Neutron field measurement at the Experimental Advanced Superconducting Tokamak using a Bonner sphere spectrometer

Accepted Manuscript Neutron field measurement at the Experimental Advanced Superconducting Tokamak using a Bonner sphere spectrometer Zhimeng Hu, Guoqiang Zhong, Lijian Ge, Tengfei Du, Xingyu Peng, Zhongjing Chen, Xufei Xie, Xi Yuan, Yimo Zhang, Jiaqi Sun, Tieshuan Fan, Ruijie Zhou, Min Xiao, Kai Li, Liqun Hu, Jun Chen, Hui Zhang, Giuseppe Gorini, Massimo Nocente, Marco Tardocchi, Xiangqing Li, Jinxiang Chen, Guohui Zhang

PII: DOI: Reference:

S0168-9002(18)30484-4 https://doi.org/10.1016/j.nima.2018.04.010 NIMA 60730

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 12 February 2018 Revised date : 5 April 2018 Accepted date : 6 April 2018 Please cite this article as: Z. Hu, G. Zhong, L. Ge, T. Du, X. Peng, Z. Chen, X. Xie, X. Yuan, Y. Zhang, J. Sun, T. Fan, R. Zhou, M. Xiao, K. Li, L. Hu, J. Chen, H. Zhang, G. Gorini, M. Nocente, M. Tardocchi, X. Li, J. Chen, G. Zhang, Neutron field measurement at the Experimental Advanced Superconducting Tokamak using a Bonner sphere spectrometer, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.04.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Neutron field measurement at the Experimental Advanced

2

Superconducting Tokamak using a Bonner sphere

3

spectrometer

4 5 6 7

Zhimeng Hua, Guoqiang Zhongb, Lijian Gea, Tengfei Dua, Xingyu Penga, Zhongjing Chena, Xufei Xiea, Xi Yuana, Yimo Zhanga, Jiaqi Suna, Tieshuan Fana,*, Ruijie Zhoub, Min Xiaob, Kai Lib, Liqun Hub, Jun Chenc, Hui Zhangd, Giuseppe Gorinie,f, Massimo Nocentee,f, Marco Tardocchif, Xiangqing Lia, Jinxiang Chena, Guohui Zhanga

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a

17

*Corresponding Author: [email protected]

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Abstract: The neutron field measurement was performed in the Experimental Advanced Superconducting Tokamak (EAST) experimental hall using a Bonner sphere spectrometer (BSS) based on a 3He thermal neutron counter. The measured spectra and the corresponding integrated neutron fluence and dose values deduced from the spectra at two exposed positions were compared to the calculated results obtained by a general Monte Carlo code MCNP5, and good agreements were found. The applicability of a homemade dose survey meter installed at EAST was also verified with the comparison of the ambient dose equivalent H*(10) values measured by the meter and BSS.

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Keywords: Bonner sphere spectrometer, EAST, neutron spectrum, neutron fluence, neutron dose

29

1 Introduction

30 31 32 33 34 35 36

Neutron radiation exists around the Experimental Advanced Superconducting Tokamak (EAST) mainly as the 2.45 MeV neutrons are emitted from the deuterium plasma via the D(d, n)3He fusion reactions [1-6]. The fusion neutrons from the plasma penetrate through the vacuum vessel walls of the tokamak and further interact with air, heating facilities and diagnostic devices in the EAST experimental hall, which result in the continuous neutron spectra from thermal region to about 2.45 MeV. The neutron spectra measured inside the EAST experimental hall are of great significance in many

School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China b Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China c China Institute of Atomic Energy, Beijing 102413, China d China National Institute of Metrology, Beijing 100029, China e Dipartimento di Fisica ‘G. Occhialini’, Università degli Studi di Milano-Bicocca, Milano 20126, Italy f Istituto di Fisica del Plasma “P. Caldirola”, Consiglio Nazionale delle Ricerche, Milano 20125, Italy

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aspects. First, neutron radiation is an important issue of the radiological protection for ensuring the personal safety at EAST. Several neutron dose survey meters were installed inside and outside of the EAST experimental hall to monitor the neutron ambient dose equivalent H*(10) which could provide dose information for the staff working area. These meters were calibrated with a 252Cf reference neutron source. However, the devices might give incorrect estimate of the H*(10) when the energy spectrum at the monitoring position differs markedly from the calibration source spectrum [7]. If the energy distributions of neutron fluence at the monitoring positions were known, the performance of neutron dose survey meters could be investigated, and the readings of them might be revised with the known response functions of the meters. Secondly, the neutron spectral information is crucial for evaluating the shielding adequacy of the room walls for housing the tokamak devices and other neutron generators to reduce the neutron dose rate to a stipulated level [8]. The background conditions of neutrons are also critical to optimize the shielding design of neutron diagnostic systems around tokamak devices [9]. At last, the neutrons, especially in the energy interval between 100 keV and 1 MeV, could also cause damage to electronics and silicon detectors [10-12]. So, the estimation of neutron energy spectra covering their entire energy ranges in the EAST experimental hall was an essential issue. At EAST, different kinds of neutron diagnostic devices have been installed and operated to perform neutron measurements. They mainly consist of four neutron flux monitors [4-6], a radial neutron camera [4-5], several proton recoil scintillation detectors [3-6, 13-14], and a time-of-flight enhanced diagnostics (TOFED) neutron spectrometer [1-2, 9, 15-19]. The neutron flux monitors comprise two 3He neutron counters and two 235U fission chambers. The radial camera has six channels of liquid scintillators with a collimator system combining polyethylene, lead and stainless steel. The investigation on the determination of fast ion velocity distribution and the evaluation of auxiliary heating performance from the neutron energy spectra were carried out with liquid scintillators and the TOFED system. Besides, a single crystal diamond detector has been installed at EAST to monitor neutron flux [20], and its application to spectra measurement was under testing. It could be concluded that these neutron diagnostic systems were mainly focused on neutron yield monitoring and the spectrum measurements of neutrons directly emitted from core plasmas. In order to measure the energy spectra in the entire energy range, other neutron spectrometers have to be chosen. The Bonner sphere spectrometer (BSS) is a good candidate for the neutron wide energy rang spectrum and fluence measurements. This kind of BSS has wide application in the field of neutron radiological protection despite its poor energy resolution, but could precisely give the integrated neutron fluence and the corresponding H*(10) values with the accuracy of better than 10% and 15%, respectively [21]. Its applicability to the neutron measurements in EAST experimental hall has been verified [22]. An active BSS based on a 3He thermal neutron counter was well calibrated with several mono-energetic neutron sources at Peking University [23] and then has been installed in the EAST experimental hall. The BSS, of which 2 / 15

83 84 93 94 95 96 97 98 99 100 101 102

94 104 105 106 107 108 109 110 111 112 113

the moderatorss were made from polyyethylene, could c coverr the neutroon energy raange from m thermal reegion to 20 MeV. The BSS S was ded dicated to measure the t fusion neutrons in the EA AST experimental hall h for the deuterium pplasma heated with on ne or more of the auxiliary heatting methoods includin ng lower hhybrid wav ve (LHW), ion cyclootron resonance heatting (ICRH H) and elecctron cyclootron reson nance heatin ng (ECRH)). The neu utron specctra and thee deduced values v incluuding the in ntegrated neeutron fluennce and H* *(10) valuues at the two t exposeed positionss were exp perimentally y determineed by the BSS. B Theese results were w also compared c too the calcu ulations obtaained by thhe Monte Carlo C code MCNP5. The H*(10 0) value at one exposeed position was also deerived from m the meaasured specttrum, and th hen was appplied to inv vestigate thee performannce of the EA AST neuttron dose suurvey meterr near the poosition.

2

Experim mental setu up

The BSS im mplemented d in the EA AST experim mental hall comprises oone bare sp phere and nine more spherical neutron n deteectors with the t diameteer varying bbetween 2.5 5 and 12 iinch, each of o which sh hares a singlle SP9 therm mal neutron n counter inn its center. The 3 o Hee gas pressure of the SP P9 counter iis 697 kPa at a 20 C. Th he fluence rresponses of the -9 BSS S to mono-eenergetic neeutrons from m 10 to 20 0 MeV weree calculatedd with a gen neral Monnte Carlo code c MCN NP5 and weere also ex xperimentallly evaluateed by six quasi q monno-energeticc neutron sources s froom 100 keV V to 20 MeV M on a Van de Grraaff acceelerator of Peking P Univ versity. Thee established d response function f maatrix is show wn in Fig.. 1. More details d of thee BSS param meters and response fu unctions can an be referen nced in thhe literaturee [23].

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Fig. 1. The responnse function n matrix of the t BSS

110 111 112 113

The neutronns were meeasured withh the BSS at a two expo osed positionns in the EA AST experimental hall. h The positions, as sshown in Fiig. 2, were 0.5 0 m below w the equatorial planne of the tookamak and d about 11.00 m and 14 4.3 m away y from the ttokamak ceenter, resppectively. The T four EA AST neutroon flux mo onitors, com mprising tw wo 3He neu utron 3 / 15

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counnters (labellled as 3He_ _1 and 3Hee_2) and tw wo 235U fisssion chambbers (labelleed as 235 U U_1 and 235U_2), U were employed tto monitor neutron n yield [4]. Theyy were verticcally installed on thee EAST staiinless-steel pplatform, an nd their layout is also sshown in Fiig. 2. Eacch of the fouur monitors,, 100 cm in height, wass made up of o a thermal neutron tub be in the center and a cylindricaal polyethyl ene moderaator with 5 cm c thicknesss and a thermal neuttron shieldd of 1 mm m thick caddmium. Thee experimeental calibraations of these t 241 monnitors weree performeed on a Am-Be source and a a 2.455 MeV quasi q monno-energeticc neutron source s on a Van de Graaff accelerator, annd the abso olute deteection efficiiencies for 2.45 2 MeV nneutrons weere determin ned to be abbout 6% forr the 3 235 Hee counter annd 0.2% forr the U fiission cham mber, respecttively [4]. IIn this reseaarch, the fusion neuttron yield was w obtainedd with the counts c of thee 3He_1 couunter multip plied 6 by tthe converssion factor of 1.05×100 which waas evaluated d with the combinatio on of Monnte Carlo simulation s and a in situ calibration n using a 2552Cf sourcee operated by b a rem mote mechannical hand.

125 129 130 131 132

Fig.. 2. The verrtical view of o the EAST T tokamak, and the arrangements of the BBS S and 3 neuttron monitoors in the experimenttal hall. In the diagraam, two H He counterss are 3 3 labeelled as Hee_1 and Hee_2, and twoo fission ch hambers are representedd by 235U_1 and 235 U U_2, respecttively. The two t exposedd positions of the BSS are also shoown.

140 141 142 143 144 145 146 147 148 149 150

Two separrate electron nic chains, as shown in n Fig. 3, weere employeed to record d the BSS S counts. Booth chains shared s a chharge sensitiive preampllifier (ORTE EC 142PC)) and an aamplifier (O ORTEC 572 2A), and theen the output unipolar pulses werre split into two channnels. The first channeel was fed iinto a singlee-channel analyzer a (OR RTEC 850)) and furthher connectted to the EAST E acquuisition system, and thee pulse num mber cumullated per 1 ms interrval was reecorded. Thhe rest of th he second chain was a multichaannel anallyzer, whichh could reco ord the pulsse height sp pectra. At EAST, E the fi first chain of the elecctronic systeem was app plied to recoord the coun nts of the BSS, while thhe second chain c wass used to determine d proportion p of the cou unts above the threshhold set in n the singgle-channel analyzer fo or the first chain and to t check thee stability oof the threshold duriing the expeeriment. 4 / 15

141 142

F 3. The schematics oof the electrronic system Fig. m for the BSSS

143

3

Experim mental resu ults and d discussion n

148 149 150 151 152 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183

EAST is a fully su uperconductting tokamaak and aim ms to invesstigate the key phyysics and enngineering problems p off the future fusion deviices for theiir high pow wered longg steady-staate operatio on up to 10000 s [1-2]. In the 2017 spring caampaign, EA AST has achieved a long steady y-state highh confinemeent plasma for more thhan 100 seco onds for tthe first tim me in the wo orld [24]. The BSS has been in nstalled in th the experim mental hall since Oct. 1 , 2016. Botth of the 2016 autum mn and 2017 spring ccampaigns were particcipated in to carry outt the neuttrons meassurement baased on the BSS. Du uring normal plasma discharges,, the phooton-neutronns can be negligible, n aand the nu uclear fusion n reactions dominated d the neuttron yield [6]. [ For theese shots, oon the cond dition that the t impurityy level is high, h plassma densityy was low w or disruuption happ pened, pho oton-neutronns couldn’tt be negllected and they were not n includedd in the norrmal dischaarges. Thesee selected shots s for experimenttal analysis have the pllasma curren nt Ip betweeen 300 kA aand 600 kA A and 19 3 19 3 the central electron densiity ne from 2×10 /m to 4×10 /m / . The auuxiliary heaating metthods of LH HW, ICRH, and ECRH H had injectting powerss up to abouut 3.5 MW W, 2.5 MW W and 0.5 MW, M respectively. The ttotal neutron n emission rate was upp to 6×1010/s. / In the present IC CRH heating g scenarioss at EAST, the fusion n reactions mainly yieeld a Gauussian peakk of 2.5 MeeV DD neuutrons, but its high en nergy neutroon tail wass not founnd on the energy speectrum [5-66, 25]. Theere are no differencess on the fu usion neuttron spectraa structure between b IC CRH and no on-ICRH heeating shotss. So, the IC CRH couppled shots were not sp pecially treaated in the data processing. In viiew of the little sharre less than 1% of the 14.0 MeV D DT fusion neutrons, n thee tokamak ccan be regaarded as a 2.5 MeV quasi q mono--energetic nneutron sourrce in wave heating sceenarios.

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3.1

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As only one o SP9 theermal neutroon counter is availablee, the ten sp spheres can’t be 3 exposed at the same time. The EAST T He_1 cou unter was ussed to norm malize the sp phere counnts and the other threee monitors w were emplo oyed to checck the workk stability of the 3 Hee_1 counter and the Bo onner spherres. The datta from several ten norrmal dischaarges werre collected for each sp phere. The normalized d sphere cou unts, i.e. rat atio between n the spheere counts and a the mon nitor countss averaged over o all the correspondding discharrges, yielld the respoonse of each h sphere to neutrons at a the two exposed possitions. The two

Measurements

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respponse curvees against sp phere diameeter are illustrated in Fig. 4. The ppile-up effect of the Bonner sphheres and the t 3He_1 ccounter waas taken into account, as done in n the literrature [26].. The higheer responsee values att position 1 indicate a more inttense neuttron field thhan that at position p 2 ddue to the sh horter distance of the pposition 1 away a from m the tokam mak center.. The relatiive larger response r vaalues for sm mall spherees at posiition 2 reveeal the larg ger share off low energ gy neutronss. The unceertainty of each respponse givenn in Fig. 4 is derived from the standard s deeviation of the normallized spheere counts and a is in thee order of 3--8%. The vaariations miight be due to the detecctors counnting statisttics, the flucctuations off plasma mo ovement and d neutron em emission den nsity proffile.

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Fig.. 4. The couunts of each sphere norm malized by the 3He_1 counter c couunts at two exposed positioons in the EAST E experiimental halll.

195 196 197 198 199 200 201 202

As an example, the time tracees of main plasma parameters off the disch harge #73999, such as plasmaa current I p, loop vo oltage Vp, electron ddensity ne, ion tem mperature Ti, LHW pow wer PLHW, IICRH poweer PICRH and d ECRH poower PECRHH, are illusstrated in Fig. 5. Also shown are the count raate time traces of the 22.5 inch Bo onner spheere and thee four EAST T neutron m monitors. Itt can be seeen that the count rate time depeendence of the 2.5 inc ch sphere reasonably matches the results of the neu utron monnitors, and their t count rates are cllosely related to the pllasma ion ttemperaturee and plassma densityy.

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Fig.. 5. Time traaces of the plasma p currrent Ip, loop voltage Vp, electron deensity ne, io on tem mperature Ti, powers of the LHW, IICRH and ECRH, E neutron count raates of the 3He 2 counnters (3He__1, 3He_2), 235 U fissionn chambers (235U_1, 235U_2) U and thhe 2.5 inch Bonnner sphere for discharg ge #73999.

201

3.2

215 216 217 218 219 220 221 222 223 224 225 226 227 228

The unfoldding procedu ure was impplemented by b using th he code MX XD_FC32 in n the UM MG 3.2 packkage [27] based b on a maximum m entropy algorithm a fo for deriving g the neuttron spectraa from the Bonner B sphhere responsses. The derrivation of neutron speectra from m the Bonnner sphere responses r iss a few-chaannel unfolding manippulation. Ass the Bonnner spheree number is less thann the energ gy bins in neutron sppectra, a priori specctrum shoulld be emplo oyed to starrt the unfollding proced dure. The ppriori spectrra at the two exposeed positionss were preddicted with the Monte Carlo codee MCNP5 [28]. [ Thee geometriees in the MCNP5 M moodel includ de the EAS ST tokamakk based on n the engiineering draawings, staiinless steel platform an nd the conccrete shieldiing walls of the experimental hall. h The ord dinary conccrete [29] was w served as a the compposition inpu ut of the shielding walls. w The so ource was ddescribed with w mono-en nergetic 2.55 MeV neuttrons and sampled uniformly u in n the vacuuum vessel of o the EAST T tokamak, as done in n the literrature [13]. The plasm ma center iss 188 cm aw way from the t tokamakk center. Fiig. 6 givees the calcculated neu utron specttra at the two exposed positioons, which are

The unfoolded neutrron spectraa and their comparison c n with calcculated resu ults

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reprresented wiith the enerrgy distribuution of neeutron fluen nce and plootted in do ouble logaarithmic cooordinates.

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Fig. 6.. The calculaated neutron spectra at tw wo exposed positions of thhe BSS

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The callculated speectra at the ttwo position ns are simillar in the shhape, though h the largger responsee values of the Bonnerr spheres att position 1 deduces ann overall hiigher flueence spectruum. The un n-collided nneutrons weere identifieed with a 2 .5 MeV en nergy peakk in the speectra. Scatteered neutronns have a wide w energy range from m thermal reegion to bbelow 2.5 MeV. M This part of speectra can be b characterrized with a thermal peak p -2 (bellow 0.4 eV)), a high en nergy peak (10 MeV V - 2.5 MeV V) and an inntermediatee flat -2 enerrgy region (0.4 ( eV – 10 MeV) bbetween theem. In orderr to quantify fy the difference of thhe two calcculated specctra, Table 1 gives thee shares of the neutronn fluence in n the fourr energy reggions mentiioned abovee, the fluen nce-average energies, aand the abso olute flueence per souurce neutron n at the twoo positions. The neutro ons betweenn 0.01 MeV V and 2.5 MeV in thhe two calcculated speectra contribute about half of thhe total neu utron flueence. The prroportion off the 2.5 M MeV neutron ns is less thaan 10%. 2.55 MeV neuttrons in tthe positionn 1 spectrum m have a rrelative high her share than that inn the positio on 2 specctrum, whicch results from fr the clooser distancce to the tok kamak portts. Compareed to the position 1, the low eneergy neutronns at positio on 2 have a higher propportion, beccause posiition 2 is clloser to the concrete shhielding wall and doesn n’t face thee tokamak ports. p As shown in Fig. F 7, the decreasing trend of th he fluence against raddial distancee for scatttered neutrrons abovee 0.01 MeV V changes slowly wh hen the exxposed position apprroaches thee concrete shielding w wall. For th he neutronss of below 0.01 MeV V the flueence increasses with thee distance, aas the shield ding walls can c reflect ssource neuttrons and also contriibutes a part of scattereed neutronss [30]. The data in Tabble 1 also reeveal that high enerrgetic neutrrons in thee position 1 spectrum m have larrger proporrtion com mpared to thhat in the po osition 2 speectrum, whiich reasonab bly correspponds to a laarger flueence-average energy vaalue at posittion 1.

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Tabble 1. The reesults obtained from thhe calculateed and expeerimental unnfolded neu utron specctra at the two t exposeed positionss, including the shares of the flueence in the four 8 / 15

248 249 250

enerrgy regionss, the flueence-averagee neutron energies, and a the abbsolute neu utron flueence per souurce neutro ons. Also liisted are th he ratios of calculationn to experim ment flueence-average energies and a absolutee fluence. Position 1

Quality Q Share of Fluence (%) Fluuence-average energy

Cal C

Mea

Cal

M Mea

< 0.44 eV

12.4 1

13.7

18.2

233.3

0.4 eV-0..01 MeV

22.0 2

29.1

27.0

333.7

0.01-2.55 MeV

57.7 5

50.3

50.4

400.0

2.5 M MeV

7.9

7.0

4.4

33.1

Value ((MeV)

0.46 0 ±0 0.00

0.32 0.39 ±0.04 ±0.00

0..23 ±00.02

Cal/M Mea

1.16±0.12

-2

Fluence per F soource neutron n

249 253 254 255 256

Position 2

Value (cm ) ×1 07 Cal/M Mea

1.39 1 ±0 0.01

1.43±0.114

1.03 1.34 ±0.13 ±0.01

1.04±0.10

0..69 ±00.07

1.49±0.115

Fig.. 7. The raddial dependeence of calcculated neu utron fluencce in the raddial lines off the twoo positions for f total neu utrons, neutr trons below 0.4 eV, neu utrons from m 0.4 eV to 0.01 MeV V, neutronss from 0.01 MeV to beelow 2.5 MeV M and 2.5 5 MeV neut utrons. The total meaasured fluennce values at a the two poositions are also illustraated. 9 / 15

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The derived unfolded spectra at the two ex xposed positions are shhown in Fig g. 8, 3 whiich are norm malized to the t counts oof He_1 co ounter. The calculated spectra are also plottted in Fig. 8, and aree adjusted inn height to have the same s total nneutron fluence withh the corresponding un nfolded oness. The shapees of the callculated andd correspon nding unfoolded specttra have geeneral goodd agreemen nt, which reesult in thaat the sharees of neuttron fluencee is consisteent betweenn them in th he four enerrgy ranges as illustrateed in Tabble 1. Howeever, there are also som me major differences d in the interrmediate reegion and above 0.4 MeV. Com mpared to thhe measured d spectra, the calculateed spectra have h largger share off high enerrgetic neutrrons and yiield higher fluence-avverage energ gies. Theese differencces can be further seenn from the quantitativee shares of the neutron ns in the four energyy ranges for the calculaated and exp perimental spectra s as giiven in Tab ble 1. In thhe MCNP5 model, thee thin stainleess steel plaate and otheer diagnostiic devises in n the tokaamak ports were not deescribed. Thhe inadequaate geometry y descriptioon might leaad to the moderationn underestim mation of soource neutro ons, and mad de the low eenergy neuttrons show w a lower proportion. p The unfoldded and calcculated spectra can yieeld the abso olute neuttron fluence, as given in Fig. 7 aand Table 1. 1 The obtaiined simulaation resultss are largger than the experimen ntal values, but agreed reasonably with them within 5% % and 50% % for positiion 1 and position p 2, respectively y. The MA AXED codee can’t givee the assoociated unccertainties of the unnfolded flu uence specttra. The uuncertaintiess of inteegrated neuttron fluencce and the ambient do ose equivallent H*(10)) were rou ughly estim mated to be b 10%. The T consideered contriibutions aree the calibbrated response funcction of thee BSS (~6% %) and thee Bonner sp phere respo onses at thee two posittions (<8% %).

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Fig.. 8. Calculatted and exp perimental nneutron specctra at the tw wo exposedd positions.

279

3.3 The H*(100) comparisson between n the calcu ulation and experimennt

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The neutronn dose in th he EAST exxperimental hall was also investigaated. At thee two exposed positioons, the ambient dose eequivalent H*(10) H valu ues were deeduced from m the convvolution off neutron sp pectra with the converrsion coefficients takenn from the ISO 10 / 15

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12789-2:2008(E) manual [21]. Table 2 presents the calculated and experimental results of neutron dose corresponding to the neutron spectra in Fig. 6. The neutrons above 0.01 MeV contribute the dominant neutron dose with proportions of more than 94%. The share of neutron dose for neutrons below 0.01 MeV is far more less than the corresponding fluence results as given in Table 1, as the conversion coefficient for neutrons above 0.01 MeV is about 400 pSv.cm2 and far greater than the coefficient of about 10 pSv.cm2 for neutrons below 0.01 MeV. The shares of neutron dose in the four energy ranges were reasonably consistent between the experiment and calculated spectra at the two exposed positions, which can be revealed by the good match of the dose-equivalent-average energy. In term of the neutrons between 0.01 MeV to 2.5 MeV, the agreement of the neutron dose between the calculated and measured spectra is better than that of the corresponding neutron fluence. The absolute H*(10) values were also compared between the calculation and the measurement. The agreement is within 20% for position 1 spectra. At position 2, the calculation made a doubling overestimation of the neutron dose. The deviation of the absolute H*(10) value between the calculation and the measurement is larger than that of the absolute fluence values. The reasons might be the overestimation of the integrated neutron fluence values by the MCNP5 calculation and large conversion coefficient for neutrons above 0.01 MeV. The value of the H*(10) per source neutron estimated by a homemade dose survey meter near the exposed position 2 was 18% higher than the result obtained by the BSS. The good consistence of the H*(10) values estimated by the BSS and the survey meter verified the reliability of the results by these two devices. The survey meter is a routine instrument for monitoring the H*(10) value at the EAST experimental hall. It has an about 1.5 m distance to the position 2 and further away from the tokamak compared to the BSS position 2. According to the MCNP5 calculation, the ratio of the H*(10) value at position 2 to the value at the survey meter position is 1.1. The survey meter reading given in Table 2 has been revised using this factor. This correction might be larger than that in reality, because the inadequate geometry description in MCNP5 model maybe enlarge the ratio of H*(10) values at these two positions. It should be pointed out that the applicability of the survey meter to EAST neutron radiation measurement should be further investigated. If the response function of the survey meter is given, then the correction factor of the survey meter reading can be obtained, and finally the performance of the survey meter at EAST can be further evaluated quantitatively.

317 318 319

Table 2. The ambient dose equivalent H*(10) results obtained from the calculated and experimental spectra of the two exposed positions, including the proportion values of H*(10) in the four energy ranges, the values of H*(10) per fluence, and the values of

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absolute H*(10) per source neutrons. Near the position 2, the H*(10) value estimated with a survey meter was also shown. Position 1

Quality

Share of H*(10) (%)

H*(10)-average energy

Absolute H*(10) per source neutron

Position 2

Cal

Mea

Cal

Mea

< 0.4 eV

0.9

1.2

1.7

2.9

0.4 eV-0.01 MeV

1.3

2.1

2.0

3.5

0.01-2.5 MeV

77.3

75.5

81.7

80.1

2.5 MeV

20.5

21.3

14.6

13.5

Value (MeV)

1.07 ±0.00

1.08 ±0.11

0.94 ±0.01

0.87 ±0.09

Cal/Mea Value(uSv) ×1011

0.99±0.10 2.26 ±0.01

Cal/Mea

1.88 ±0.19

1.20±0.12

Survey meter (uSv) ×1011

#

Meter/Mea

# #

1.085±0.109 1.31 ±0.01

0.65 ±0.07

2.00±0.20 #

0.77 ±0.13

1.18±0.20

322

4

Conclusions and outlook

323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345

A BSS was successfully employed to investigate the EAST neutron field in a single and coupled heating scenarios including LHW, ICRH and ECRH. In the EAST experimental hall, the neutron spectra at two positions were unfolded from the responses of Bonner spheres. The measured neutron spectra match the calculated results well, even though some disagreement between them was found. The BSS gave an over-moderated neutron spectra compared to the calculated ones at the two exposed positions. The Monte Carlo calculation overestimated the integrated neutron fluence by from 5% to 50%, and the deduced H*(10) values were overestimated by from 20% to a factor of 2. These discrepancies can be explained by the inadequate geometry description in the MCNP5 model of EAST. In order to improve the estimation accuracy by Monte Carlo simulation to characterize the neutron fields at EAST, besides the tokamak device, stainless steel platform and shielding wall all the diagnostic devices and other facilities in the experimental hall should also be included in the MCNP5 geometry description. Additionally, the response function of the neutron dose survey meter installed at EAST, along with the neutron field near them, should be characterized to further investigate the applicability of the neutron dose survey meter installed at EAST. This work was focused on the low yield neuron field measurements for the EAST deuterium operations. However, when the EAST plasma is heated with NBI, the neutron emission rate can be up to 1014 s-1, and the present active BSSs will suffer from saturation problem and can’t work normally. As staff are forbidden to enter the EAST experiment hall during experimental operations, the passive BSSs are also not a choice. A new active BSS has been developed using a single crystal chemical vaper 12 / 15

346 347 348 349 350

deposition diamond detector as the thermal neutron sensor and can work well for the EAST high neutron yield campaigns. The new BSS is also a good candidate of neutron spectrometers for neutron field measurement in the International Thermonuclear Experimental Reactor (ITER) and the China Fusion Engineering Test Reactor (CFETR).

351

Acknowledgments

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This work was supported by the National Key Research and Development Program of China (Nos. 2016YY0200804 and 2017YFF0206205), the State Key Program of National Natural Science of China (No. 11790324), and the National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB106004 and 2012GB-101003). The authors are very grateful to the EAST operation team for their help during the experimental campaigns.

358

References

359 360

[1] Xing Zhang, et al., Diagnosing NB plasmas on the EAST tokamak with new time-of-flight neutron spectrometer, Nuclear Fusion, 54 (2014) 104008.

361 362

[2] Z. Chen, et al., Simulation of neutron emission spectra from neutral beam-heated plasmas in the EAST tokamak, Nuclear Fusion, 53 (2013) 063023.

363 364

[3] Peng Xingyu, et al., Application of a BC501A Liquid Scintillation Detector with a Gain Stabilization System, Plasma Science and Technology, 18 (2016) 23.

365 366

[4] G. Q. Zhong, et al., Status of neutron diagnostics on the experimental advanced superconducting tokamak, Review of Scientific Instruments, 87 (2016) 11D820.

367 368

[5] Guoqiang Zhong, et al., The behavior of neutron emissions during ICRF minority heating of plasma at EAST, Plasma Phys. Control. Fusion, 58 (2016) 075013.

369 370

[6] Zhong Guoqiang, et al., Measurement of Neutron Flux at the Initial Phase of Discharge in EAST, Plasma Science and Technology, 13 (2011) 162.

371 372 373

[7] International Standard Organization (ISO), Reference radiation fields — Simulated workplace neutron fields - Part 1: Characteristics and methods of production, ISO 12789-1:2008(E), 2008.

374 375 376

[8] P. Srinivasan, et al., Assessment of radiation shield integrity of DD/DT fusion neutron generator facilities by Monte Carlo and experimental methods, Nucl. Instr. Meth. Phys. Res. B, 342 (2015) 125.

377 378 379

[9] T. F. Du, et al., Design of the radiation shielding for the time of flight enhanced diagnostics neutron spectrometer at Experimental Advanced Superconducting Tokamak, Review of Scientific Instruments, 85 (2014) 11E115.

380

[10] Yuri Fisyak, et al., Thermal neutron flux measurements in the STAR experimental 13 / 15

381

hall, Nucl. Instr. Meth. Phys. Res. A, 756 (2014) 68.

382 383 384

[11] K. Ogawa, et al., Investigation of irradiation effects on highly integrated leading-edge electronic components of diagnostics and control systems for LHD deuterium operation, Nuclear Fusion, 57 (2017) 086012.

385 386 387

[12] Takeo Nishitani, et al., Radiation Field Estimation for the Diagnostic and Control Components by Monte Carlo Neutronics Calculations with LHD 3-Dimensional Modeling, Plasma and Fusion Research, 11 (2016) 2405057.

388 389

[13] X. Yuan, et al., Neutron energy spectrum measurements with a compact liquid scintillation detector on EAST, Journal of Instrumentation, 8 (2013) P07016.

390 391 392

[14] Xing Zhang, et al., A compact stilbene crystal neutron spectrometer for EAST D-D plasma neutron diagnostics, Review of Scientific Instruments, 84 (2013) 033506.

393 394 395

[15] Xing Zhang, et al., Second generation fusion neutron time-of-flight spectrometer at optimized rate for fully digital data acquisition, Review of Scientific Instruments, 85 (2014) 043503.

396 397 398

[16] Z. J. Chen, et al., Data acquisition system with pulse height capability for the TOFED time-of-flight neutron spectrometer, Review of Scientific Instruments, 85 (2014) 11D803.

399 400 401 402

[17] X. Y. Peng, et al., Light output function and assembly of the time-of-flight enhanced diagnostics neutron spectrometer plastic scintillators for background reduction by double kinematic selection at EAST, Review of Scientific Instruments, 85 (2014) 11E112.

403 404 405

[18] Z. Q. Cui, et al., Design of a magnetic shielding system for the time of flight enhanced diagnostics neutron spectrometer at Experimental Advanced Superconducting Tokamak, Review of Scientific Instruments, 85 (2014) 11D829.

406 407 408

[19] Zhang Xing, et al., The Design and Optimization of a Neutron Time-of-Flight Spectrometer with Double Scintillators for Neutron Diagnostics on EAST, Plasma Science and Technology, 14 (2012) 675.

409 410 411

[20] Du Tengfei, et al., Time Dependent DD Neutrons Measurement Using a Single Crystal Chemical Vapor Deposition Diamond Detector on EAST, Plasma Science and Technology, 18 (2016) 950.

412 413 414

[21] International Standard Organization (ISO), Reference radiation fields — Simulated workplace neutron fields - Part 2: Calibration fundamentals related to the basic quantities, ISO 12789-2:2008(E), 2008.

415 416 417 418

[22] Z. M. Hu, et al., Monte Carlo simulation of a Bonner sphere spectrometer for application to the determination of neutron field in the Experimental Advanced Superconducting Tokamak experimental hall, Review of Scientific Instruments, 85 (2014) 11E417. 14 / 15

419 420 421

[23] Z. Hu, et al., Experimental determination of the response functions of a Bonner sphere spectrometer to monoenergetic neutrons, Journal of Instrumentation, 12 (2017) T06002.

422

[24] http://english.ipp.cas.cn/syxw/201707/t20170705_179369.html.

423 424

[25] Li Xiaoling, et al., Neutron Yields Based on Transport Calculation in EAST ICRF Minority Heating Plasmas, Plasma Science and Technology, 15 (2013) 950.

425 426

[26] Marco Marinelli, et al., Synthetic single crystal diamond as a fission reactor neutron flux monitor, Applied Physics Letters, 90 (2007) 183509.

427 428 429

[27] M. Reginatto, The ‘few-channel’ unfolding programs in the UMG package: MXD_FC32, GRV_FC32 and IQU_FC32, Physikalisch-Technische Bundesanstalt, Braunschweig, Germany, 2004.

430 431 432

[28] X-5 Monte Carlo team, MCNP - a general Monte Carlo N-particle transport code, version 5, Report LA-UR-03-1987, Los Alamos National Laboratory, Los Alamos U.S.A., 2003.

433 434

[29] R. G. Williams III, et al., Compendium of material composition data for radiation transport modeling, Pacific Northwest National Laboratory, Washington, USA, 2006.

435 436

[30] J. B. Hunt, The calibration of neustron sensitive spherical devices, Radiation Protection Dosimetry, 8 (1984) 239.

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Highlights (for review)

Highlights 1) A Bonner sphere spectrometer was employed to measure the neutron field at EAST tokamak. 2) The neutron spectra at two exposed positions were obtained with the spectrometer. 3) The Monte Carlo model of EAST was established to predict the neutron fields. 4) The neutron spectra and deduced integrated fluence and H*(10) values were compared between calculation and experiments. 5) The applicability of a homemade dose survey meter installed at EAST was verified.