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研究堆慢正电子源构建中的关键机理问题

王冠博 李润东 杨鑫 曹超 张之华

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研究堆慢正电子源构建中的关键机理问题

王冠博, 李润东, 杨鑫, 曹超, 张之华

Key physics mechanism of the research reactor based slow positron source

Wang Guan-Bo, Li Run-Dong, Yang Xin, Cao Chao, Zhang Zhi-Hua
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  • 研究堆慢正电子源是获得高强度慢正电子束流的有效方式,国际上已建成多座装置并获得广泛应用. 与常规同位素慢正电子源相比,研究堆慢正电子源的物理过程复杂,影响末端束流强度的因素众多,对其进行深入研究与合理建模是未来在中国绵阳研究堆(CMRR)上构建慢正电子源的基础. 本文厘清了研究堆慢正电子产生的关键过程与物理机理,建立了预测末端正电子束流强度的理论模型,找到了影响其末端强度的主要物理量:快正电子体产生率、慢化体有效表面积、慢化体扩散距离、慢正电子从表面被提取到靶环末端的效率、及束流系统提取效率. 用多种实验结果对模型进行校验,包括多个同位素慢正电子源的效率测量值,以及PULSTAR研究堆慢正电子源测量结果,充分验证了模型正确性. 根据模型对各物理量的影响因素进行了分析,找到了需着重关注的影响因素,对未来源/靶结构的设计给出建议.
    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.
      通信作者: 李润东, amdom@sohu.com
    • 基金项目: 国家自然科学基金(批准号:11405151,11475152)和中国工程物理研究院中子物理实验室基金(批准号:2014BC02)资助的课题.
      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).
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    Hugenschmidt C, Lwe B, Mayer J, Piochacz C 2008 Nucl. Instrum. Meth. A 593 616

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    Hugenschmidt C, Schreckenbach K, Habs D 2012 Appl. Phys. B 106 241

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    Zecca A 2002 Appl. Surf. Sci. 19 4

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    Suzuki R, Amarendra G, Ohdaira T, Mikado T 1999 Appl. Surf. Sci. 149 66

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    [27]

    Weng H M, Ling C C, Beling C D, Fung S, Cheung C K 2004 Nucl. Instrum. Meth. B 225 397

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    Brusa R S, Naia M D, Galvanetto E, Scardi P, Zecca A 1992 Mater. Sci. Forum 105 1849

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    Gramsch E, Throwe J, Lynn K G 1987 Appl. Phys. Lett. 51 1862

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    Reurings F, Laakso A, Rytsl K, Pelli A 2006 Appl. Surf. Sci. 252 3154

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    Zafar N, Chevallier J, Laricchia G, Charlton M 1989 J. Phys. D: Appl. Phys. 22 868

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    Zafar N, Chevallier J, Jacobsen F M, Charlton M, Laricchia G 1988 Appl. Phys. A 47 409

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  • [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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出版历程
  • 收稿日期:  2016-11-01
  • 修回日期:  2017-01-22
  • 刊出日期:  2017-04-05

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