Key physics mechanism of the research reactor based slow positron source

Wang Guan-Bo; Li Run-Dong; Yang Xin; Cao Chao; Zhang Zhi-Hua

doi:10.7498/aps.66.082802

Abstract

In the world there have been built five reactor based slow positron sources producing very intense beams, of which, the NEPOMUC source generates the highest intensity about 3109 e+/s after updated. The beam intensity depends on the power of the core, the converter material, and the moderator geometry. It is important to have good knowledge of the influencing factors and relevant processes for building a positron source in China Mianyang Research Reactor (CMRR). In this paper, the basic mechanism and several pivotal processes are studied and modeled, including the high energy ray induced fast positron generated in target, the moderation of fast positron to slow positron, the emission of slow positron from surface, the extraction of slow positron from surface to external grid, and finally the focusing and transport by beam optic system. The beam intensity at the end of the solenoid can be deduced as I = Emth 12, where 1 is the slow positron extraction efficiency from moderators, 2 is the efficiency of lens extraction and solenoid transportation, and Emth is the slow positron emission rate from surface. The value of Emth can be expressed as Emth= AP 2L+e+pbmod, where A is the effective surface area of the moderator, P is the generating rate of the fast positron in unit volume, L+ is the slow positron diffusion length, e+ is the branching ratio of surface positron ( 0.25), i.e. the ratio of positrons reaching the surface to that emitted freely, pbmod ( 0.4) is the probability of the emitted moderated positron. Therefore, attention should be paid to the values of P, L+, 2 and A to enhance the beam intensity. P is in proportion to the neutron absorption rate by cadmium, which requires higher neutron flux of incidence. L+ is sensitive to the moderator material and its annealing condition. For the well annealed single crystal tungsten, the value of L+ is about 100 nm, while for that annealed at 1600 ℃, it decreases to only 40 nm. The value of 1 is related to the moderator depth/width ratio, the extraction voltage, and the moderator back layout. Although deeper ring can enlarge the moderator area A, the average extraction efficiency 1 decreases obviously. Considering the product of 1 and A, the recommended depth/width ratio is 3 : 1. Validations are performed by employing two types of experimental results, including several isotope slow positron sources and the PULSTAR reactor based source. The calculated efficiencies of isotope sources match well with the experimental measured results, which verifies our basic model and parameters. With these parameters and models, the intensity of PULSTAR reactor based positron source at system exit is calculated to be 5.8108e+/s, which matches well with the reported measured value of (0.5-1.1)109e+/s. Some suggestions are made and will be considered in our future design of positron source.

Keywords:

research reactor based positron source /
mechanism and models /
positron beam intensity

Authors and contacts

1.
Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China

Corresponding author: Li Run-Dong, amdom@sohu.com

Funds: Project supported by the National Natural Science Foundation of China (Grant Nos. 11405151, 11475152) and the Key Laboratory of Neutron Physics, China Academy of Engineering Physics (Grant No. 2014BC02).

References

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[22]	Brandt W, Paulin R 1977 Phys. Rev. B: Condens. Matter 15 2511
[23]	Suzuki R, Amarendra G, Ohdaira T, Mikado T 1999 Appl. Surf. Sci. 149 66
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Cited By

[1]	Golge S, Vlahovic B 2012 Proceedings of IPAC New Orleans, Louisiana, USA, May 20-25, 2012 p1464
[2]	Hugenschmidt C, Brunner T, Legl S, Mayer J, Piochacz C 2007 Phys. Status Solidi 4 3947
[3]	Hugenschmidt C, Ceeh H, Gigl T, Lippert F 2013 J. Phys.: Conf. Ser. 443 012079
[4]	Hawari A I, Gidley D W, Moxom J, Hathaway A G, Mukherjee S 2011 J. Phys.: Conf. Ser. 262 012024
[5]	Hugenschmidt C, Piochacz C, Reiner M, Schreckenbach K 2012 New J. Phys. 14 778
[6]	Hugenschmidt C 2011 J. Phys.: Conf. Ser. 262 12002
[7]	Falub C V, Eijt S W H, Mijnarends P E, Schut H, van Veen A 2006 Nat. Mater. 5 23
[8]	Hugenschmidt C, Qi N, Stadlbauer M, Schreckenbach K 2009 Phys. Rev. B 80 308
[9]	Hengstler-Eger R M, Baldo P, Beck L, Dorner J, Ertl K 2012 J. Nucl. Mater. 423 170
[10]	Wu Y C, Hu Y, Wang S J 2008 Prog. Phys. 28 83 (in Chinese) [吴奕初, 胡懿, 王少阶 2008 物理学进展 28 83]
[11]	Wu Y C 2005 Prog. Phys. 25 258 (in Chinese) [吴奕初 2005 物理学进展 25 258]
[12]	Wang B Y, Ma Y Y, Wang P, Cao X Z, Qin X B, Zhang Z, Yu R S, Wei L 2008 Chin. Phys. C 32 156
[13]	Cao X Z, Wang B Y, Wang P, Ma Y Y, Qin X B, Wei L 2006 High Energ. Phys. Nucl. 30 1196 (in Chinese) [曹兴忠, 王宝义, 王平, 马雁云, 秦秀波, 魏龙 2006 高能物理与核物理 30 1196]
[14]	Triftshuser G, Kogel G, Triftshuser W 1997 Appl. Surf. Sci. 116 45
[15]	Straer B, Springer M, Hugenschmidt C, Schreckenbach K 1999 Appl. Surf. Sci. 149 61
[16]	Hugenschmidt C, Koģel G, Repper R, Schreckenbach K, Sperr P, Strar B, Triftshuser W 2002 Nucl. Instrum. Meth. B 192 91
[17]	Hugenschmidt C, Lwe B, Mayer J, Piochacz C 2008 Nucl. Instrum. Meth. A 593 616
[18]	Moxom J, Hathaway A G, Bodnaruk E W, Hawari A I, Xu J 2007 Nucl. Instrum. Meth. A 579 534
[19]	Hugenschmidt C, Schreckenbach K, Habs D 2012 Appl. Phys. B 106 241
[20]	Zecca A 2002 Appl. Surf. Sci. 19 4
[21]	Seeger A, Britton D T 1999 Appl. Surf. Sci. 149 287
[22]	Brandt W, Paulin R 1977 Phys. Rev. B: Condens. Matter 15 2511
[23]	Suzuki R, Amarendra G, Ohdaira T, Mikado T 1999 Appl. Surf. Sci. 149 66
[24]	Jrgensen L V, Labohm F, Schut H, van Veen A 1998 J. Phys.: Condens. Matter 10 8743
[25]	Hugenschmidt C, Koģel G, Repper R, Schreckenbach K, Sperr P, Triftshuser W 2002 Nucl. Instrum. Meth. B 198 220
[26]	Teng M K, Shen D X 2000 Positron Annihilation Spectroscopy and Its Applications (Beijing: Atomic Energy Press) pp5-6 (in Chinese) [滕敏康, 沈德勋 2000 正电子湮没谱学及其应用 (北京: 原子出版社) 第5-6页]
[27]	Weng H M, Ling C C, Beling C D, Fung S, Cheung C K 2004 Nucl. Instrum. Meth. B 225 397
[28]	Brusa R S, Naia M D, Galvanetto E, Scardi P, Zecca A 1992 Mater. Sci. Forum 105 1849
[29]	Gramsch E, Throwe J, Lynn K G 1987 Appl. Phys. Lett. 51 1862
[30]	Reurings F, Laakso A, Rytsl K, Pelli A 2006 Appl. Surf. Sci. 252 3154
[31]	Zafar N, Chevallier J, Laricchia G, Charlton M 1989 J. Phys. D: Appl. Phys. 22 868
[32]	Zafar N, Chevallier J, Jacobsen F M, Charlton M, Laricchia G 1988 Appl. Phys. A 47 409
[33]	Hathaway A G, Skalsey M, Frieze W E, Vallery R S, Gidley D W 2007 Nucl. Instrum. Meth. A 579 538

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