Copyright © 2019 The Authors. Ionising radiation detectors based on wide band-gap materials have the potential to operate at temperatures higher than 
. Such detectors are important in applications such as monitoring near nuclear reactors and in deep oil and gas well borehole logging. We discuss the development of alpha particle detectors, based on CVD diamond, which operate with high charge collection efficiency and energy resolution at temperatures up to 
. Four nominally identical commercial, electronic grade, CVD diamonds have been coated with a thin metal conductive layer in our laboratory and then attached to ceramic PCB. We present the I–V characteristics, the charge collection efficiency and the energy resolution for alpha particles from a mixed 
 source, for the four sensors operating at temperatures from 20 to 
. Monte Carlo simulations of the energy spectra and charge collection efficiency and experimental measurements of these are presented. Energy resolutions between 1.6% and 4.0% at elevated temperatures with charge collection efficiency exceeding 96% were measured. The potential for thermal neutron detection is discussed.EPSRC ref: EP/L504671/1  High temperature radiation hard detectors (HTRaD

Fern, GR

Hobson, PR

Metcalfe, A

Smith, DR

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

Brunel University Research Archive

Nuclear Inst. and Methods in Physics Research, A 958 (2020) 162486Contents lists available at ScienceDirectNuclear Inst. and Methods in Physics Research, Ajournal homepage: www.elsevier.com/locate/nimaPerformance of four CVD diamond radiation sensors at high temperatureGeorge R. Fern, Peter R. Hobson, Alex Metcalfe, David R. Smith ∗Brunel University London, College of Engineering, Design and Physical Sciences, Uxbridge, UB8 3PH, UKA R T I C L E I N F OKeywords:DiamondRadiation sensorHigh temperatureNeutronA B S T R A C TIonising radiation detectors based on wide band-gap materials have the potential to operate at temperatureshigher than 200 ◦C. Such detectors are important in applications such as monitoring near nuclear reactors andin deep oil and gas well borehole logging. We discuss the development of alpha particle detectors, based onCVD diamond, which operate with high charge collection efficiency and energy resolution at temperatures upto 225 ◦C. Four nominally identical commercial, electronic grade, CVD diamonds have been coated with a thinmetal conductive layer in our laboratory and then attached to ceramic PCB. We present the I–V characteristics,the charge collection efficiency and the energy resolution for alpha particles from a mixed 239Pu, 241Am, 244Cmsource, for the four sensors operating at temperatures from 20 to 250 ◦C. Monte Carlo simulations of theenergy spectra and charge collection efficiency and experimental measurements of these are presented. Energyresolutions between 1.6% and 4.0% at elevated temperatures with charge collection efficiency exceeding 96%were measured. The potential for thermal neutron detection is discussed.1. IntroductionRadiation monitoring near nuclear reactors and in deep oil and gasborehole logging requires sensors capable of operating at temperatures> 200 ◦C. We are developing alpha particle detectors, based on CVDdiamond, which operate with high charge collection efficiency (CCE)and energy resolution at these temperatures [1,2].In recent years there has been significant research directed to-wards developing sensors, based primarily on diamond, which canact as neutron detectors at elevated temperatures. Some studies havedemonstrated CVD diamond sensors operating continuously for periodsexceeding 100 h with a variety of different metallisations and with biaspotentials up to 300 V [3,4]. The latter developed a complete packagedsensor, specifically for bore-hole logging and demonstrated operationfor more than 100 h at 200 ◦C and with a spectroscopic resolution of3.3%. Recently a sensor has been shown to operate reliably at 300 ◦Cwith an energy resolution of 3.5% which was essentially independentof temperature [5]. Other wide band-gap materials have also beeninvestigated, notably SiC which has been demonstrated to operate, ina self-biassed mode, up to 100 ◦C [6].A notable feature of most papers is that either only one sensoris described and characterised or many are but each with a differ-ent metallisation or metal thickness or some other sensor-to-sensorvariation. One of the aims of the work presented here was to makeand characterise a number of nominally identical sensors based onCVD diamond from a single producer and subsequently cleaned andmetallised by us identically.∗ Corresponding author.E-mail address: David.Smith@brunel.ac.uk (D.R. Smith).2. Modelling the CCE and energy resolutionTo determine the ultimate limits on detector performance in termsof resolution and CCE, a simple model of our experimental setup wasdeveloped. An approximation of the alpha source, diamond sensor andthe PCB substrate was constructed in FLUKA [7]. Our triple isotopesource, 239Pu, 241Am, 244Cm with activity of 1 kBq per isotope, wasmodelled as an isotropic cylinder of 0.22 μm thickness and 3.5 mmradius. For the purposes of simulating energy spectra, the volumebetween the metalised contacts on either side of the diamond wastreated as having 100% CCE, the rest of the volume being deemed tohave 0% CCE. This assumption disregards edge effects but they wereconsidered to be a minor contributor to the overall detector resolutionwhen compared to the main sensor geometry effects. The resultingspectral peaks generated by the FLUKA model were fitted with Gaussianfunctions. The extracted CCE and energy resolution (FWHM) are shownin Table 1.3. Experimental detailsA set of four sensors of 2×2×0.5mm3, were fabricated using singlecrystal electronic grade CVD diamond (Element Six Ltd). We depositedmetal coatings on the square surfaces of the diamond at a pressure of10-3 mbar. The sensor capacitance was measured to be 0.34 pF at 1MHz. The response of the sensors to alpha particles was evaluated at apressure of < 10-4 mbar to minimise alpha particle energy loss.https://doi.org/10.1016/j.nima.2019.162486Received 30 March 2019; Received in revised form 22 July 2019; Accepted 30 July 2019Available online 2 August 20190168-9002/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).G.R. Fern, P.R. Hobson, A. Metcalfe et al. Nuclear Inst. and Methods in Physics Research, A 958 (2020) 162486Fig. 1. PCB with co-axial cable and wire-bonded diamond sensor. The hole throughthe PCB is for mounting it to the copper heating block.Table 1CCE and energy resolution from simulation.Alpha energy CCE Energy resolutionMeV % (FWHM) %5.16 99.0 ± 0.5 1.22 ± 0.0055.49 99.7 ± 0.5 1.07 ± 0.0065.81 99.2 ± 0.5 1.04 ± 0.005Table 2Sensor characteristics.Sensor Type SurfaceroughnessRaman peak Raman peakwidthnm cm−1 cm−1BSC5 Grid etched 5.38 1332.55 2.21BSC6 Normal 5.78 1332.88 2.07BSC7 Normal 5.24 1332.40 2.22BSC8 Normal 8.06 1332.23 2.07Table 3I–V summary data. SS = Symmetrical Double Schottky and SA = Asymmetrical DoubleSchottky.Sensor 𝛥 0 V to 100 V 𝛥 0 V to −100 V BehaviourpA pABSC5 1.43E+05 −2.47E+05 SSBSC6 22.5 −17.1 OhmicBSC7 42.4 −55.7 OhmicBSC8 749.1 −648.8 SAThe sensors were mounted on copper-coated, 1 mm thick aluminaceramic PCB (Rubalit 708S from CeramTec) with vias to connect theground planes on both surfaces. Silver-filled epoxy suitable for tem-peratures up to 260 ◦C (Duralco 120, from Cotronics Corp.) was usedboth to fix the sensor to the board, and to connect a coaxial cable tothe board. The electrical connection from the top of the sensor to thePCB was completed by a set of ultrasonically bonded 25 μm diameteraluminium bond wires from the edge of the top surface metallisationto a pad on the PCB, connected to the coaxial cable (see Fig. 1). Thecharge induced in the sensor was amplified by a Canberra 2004 pre-amplifier operating at 1 V/pC and a 2021 spectroscopy amplifier witha shaping time of 1 μs.The surface roughness of the diamond was determined using aZygo NewView 5000 and the Raman peak and its width were mea-sured (see Table 2). The narrow peaks indicate a low concentration ofsubstitutional nitrogen defects [8].4. ResultsThe I–V characteristics tests of each sensor were measured using aKeithley 617 electrometer. Subtracted from the data, and attributed toa leakage current in our measurement system, was a current offset ofapproximately -130 pA at 0 V bias. The results are shown in Table 3.Using our triple isotope source in vacuum, the energy resolution ofthe primary peaks was measured for both positive and negative biasvoltages at temperatures up to 250 ◦C. Fig. 2 shows a typical datasetFig. 2. Variation of response to triple alpha source as a function of temperature forsensor BSC6.Fig. 3. CCE as a function of positive and negative bias for sensor BSC8.Fig. 4. Alpha particle energy resolution as a function of positive and negative bias forsensor BSC8.for the sensors with moderate leakage currents. The large, suppressedpeak close to 0 MeV is the electronic noise in the absence of the source.We determined the variation of CCE and energy resolution withbias. In Figs. 3 and 4 we show the response of one sensor as a functionof bias voltage and alpha particle energy respectively. The error barsshown are the one sigma boundaries.Table 4 summarises the highest temperature at which each sensoroperated spectroscopically. The CCE and energy resolution at the peaktemperature are those quoted in the table. Sensor BSC5, which wasplasma etched with a surface grid pattern, had such a high leakagecurrent even at 100 V bias (see Table 3) that no alpha peaks couldbe resolved even at room temperature. The failure of sensor BSC5 due2G.R. Fern, P.R. Hobson, A. Metcalfe et al. Nuclear Inst. and Methods in Physics Research, A 958 (2020) 162486Table 4CCE and alpha particle energy resolution. In this table the highest temperature at whichno alpha particle energy-dependent effects on CCE or energy resolution was observedis stated for each sensor.Detector Bias Mode PeaktemperatureCCE Energy resolutionV ◦C % (FWHM) %BSC6 300 hole 225 99 ± 5 1.6 ± 0.5BSC6 −300 electron 225 97 ± 3 2.1 ± 0.5BSC7 200 hole 200 98 ± 2 4.0 ± 0.7BSC7 −200 electron 225 96 ± 2 2.0 ± 0.5BSC8 300 hole 225 98 ± 3 3.3 ± 0.7BSC8 −300 electron 225 97 ± 3 2.6 ± 0.5to high leakage is attributed to misalignment in the shadow masking,which permits direct conduction down the comparatively low resis-tance sides of the diamond crystal. Due to limitations in the mountingof BSC5 for metalisation it is possible for these areas to retain a degreeof hydrogen termination [9] which may contribute to this effect.In electron sensitive mode each of the three unetched sensors wasfound to function well, with good performance up to a temperatureof 225 ◦C but losing useful spectroscopic performance past this, see forexample Fig. 2. The energy resolution, in hole mode, for sensor BSC7shows a dependence on the alpha particle energy at a temperature of225 ◦C varying from 2.1% for alphas with 5.16 MeV energy to 7.0%for alphas with 5.81 MeV energy. These limitations on performance,with observable impact around 180 ◦C, correspond with findings inthe wider literature on thermally stimulated current and thermolumi-nescence [10,11]. Trapping–detrapping effects are thought to be themain limiting factor on the operation of diamond sensors at elevatedtemperature and these have been investigated elsewhere by meansof transient current [12,13], thermally stimulated current [14] andthermoluminescence techniques. Identified in these works are deeptraps with activation energies that indicate their contribution becomessignificant in the temperature range studied here. A series of deeptrapping levels with activation energies of 0.97, 1.14, 1.29 and 1.4 eVhave been identified [15]. This corresponds to a temperature range of127 − 327 ◦C with a particularly strong trapping activation energy atan equivalent temperature of 227 ◦C. This corresponding well with theobserved degradation in performance observed here. Due to the shortrange of alpha particles, contributions to the trapping caused by themetal-diamond interface will have an exacerbated effect on the spacecharge compared to those present in the bulk.5. Enhancing neutron detection efficiencyMicropatterning of the CVD diamond surface by plasma etching isexpected to enhance the thermal neutron detection efficiency. We havetested the patterning technique experimentally using a ≈ 35 μm pitchgrid (see Fig. 5).Using FLUKA [7] we simulated the response to neutrons for adiamond with pits replicating those shown in Fig. 5 filled with 10B. Inthe simulation we varied the width of the diamond ridge that surroundseach of the etched pits and with a width of approximately 4 μm wepredict a neutron detection efficiency exceeding 25% as shown inFig. 6.6. SummaryRadiation sensors intended for high temperature operation havebeen constructed and tested under alpha irradiation at temperatures upto 250 ◦C. Of those not immediately rendered non-functional during thefabrication process (BSC5) all were found to deliver workable detectionperformance at up to 225 ◦C with a consistent degradation when heatedpast this, attributed to a sharp increase in thermally induced trapping–detrapping past this point. The sensors were stable throughout theapplied heat cycles.Fig. 5. SEM photograph of part of a grid, plasma etched into the surface of sensorBSC5.Fig. 6. MC simulation predicting the enhancement of neutron detection.A surface profile on a diamond sensor which we generated usingplasma etching, has the potential to improve the neutron detectionefficiency above that of a planar sensor.References[1] A. Metcalfe, G.R. Fern, P.R. Hobson, T. Ireland, A. Salimian, J. Silver, D.R.Smith, G. Lefeuvre, R. 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Performance of four CVD diamond radiation sensors at high temperature

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Performance of four CVD diamond radiation sensors at high temperature

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