Single-crystal Diamond Detectors (SDD) are good candidates as high-energy neutron detectors in the extreme conditions of the next generation thermonuclear fusion facilities like the ITER experiment, due to their high radiation hardness, fast response time and small size. Neutron detection in SDDs is based on the collection of electron-hole pairs produced by charged particles generated by neutron interaction on 12C. In this work the SDD response to neutrons with energies between 2.8 and 3.8MeV was determined at the Legnaro CN accelerator at the INFN Laboratories in Legnaro (PD, Italy). This work is relevant for the characterization of SDDs response functions, which are key points for Deuterium-Deuterium and Deuterium-Tritium plasma diagnostic

Calvani, P.

Cazzaniga, C.

Croci, G.

Girolami, M.

Gorini, G.

Grosso, G.

Perelli Cippo, E.

Rebai, M.

Tardocchi, M.

Trucchi, D. M.

English

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DOI 10.1393/ncc/i2015-15195-9Colloquia: UCANS-VIL NUOVO CIMENTO 38 C (2015) 195Single-crystal Diamond Detector for DT and DD plasmasdiagnosticM. Rebai(1)(∗), C. Cazzaniga(2), M. Tardocchi(3), G. Grosso(3), G. Croci(1),E. Perelli Cippo(3), P. Calvani(4), M. Girolami(4), D. M. Trucchi(4)and G. Gorini(2)(1) University of Milano Bicocca - Piazza della Scienza 3, 20126 Milano, Italy(2) STFC, Rutherford Appleton Laboratory - Didcot, OX11 0QX, UK(3) IFP-CNR - Via Cozzi 53, 20125 Milano, Italy(4) CNR-IMIP - Via Salaria, Monterotondo Scalo (RM), Italyreceived 22 February 2016Summary. — Single-crystal Diamond Detectors (SDD) are good candidates ashigh-energy neutron detectors in the extreme conditions of the next generation ther-monuclear fusion facilities like the ITER experiment, due to their high radiationhardness, fast response time and small size. Neutron detection in SDDs is basedon the collection of electron-hole pairs produced by charged particles generated byneutron interaction on 12C. In this work the SDD response to neutrons with ener-gies between 2.8 and 3.8MeV was determined at the Legnaro CN accelerator at theINFN Laboratories in Legnaro (PD, Italy). This work is relevant for the character-ization of SDDs response functions, which are key points for Deuterium-Deuteriumand Deuterium-Tritium plasma diagnostic.1. – IntroductionDiamond Detectors, due to their high radiation hardness, fast response time andsmall size, are good candidates as fast neutron detectors in those environments wherethe high neutron flux is an issue, such as in the thermonuclear fusion environment,i.e. the ITER experiment. By measuring the neutrons emitted from the plasma muchinformation can be obtained on the plasma itself, e.g. density and temperature. Forexample, in a Deuterium-Tritium (DT) plasma, the 14MeV neutron peak width wouldbe proportional to the plasma temperature [1]. For a diamond-based diagnostic thespectroscopic channel which can be used for 14MeV neutrons measurements in fusion(∗) E-mail: marica.rebai@mib.infn.itCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 M. REBAI et al.plasmas is the 12C(n,α)9Be reaction. The possibility of using the SDD prototype in fusionenvironments for spectroscopic neutron measurements was successfully investigated withDeuterium-Deuterium (DD) plasmas at JET [2].In order to fully characterize the SDD response to neutrons a single pixel prototypewas tested with 14MeV neutrons at the FNG neutron source (ENEA-Frascati, Italy) [3,4]. Here, the characterization of the SDD prototype with neutrons with energies from2.8MeV to 3.8MeV will be described.2. – Experimental set-up2.1. The SDD detector . – Neutron detection in SDDs is based on the collection ofelectrons and holes generated in the detector active volume (13 eV are needed to producean e-h pair) by charged particles produced via neutron-induced nuclear reactions on 12C.The main reactions occurring in carbon are:• the n-α reaction: 12C(n,α)9Be (Qvalue = −5.7MeV, Ethr = 6.17MeV);• the elastic neutron scattering channel: 12C(n,n′)12C;• the n-3α reaction (carbon breakup): 12C(n,n′)3α (Qvalue = −7.23MeV, Ethr =7MeV).The first reaction, due to the reaction kinematics, represents the spectroscopic channel:in this case, the deposited energy, ideally, equals the incoming neutron minus the reactionQ-value. The other two give rise to a continuum up to the energy of the 12C(n,n′)3αreaction shoulder (En–Qvalue). For neutrons with an energy lower than 7MeV, such asneutron from DD plasmas, the elastic channel is the only reaction channel which canoccur.The detector was designed and built at the CNR-IFP in Milan and at the CNR-ISMinstitute in Rome (Italy) [5-12]. The SDD is made of a single-crystal diamond sample(4.5× 4.5× 0.5mm3) with boron concentration [B] < 5 ppb and nitrogen concentration[N ] < 1 ppb), provided by Element Six Ltd. [13]. Ohmic contacts were obtained on topand bottom surfaces of the sample by subsequent sputtering depositions of a multilayermetal structure (patent pending), followed by a final gold layer deposition, in order toimprove weldability with microwires and to prevent oxidation of the underlying structure.The contact thickness is 200 nm with a lateral dimension of 4.2× 4.2mm2.A dedicated 1mm thick alumina Printed Circuit Board (PCB) was designed andfabricated; the bottom surface of the diamond sample was glued with a thin layer ofconductive silver paste on the pad, whereas top surface were wire-bonded (by means of25μm thick Al/Si wires) on the ground plane. The alumina PCB is housed inside aproperly designed and developed aluminum metal case in order to shield it from elec-tromagnetic interference and to give the detector the mechanical resistance. The case isequipped with a SMA connector for detector biasing and signal collecting, on the top ofit an hole was performed in order to allow the alpha particle injection.2.2. The electronic chain. – The SDD was coupled though a 5 cm RG62 cable to aCIVIDEC c6 fast charge preamplifier [14], which has a rise time of 3.5 ns and a shapingtime of 25 ns. A CAEN HV Module NDT1470 was used to bias the detector [15]. A biasvoltage of +400V was applied, giving an electric field of 0.8V/μm in the diamond bulk.The preamplifier output was fed into a C1 broadband amplifier in order to enhance theheight of the signal of about a factor 10: the signal is then fed into a waveform digitizerSINGLE-CRYSTAL DIAMOND DETECTOR FOR DT AND DD PLASMAS DIAGNOSTIC 3Table I. – Neutron energies calculated analytically and used for SDD neutron calibration.Measure Proton Energy (MeV) Angle (degrees) Neutron energy (MeV)10 5 0 3.312 4.5 0 2.817 5.5 0 3.8CAEN DT5730 (14 bit and 500MS/s sampling rate). The digitizer is equipped with asoftware able to process on-line the data and sort them into an energy histogram. Thechoice of the electronic chain was made in order to combine a good energy resolution(< 4%) with the possibility to measure the neutron flux at high rates (>1MHz) [16,17].2.3. The beam. – The measurements were performed at the Legnaro National Labo-ratories on April 2015. Neutrons were produced by protons impinging on a 7LiF target.Proton beam energy was in the range (4.5–5.5MeV) and the expected neutron energiesat 0 degrees are reported in table I. The distance between the SDD and the target wasabout 4 cm in order to maximize the count rate.3. – ResultsThe most important result achieved during the experiment is the evaluation of theSDD response to quasi-monoenergetic neutrons. This response can be described by ana-lyzing the spectra in fig. 1. The plot on the left shows the Pulse Height Spectra obtainedby making the histogram from data collected by the digitizer: three neutron energiesare here reported. The x-axis was calibrated by using an 241Am α emitter, which emitsα particles of 5.5MeV. The y-axis is reported in counts/second/bin. The bin width is50 eV, while the data acquisition time is 6200 s, 7500 s and 8550 s for measure 10, 12 and17, respectively. As said, the only interaction channel between neutrons and 12C atomsis the elastic recoil: through this reaction the energy deposited into the detector Ed isequal to 0.298 · cos2 θ ∗En, where θ is the recoiling angle. The energy deposited reachesa maximum when the recoiling angle is zero, therefore by evaluating the position of theelastic shoulder the neutron energy can be measured. The three shoulders were fitted(fig. 1 right) with a step function convoluted with a Gaussian in order to evaluate theenergy deposited into the detector and the energy resolution. In measure 17, red line, theshoulder position is at Ed = 1.15MeV, which gives a neutron energy of En = 3.86MeV,Fig. 1. – Left: Pulse Height spectra for neutrons of 2.8, 3.3 and 3.8MeV normalized respect tothe acquisition time: 6200 s, 7500 s and 8550 s, respectively; the Energy Bin Width is 50 eV. Onthe right the shoulders fit with a step function convoluted with a Gaussian is shown.4 M. REBAI et al.with a deviation standard of 40 keV. In runs 10 (green line) and 12 (blue line) we mea-sured a shoulder position of 0.98MeV and 0.81MeV giving neutron energies of 3.29MeVand 2.71MeV, respectively. The agreement between the measured neutron energy andthe expected one, shown in table I, is very good. The energy resolution measured atneutron energies of 2.8MeV, 3.3MeV and 3.8 is 5.6%, 3.4% and 2.4%, respectively: sucha result is very good and matches the requirements for being a plasma diagnostic (< 5%).4. – ConclusionsThis work is the starting point for a complete characterization of SDDs in the energyrange 0–20MeV which is a key point for Deuterium-Deuterium and Deuterium-Tritiumplasma diagnostic with Single-crystal Diamond Detectors. A matrix of 12 SDD pixelshas been installed at JET (Joint European Thorus) at Culham, GB [18] to be a plasmadiagnostic in the next DD campaign in 2015 and in the DT campaign planned for 2018.REFERENCES[1] Faust W. R. and Harris E. G., Nucl. Fusion, 1 (1960) 62C; Hellesen et al., Nucl.Fusion, 50 (2010) 084006.[2] Cazzaniga C. et al., Rev. Sci. Instrum., 85 (2014) 043506.[3] Cazzaniga C. et al., Nucl. Fus. Rev. Sci. Instrum., 85 (2014) 11E101.[4] Martone M. et al., J. Nucl. Mater. B, 212-215 (1994) 1661.[5] Trucchi M. et al., IEEE Electron Device Lett., 33 (2012) 615.[6] Girolami M. et al., IEEE Electron Device Lett., 33 (2012) 224.[7] Bellucci A. et al., IEEE Electron Device Lett., 34 (2013) 695.[8] Girolami M. et al., Appl. Phys. Lett., 103 (2013) 083502.[9] Trucchi D. M. et al., Diamond Relat. Mater., 14 (2005) 575.[10] Conte G. et al., J. Appl. Phys., 93 (2003) 6078.[11] Conte G. et al., Phys. Status Solidi A, 201 (2004) 249.[12] Trucchi D. M. et al., Nucl. Instrum. Methods A, 718 (2013) 373.[13] Element Six Ltd, http://www.e6.com.[14] CIVIDEC, see http://www.cividec.at.[15] CAEN, see http://www.caen.it.[16] Rebai M. et al., JINST, 7 (2012) C05015.[17] Rebai M. et al., JINST, 8 (2013) P10007.[18] Joint European Thorus (JET), see http://www.efda.org/jet/.

Single-crystal Diamond Detector for DT and DD plasmas

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Single-crystal Diamond Detector for DT and DD plasmas diagnostic

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