Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike LicenceIn astrophysics, the determination of the optical alpha-nucleus potential for low alpha-particle energies, crucial in understanding the origin of the stable isotopes, has turned out to be a challenge. Theory still cannot predict the optical potentials required for the calculation of the astrophysical reaction rates in the Hauser-Feshbach statistical model and there is scant experimental information on reactions with alpha particles at the relevant astrophysical energies. Measurements of (n,alpha) cross-sections offer a good opportunity to study the alpha channel. At the n_TOF experiment at CERN, a prototype detector, based on the chemical vapor deposition (CVD) diamond technology, has been recently developed for (n,alpha) measurements. A reference measurement of the 10B(n,alpha)7Li reaction was performed in 2011 at n_TOF as a feasibility study for this detector type. The results of this measurement and an outline for future experiments are presented here

Badurek, G.

Berthoumieux, E.

Calviani, M.

Chiaveri, E.

Dobos, D.

Griesmayer, E.

Guerrero, C.

Jericha, E.

Kaeppeler, F.

Leeb, H.

Rauscher, T.

Vlachoudis, V.

Weiss, C.

English

University of Hertfordshire Research Archive

PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-sectionmeasurementsC. Weiss∗1,2, G. Badurek1, E. Berthoumieux3, M. Calviani2, E. Chiaveri3, D. Dobos2,E. Griesmayer1, C. Guerrero2, E. Jericha1, F. Käppeler4, H. Leeb1, T. Rauscher5 andV. Vlachoudis21 Vienna University of TechnologyVienna, Austria2 CERNGeneva, Switzerland3 CEA SaclayIRFU, F-91191 Gif-sur-Yvette, France4 Karlsruhe Institute of Technology, Campus NordKarlsruhe, Germany5 University of BaselBasel, SwitzerlandE-mail: christina.weiss@cern.ch, badurek@ati.ac.at,eric.berthoumieux@cern.ch, marco.calviani@cern.ch,enrico.chiaveri@cern.ch, daniel.dobos@cern.ch,erich.griesmayer@cern.ch, carlos.guerrero@cern.ch,jericha@ati.ac.at, franz.kaeppeler@kit.edu, leeb@ati.ac.at,thomas.rauscher@unibas.ch, vasilis.vlachoudis@cern.chIn astrophysics, the determination of the optical α-nucleus potential for low α-particle energies,crucial in understanding the origin of the stable isotopes, has turned out to be a challenge. Theorystill cannot predict the optical potentials required for the calculation of the astrophysical reactionrates in the Hauser-Feshbach statistical model and there is scant experimental information onreactions with α particles at the relevant astrophysical energies. Measurements of (n, α) cross-sections offer a good opportunity to study the α channel.At the n_TOF experiment at CERN, a prototype detector, based on the chemical vapor deposition(CVD) diamond technology, has been recently developed for (n, α) measurements. A referencemeasurement of the 10B(n, α)7Li reaction was performed in 2011 at n_TOF as a feasibility studyfor this detector type.The results of this measurement and an outline for future experiments are presented here.VI European Summer School on Experimental Nuclear Astrophysics, ENAS 6September 18-27, 2011Acireale Italy∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. Weiss1. IntroductionAt the neutron time-of-flight facility(n_TOF) at CERN [1] the cross-sections ofneutron-induced nuclear reactions are mea-sured over a wide neutron energy (En) range.For the investigation of (n, α) reactions, a pro-totype detector, based on the chemical vapordeposition (CVD) diamond technology, wasdeveloped and tested in the neutron beam in2011.The experimental observable in the measure-ment of the neutron cross-section of a given re-action is the reaction yield Y (En). For the (n,α) reaction it is given by:Yα(En) =(1− e−nσt(En))σα(En)σt(En), (1.1)where σt(En) and σα(En) are the total and the(n, α) cross-sections of the isotope under in-vestigation, respectively. The number of atomsper projected area of this isotope is given by n.The reaction yield can be experimentally ob-tained by:Yα(En) =C(En)−B(En)εα(En) ·Φn(En) ·BIF . (1.2)Here C(En) are recorded counts during themeasurement of the sample and B(En) arebackground events. Φn(En) is the neutron flu-ence at the experiment, BIF the beam intercep-tion factor for the experiment and εα(En) is theefficiency of the experimental setup.The aim of this measurement was to identifyεα(En) of the prototype detector, its responseto the neutron beam and possible aspects thatneed to be improved in future applications ofCVD diamond detectors for cross-section mea-surements.2. CVD diamond as detector materialDiamond is a semi-conducting materialwith a band gap of 5.5 eV, which makes ita very low-noise and light-insensitive detectormaterial. The ionization energy of 13.6 eV re-sults in a comparably low charge yield. In com-bination, diamond is a suitable material evenfor single minimum ionizing particle (MIP) de-tection [2]. The mobility of the charge carri-ers is 1700 and 2100 cm2/Vs for electrons (e−)and holes (h), respectively [3]. This makes di-amond a detector material with a very fast re-sponse and a particle discrimination capabilityin the nano-second range. Diamond has a dis-placement threshold energy of 43 eV/atom. Itis a radiation-hard material, suited for applica-tions in accelerator environments.CVD diamonds are grown in plasma reactorson a suitable seed. To date, commercially avail-able detector-grade CVD diamond materialsare single-crystal (sCVD) and poly-crystalline(pCVD) diamonds.Homoepitaxial sCVD diamonds are grown onhigh-temperature, high-pressure synthetic dia-monds and are therefore restricted in size toabout 5 x 5 mm2.For pCVD material, diamond nano-crystalsFigure 1: Schematics of crystal growth for pCVDdiamond material. The number of grain bound-aries reduces significantly with the material thick-ness. The diamond substrate is abraded and pol-ished from both sides to the thickness indicatedwith white background, which is used as detectormaterial.2PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. Weisswhich are deposited on a silicon crystal areused as the seed. Due to the non-uniformity ofthe seed, the grown diamond material consistsof randomly oriented crystals which are sep-arated from each other through grain bound-aries. These boundaries, which are potentiallocations for the accumulation of material im-purities (such as N, B, Si and others), can actas charge sinks, reducing the charge collectionefficiency (CCE) of the material in use and in-troducing space charge effects. Only throughthe growth of a very thick substrate, where thetop layer is used as detector material, see whitearea in Figure 1, these effects can be signifi-cantly reduced.In recent years there have been efforts to growheteroepitaxial diamonds for large-area detec-tor applications, which have comparable ma-terial properties to homoepitaxial sCVD dia-mond. In December 2011 it was reported [4]that diamond substrates, grown on large areairidium crystals (DOI), have a CCE of up to96% and an energy resolution δE/E of 1.5%,which is already close to the values for sCVDdiamond [5].3. The n_TOF experiment at CERNThe n_TOF facility was constructed fordedicated cross-section measurements of neu-tron induced reactions. A Pb spallation targetwhich is impinged by a pulsed proton beam of20 GeV/c, is used as neutron source. The ex-perimental area, where the measurements takeplace, is located 185 m downstream from thetarget. The layout of the facility at CERN canbe seen in Figure 2.A sweeping magnet, positioned after a firstcollimator for the neutron beam, ensures thatcharged particles from the spallation processare removed from the neutron beam. The pho-ton background, caused by the spallation pro-cess and the subsequent thermalization of theFigure 2: Layout of the n_TOF facility at CERN.Figure 3: Neutron fluence at n_TOF, for a collima-tor of 1.8 cm diameter.neutrons, remains within the neutron beam. Di-rectly before the experimental area a secondcollimator is positioned, the inner diameter ofwhich can be adjusted according to the experi-mental requirements.The characteristics of the facility are the highinstantaneous neutron flux over a wide energyrange from thermal up to GeV, see Figure 3,with the neutron energy being determined ex-perimentally via the time-of-flight technique.The long flight path and the narrow protonbeam pulse width of 6 ns rms provide an en-ergy resolution of the order of 10−3 in the en-ergy region of astrophysical interest.3PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. Weiss4. 10B(n, α)7Li test measurementThe experimental program at n_TOF hasrecently been extended from capture and fis-sion cross-section measurements to the investi-gation of neutron-induced charged particle (cp)reactions. A critical parameter for the mea-surement of (n, cp) reactions is the thicknessof the sample containing the isotope of inter-est, as the produced particles should have onlya minor energy loss before escaping the sam-ple. This implies small mass samples. In or-der to have acceptable statistics during a givenmeasurement, the solid angle covered by thedetector should be maximized. This can beachieved by placing the detector directly in theneutron beam. Hence the detector in use for(n, cp) cross-section measurements should, inaddition to having an adequate size to coverthe neutron beam, combine a high particle de-tection efficiency with a low neutron-inducedbackground.CVD diamond has proven to have a chargedparticle detection efficiency close to 100% [6].The (n, α) reaction on 12C, being the first sig-nificant background reaction channel to open,has a threshold energy of 6.2 MeV and the highradiation tolerance of this material allows thepositioning of the detector in the neutron beam.For this measurement sCVD diamonds wouldhave been preferred for their good spectro-scopic properties, but only pCVD diamondsubstrates were available in adequate sizes inorder to cover the n_TOF neutron beam with afull width at half maximum (FWHM) of 2 cm,when using the collimator of 1.8 cm in diame-ter.To validate the functionality of the detectorsetup, the well known 10B(n, α)7Li reaction,standard up to 1 MeV [7], has been chosen fora first measurement.The objectives of the test measurement werethe following:• To determine the detection efficiency ofthe pCVD diamond detector for the (n,α) reaction.• To investigate the background level ofthe CVD diamond in the region of inter-est.• To identify aspects to be improved for afuture CVD diamond detector.4.1 Prototype detector and experimentalsetupFor the prototype detector a pCVD dia-mond of 20 x 20 x 0.5 mm3 was used [8].The raw pCVD material showed graphite in-clusions, which locally alter the material prop-erties of the diamond.The diamond was metallized with 100 nmthick, segmented Al electrodes in collaborationwith CEA Saclay [9], implementing a 4-paddetector.The diamond was mounted on a printed circuitboard sandwich-structure. The electrical con-tacts to the 4 pads and the ground electrodewere achieved by mechanical clamping. Pad1 and 3 where biased with -400 V, pad 2 and 4with -396 V. This introduced a transversal fieldbetween adjacent electrodes to minimize field-free regions in the diamond detector.The signals of each pad were read out on thehigh voltage (HV) side and amplified with fast,low-noise charge amplifiers [10].The whole assembly was positioned in the neu-tron beam. The 10B4C sample was mountedin front of the diamond detector, leaving an airgap of 1 mm to the detector. This gave a solidangle coverage close to 50%.4.2 Auxiliary measurementsThe 10B4C sample was characterized atthe Centro Nacional de Aceleradores (CNA)in Sevilla/Spain through Rutherford back-scattering (RBS), which gave a thickness of4PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. Weiss 0 50 100 150 200 250 300 350 400 450 0  50  100  150  200CountsMaximum Amplitude [mV]Amplitude spectrum measurement with 241AmsCVD, Amax = 184 mVpCVD from 10B measurement, Amax = 75 mVFigure 4: Amplitude spectrum of the pCVD de-tector (red) during the calibration with the 241Amsource. For qualitative comparison the spectrum ofa sCVD diamond detector is shown (green).1.08·10−5 10B atoms/barn [12].The geometrical efficiency and the effect ofthe sample thickness on the outgoing α parti-cles was simulated using the FLUKA Monte-Carlo code. This simulation showed that 32.7%of all produced α particles enter the diamonddetector with a minimum energy of 500 keV.This energy has been chosen as the lower limitfor discrimination of the α particles from thein-beam γ-background at n_TOF, which has acomparable energy.The neutron fluence was monitored via an Au-foil activation, resulting in BIF = 0.89 [11].The 11% losses of the available neutrons arerelated to the smaller size of the detector com-pared to the beam size.The diamond detector has been calibrated us-ing a 241Am α-source and a 90Sr β -source. Theamplitude spectrum from the calibration with241Am can be seen in Figure 4 in comparisonto a sCVD diamond detector. The spectrumrecorded with the sCVD diamond shows the αpeak around 155 mV. As expected, the pCVDdiamond exhibits less amplitude, due to the re-duced CCE of about 40% [8]. Its spectrumdoes not show the alpha peak, which demon-strates insufficient spectroscopical properties.With the trigger set to 6 mV, the mean value ofthe amplitude spectrum of the pCVD diamonddetector is 16.8 mV and the standard deviation8.2 mV, which gives an energy resolution of49%. Upon calibration with a 90Sr source thecounting rate was of comparable magnitude forboth materials.4.3 Experimental dataThe amplitude spectra of the α particlesproduced via the 10B(n, α) reaction and therecorded background have been compared foreach of the 4 pads (DIAM1-4). An amplitudethreshold for the experimental data was set foreach of the detector pads, taking the differentgains of the preamplifiers into account, dis-missing noise events, and rejecting part of thebackground.During the experimental campaign, to recordthe background, a measurement without the10B sample was performed. The backgroundspectra as a function of neutron energy can beseen in Figure 5, for each of the detector pads.Figure 5: Recorded background for all 4 detec-tor pads (DIAM1-4) as a function of neutron en-ergy. The averaged background is shown in black.The red curve corresponds to the theoretical back-ground, taking the thickness of the diamond detec-tor and all the non-elastic neutron-induced reactionson 12C into account.5PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. WeissThe contribution of the n_TOF specific in-beam γ-background can be seen in the energyregion from 100 eV to a few hundred keV. Thedifference between the pads is related to theirrelative position in the Gaussian neutron beamprofile.The elastic scattering of neutrons on 12C,which is present throughout the neutron spec-trum, is visible at neutron energies above 300keV.At low energies, where the spectrum follows a1/E characteristic, the experimental data is 10times higher than expected, see Figure 5. Thiscan be explained by either B or N impuritiesin the diamond material. B would contributethrough the (n, α) and N through the (n, p) re-action.The experimental spectrum recorded duringFigure 6: Experimental spectrum for the detectorDIAM4. The measured 10B spectrum is plottedin black. The spectrum recorded during the back-ground measurement is shown in green.the 10B measurement in comparison with thebackground spectrum for one of the detectorpads can be seen in Figure 6. The backgroundfrom in-beam γ-rays dominates for En ≥ 100eV. With a better energy resolution of the de-tector material it would be possible to reducethe background significantly, as a classificationof the measured particles would be possiblethrough the amplitude spectrum.Figure 7: Recorded 10B(n, α)7Li events in total(black) for neutron energies up to 1 keV. For thecalculated, expected spectrum (green), the neutronfluence, sample thickness and efficiency are takenfrom the auxiliary measurements (see text for de-tails). The ratio of the two graphs shows that about9% of the expected 10B(n, α)7Li events could bedistinguished with the prototype detector.4.4 ResultsThe experimental result for each of the de-tectors is calculated as the difference betweenthe recorded 10B and the background spectrum,corresponding to C(En)− B(En) in equation(1.2).In Figure 7 the result of the measurement isplotted in comparison with the expected result,normalized to pulses of 7·1012 protons on tar-get. The green graph in Figure 7 includes theresults of the auxiliary measurements for theneutron fluence, the sample thickness and theefficiency of the detector setup.9% of the expected counting rate was actuallymeasured, giving an overall efficiency ε = 3%.6PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. Weiss5. ConclusionThe results of a measurement of the 10B(n,α)7Li reaction with a prototype detector basedon the CVD diamond technology were pre-sented.The overall efficiency of the measurement wasε = 3% instead of the expected 50%, which area consequence of the 2pi geometry of the ex-perimental setup. This discrepancy can be ex-plained by the insufficient energy resolution of49% for the detector material in use, which in-hibited the rejection of a significant part of therecorded background, caused by both in-beamγ and elastic scattering of neutrons on 12C. Aclassification of particles through the amplitudespectrum of the experimental data was not pos-sible.For future applications of CVD diamonds inthe field of (n, cp) cross-section measurements,the following points need to be implemented:• The detector material needs to have anenergy resolution comparable to sCVDdiamond material, so that backgroundevents can be identified in the amplitudespectrum and dismissed from the spec-trum.• The impurities in the diamond materialshould be as few as possible, in order toreduce neutron-induced background.• The detector material should not havedefects like graphite inclusions, in orderto have homogeneous material propertiesthroughout the detector.• For additional background minimiza-tion, the thickness of the diamond shouldbe reduced.• The size of the detector should be en-larged in order to cover more of the avail-able neutron beam.• The whole assembly should be placed invacuum, to minimize the energy loss ofthe produced particles on their way to thedetector.• To implement a geometry close to 2*2pi ,a sample should be positioned on eitherside of the detector.For the near future, a measurement that shouldfulfill all these requirements has been alreadyproposed at CERN [13]. The 59Ni(n, α)56Fecross-section with a large area, 250 µm thickDOI diamond [4] will be measured.References[1] C. Rubbia et al., A high Resolution Spallationdriven Facility at the CERN-PS to measureNeutron Cross Sections in the Interval from 1eV to 250 MeV, CERN/LHC/98-02 (EET),(1998); CERN/LHC/98-02 (EET)-Add.1,(1998).[2] H. Frais-Koelbl et al., A Fast Low-NoiseCharged-Particle CVD Diamond Detector,IEEE Transactions on Nuclear Science,Vol.51, No. 6 (2004).[3] H. Pernegger et al., Charge-carrierproperties in synthetic single-crystaldiamond using the transient-currentTechnique J.Appl.Phys. 97, 073704 (2005).[4] http://www-carat.gsi.de/.[5] E. Griesmayer et al., High-Resolution Energyand Intensity Measurements with CVDDiamond at REX-ISOLDE, CERNBE-Note-2009-028 (2009).[6] M. Rebisz et al., CVD diamond dosimetersfor heavy ion beams, Diam.&Rel.Mat. 16,1070-1073 (2007).[7] A. D. Carlson et al., International Evaluationof Neutron Cross Section Standards, NuclearData Sheets 110, 3215-3324 (2009).[8] Diamond Detectors Ltd.,http://www.diamonddetectors.com/.7PoS(ENAS 6)015A CVD diamond detector for (n, α) cross-section measurements C. Weiss[9] P. Bergonzo, M. Pomorski, LIST CEASaclay.[10] CIVIDEC Instrumentation GmbH,www.cividec.at.[11] N. Sabbi, Private communication, June 2011.[12] J. Praena, Private communication, October2011.[13] C. Weiss et al., The (n, α) reaction in thes-process branching point 59NiCERN-INTC-2012-011, INTC-P-327.8

A Fast Low-Noise Charged-Particle CVD Diamond Detector,

A high Resolution Spallation driven Facility at the CERN-PS to measure Neutron Cross Sections

Charge-carrier properties in synthetic single-crystal diamond using the transient-current Technique J.Appl.Phys. 97,

CVD diamond dosimeters for heavy ion beams,

P o S(ENAS 6)015 A CVD diamond detector for (n, α) cross-section measurements C.

Private communication,

The (n, α) reaction in the s-process branching point 59Ni CERN-INTC-2012-011,

A CVD diamond detector for (n,alpha) cross section measurements

A CVD diamond detector for (n,alpha) cross section measurements

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