy spectra of the unscattered and scattered photons, and use the detailed energy information that comes with the list-mode data for the estimation of the scattered events distribution in the data space. Also, inside the maximum-likelihood expectation maximization (ML-EM) image reconstruction algorithm, we introduce energy-dependent factors that individualize the correction terms for each event, given its position and energy information. The central piece of our approach is the two-dimensional detector energy response model represented as a linear combination of four components, each one representing a particular state a PET event can be found in: both photons unscattered, the second scattered while the first not, the first photon scattered while the second not and both photons scattered. For a set of events collected in the vicinity of a point in the projection space, the coefficient of each component is determined by applying a statistical estimator. As a result we obtain the number of scattered events that are in the given set. The model also gives us the variation of scatter fraction with the photon pair energies for that particular position in the data space. A simulation study that demonstrates the proposed methods is presented.

Hard X- and γ-ray measurements with a large volume coplanar grid CdZnTe detector

The recent introduction of coplanar grid techniques has led to resurgence in interest in developing large volume compound semiconductor detectors that have a reasonable γ-ray response but also good spectroscopic resolution. We report the results of a series of hard X- and γ-ray measurements on a large 15×15 mm2, 10 mm thick CdZnTe coplanar grid detector. The measurements were carried out at the HASYLAB and ESRF synchrotron radiation facilities using highly monochromatic pencil beams across the energy range 20–800 keV. Additional full-area measurements were carried out using radioactive sources. All measurements were carried out at room temperature. The measured energy resolution under full-area illumination was 8 and 12 keV FWHM at 59.95 and 662 keV, respectively. Under pencil beam illumination, the measured resolutions were essentially the same. The detector energy response function was found to be linear over the energy range 20–1400 keV with an average rms nonlinearity of 1.4%, consistent with statistics. The spatial uniformity of the detector was evaluated at 30, 60 and 180 keV by raster scanning a 20×20 μm2 monoenergetic X-ray beam across the active area. Apart from a few localized areas, the detector response was found to uniform at the few percent level, consistent with statistics. At 180 keV, the nonuniformity in the energy response due to the grids was estimated to be at the 0.7% level.

Hard X- and γ-ray measurements with a 3×3×2mm3 CdZnTe detector

Abstract We report the results of a series of X- and γ-ray measurements on a 3 × 3 mm 2 , 2 mm thick CdZnTe detector carried out at the HASYLAB and ESRF synchrotron radiation facilities. The detector energy response function was found to be linear over the energy range 10–100 keV with an average rms non-linearity of 0.6%, consistent with statistics. Under full area illumination, the FWHM energy resolution was 270 eV at 5.9 keV and rises to 930 eV at 59.54 keV. Under pencil beam illumination, the measured energy resolution at 10 keV was 310 eV FWHM and rises to ∼1000 eV at 100 keV. At 60 keV the resolution was found to be ∼30% lower than that measured under uniform illumination, indicating a degree of non-uniform crystallinity and stoichiometry in the bulk. For energies

Study of systematic effects in beta decay measurements with AgReO 4 calorimeters

AgReO 4 microcalorimeters are planned to be used in the next generation of calorimetric neutrino mass experiments with sensitivity down to 0.2 eV. The understanding and characterization of all possible sources of systematic uncertainties is crucial. In this work, we focus on two of these sources which have been studied in the ten detectors of the Milano AgReO 4 array experiment (MIBETA): (1) the solid-state beta environmental fine structure (BEFS) observed for the first time in AgReO 4; (2) the detector energy response for internal beta events, which has been investigated with a dedicated measurement using a 44Ti gamma source. The possible effects on neutrino mass experiments due to the incomplete understanding of these two aspects are briefly discussed.

Pixelated CdZnTe drift detectors

A technique, the so-called Drift Strip Method (DSM), for improving the CdZnTe detector energy response to hard X-rays and gamma-rays was applied as a pixel geometry. First tests have confirmed that this detector type provides excellent energy resolution and imaging performance. We specifically report on the performance of 3 mm thick prototype CZT drift pixel detectors fabricated using material from eV-products. We discuss issues associated with detector module performance. Characterization results obtained from several prototype drift pixel detectors are presented. Results of position and energy resolution measurements, charge sharing and IV-measurements are discussed.

Hard X-ray response of CdZnTe detectors in the swift burst alert telescope

The Burst Alert Telescope (BAT) onboard the Swift Gamma-ray Burst Explorer (scheduled for launch in May of 2004) has a coded aperture mask and a detector array of 32,768 Cd/sub 0.9/Zn/sub 0.1/Te/sub 1.0/ (4 /spl times/ 4 mm/sup 2/ large, 2 mm thick) semiconductor devices. Due to small mobility and short lifetime of carriers, the electron-hole pairs generated by irradiation of gamma-rays cannot be completely collected. Since the shape of the measured spectra has the broad low-energy tail, it is very useful for us to estimate the obtained spectra to fit the model which considers the charge transport properties depended on the depth of the photon interaction (G. Sato, 2002). The energy calibration of the BAT array and coded mask experiments were carried out at NASA/Goddard Space Flight Center between December 2002 and March 2003. We applied the model fitting to the calibration spectra, to yield the mobility-lifetime products for each detector and these values differ by over 2 orders of magnitude among detectors. Also using the mobility-lifetime parameters, we can identify the detector energy response as a function of the temperature and illumination angle. But we figure out a difference between the model and the obtained data. To determine the difference between the model and the measured data, we conducted the detailed check experiment for a single CdZnTe, to show that the cause of the excess is due to the areal nonuniformity of the mobility-lifetime parameter.

Energy response of an aluminium oxide detector in kilovoltage and megavoltage photon beams: an EGSnrc Monte Carlo simulation study

A Monte Carlo study of the energy response of an aluminium oxide (Al(2)O(3)) detector in kilovoltage and megavoltage photon beams relative to (60)Co gamma rays has been performed using EGSnrc Monte Carlo simulations. The sensitive volume of the Al(2)O(3) detector was simulated as a disc of diameter 2.85 mm and thickness 1 mm. The phantom material was water and the irradiation depth chosen was 2.0 cm in kilovoltage photon beams and 5.0 cm in megavoltage photon beams. The results show that the energy response of the Al(2)O(3) detector is constant within 3% for photon beam energies in the energy range of (60)Co gamma rays to 25 MV X rays. However, the Al(2)O(3) detector shows an enhanced energy response for kilovoltage photon beams, which in the case of 50 kV X rays is 3.2 times higher than that for (60)Co gamma rays. There is essentially no difference in the energy responses of LiF and Al(2)O(3) detectors irradiated in megavoltage photon beams when these Al(2)O(3) results are compared with literature data for LiF thermoluminescence detectors. However, the Al(2)O(3) detector has a much higher enhanced response compared with LiF detectors in kilovoltage X-ray beams, more than twice as much for the case of 50 kV X rays.

Energy response of a PbWO4 scintillation detector to gamma and neutron radiation

A scintillation detector based on PbWO4 is used to measure gamma rays in a mixed neutron/gamma field from a high intensity pulsed reactor. In order to calibrate gamma sensitivities, a series of monoenergetic radioisotope sources are manufactured in a reactor. Two high-energy gamma sources were obtained from nuclear reactions 12C(p, γ)13N and 19F(p, αγ)16O by means of the 600 kV high voltage accelerator. The energy response of the detector to gamma rays in the energy range 0.15 MeV ⩽ En ⩽ 6.13 MeV was measured and the results are in good agreement with the MCNP simulations. In order to get the absolute sensitivities of the detector to neutrons, one experiment was carried out using 2.5 and 14.1 MeV neutrons from D(d, n)3He and T(d, n)4He reactions and another experiment was carried out using 0.565, 1.2, 1.8, 2.5 MeV neutrons from the T(p, n)3He reaction. The result shows the detector has a high gamma-to-neutron discrimination by comparing the gamma sensitivity at 1.25 MeV with the neutron sensitivity at 1.2 MeV. It is an appropriate gamma detector for pulsed gamma rays in a mixed neutron/gamma field.

X-Ray Energy Responses of Silicon Tomography Detectors Irradiated with Fusion Produced Neutrons

In order to clarify the effects of fusion-produced neutron irradiation on silicon semiconductor x-ray detectors, the x-ray energy responses of both n- and p-type silicon tomography detectors used in the Joint European Torus (JET) tokamak (n-type) and the GAMMA 10 tandem mirror (p-type) are studied using synchrotron radiation at the Photon Factory of the National Laboratory for High Energy Accelerator Research Organization (KEK). The fusion neutronics source (FNS) of Japan Atomic Energy Research Institute (JAERI) is employed as well-calibrated D-T neutron source with fluences from 1013 to 1015 neutrons/cm2 onto these semiconductor detectors. Different fluence dependence is found between these two types of detectors; that is, (i) for the n-type detector, the recovery of the degraded response is found after the neutron exposure beyond around 1013 neutrons/cm2 onto the detector. A further finding is followed as a “re-degradation” by a neutron irradiation level over about 1014 neutrons/cm2. On the other hand, (ii) the energy response of the p-type detector shows only a gradual decrease with increasing neutron fluences. These properties are interpreted by our proposed theory on semiconductor x-ray responses in terms of the effects of neutrons on the effective doping concentration and the diffusion length of a semiconductor detector.

Determination of the modulation transfer function using the edge method: Influence of scattered radiation

The edge method for measuring the modulation transfer function (MTF) has recently gained popularity due to its simplicity and appropriateness particularly for digital imaging systems. Often edge test devices made of rather thin metal sheets are used, which are semitransparent to x rays and may generate scattered radiation. The effect of this scattered radiation on the determined MTF was investigated both theoretically (assuming an ideal detector) and experimentally using a CsI-based digital detector. It was found that the MTF increases due to the scattered radiation for all spatial frequencies larger than 0 mm(-1). The theoretical model developed in this study predicts that the maximum error compared to the true detector MTF is given by S/A, where A is the attenuated fraction and S is the scattered fraction reaching the detector, relative to the incident radiation. Theoretical and experimental results are in good agreement for radiation qualities corresponding to general radiography (RQA3, RQA5, and RQA7), whereas for chest beam quality (RQA9) the experimentally observed MTF error is larger than predicted by the simple model, possibly because the energy response of the CsI-based detector differs from that of an ideal one. The theoretical MTF error reaches a value of 18% for a 0.25 mm thick lead edge of RQA9. Since the MTF enters squared into the determination of the detective quantum efficiency (DQE), an error of at least 36% in DQE may result when using this edge test device. In conclusion, the use of fully absorbing edge material is advised for MTF determination with the edge method.