國聖核電廠微中子實驗室以碘化銫晶體陣列對微中子與電子彈性散射之量測

Measurement of Neutrino-electron elastic scattering with a CsI(Tl) crystal array at the Kuo-Sheng Reactor Neutrino Laboratory

林興德(Lin Shin-Ted)

指導教授 ： 王名儒

共同指導教授 ： 王子敬(Tsz-King Wong)

國立臺灣大學/理學院/物理研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.02093

摘要

研究微中子和電子彈性散射作用相當適合於核電廠中測量，此作用在標準模型中包含了弱作用的帶電之W波色子和中性電之Z波色子傳遞的效應，一個對反彈電子的能譜之量測是可以決定Weinberg角和其中性和帶電作用之破壞性干涉項•本實驗將測量未被測試的MeV能區•而量測此作用的策略將專注於3至8 MeV之間，其原因有二：一為此能區高於自然放射線的峰值，對背景有顯著的降低；二為對核電廠的反微中子能區有較精確的瞭解•而本實驗以200公斤的碘化銫陣列偵測器則可補償於作用截面對能量的遞減。選擇單獨的事件是此反應的候選人，其中排除的來源有宇宙射線、光子造成之康普敦效應、波形鑑別、晶體兩端的邊界。在瞭解背景的分析中，對放射性Th和U衰變的內部晶體含量有很靈敏的測量，多重擊發晶體事件的研究也使其背景有充分的理解。收集九個半月的數據，其中包含50天的關機時間。 透過開機與關機的數據相減，最佳擬合Weinberg角的初步結果為0.37正負0.13在3至8 MeV；於3.5至8 MeV最佳值則為0.30。而更進一步對背景直接壓低的方法可為波形做Chi平方函數的計算，比較由Th衰變裡的電子訊號與實驗中背景之差異、使用全面性的擬合方法、累積更多的數據。再加上對多重擊發事件對背景的充分解釋，這預期的背景可使微中子和電子作用之量測達到小於20%的精確；亦可有對Weinberg角有8%精確量測，且對其中性和帶電的干涉項有25%之敏感度

關鍵字

核電廠微中子 ； 電子微中子散射面積 ； 溫伯格角度 ； 鈾釷衰變 ； 碘化銫晶體

並列摘要

Reactors are well suited to study ee scattering. Both charged current (CC) and neutral weak currents (NC) are involved in the Standard Model. A measurement of the differential cross section allows to determine the Weinberg angle sin2 W. and to observe the interference which is expected to be destructive for reasonable values of sin2 W. This thesis will focus on untested few MeV range.
To measure the ee scattering, the strategy is to choose on high (>3 MeV) recoil energy above those from natural radioactivity, with the additional merits of having good accuracies in the knowledge of the reactor anti-neutrino spectra. The large target mass compensates the drop in the cross-sections. Selecting lone events,basically neutrino ones, adopted veto of cosmic ray,"Single Hit" and pulse shape discrimination(PSD). According to the PSD capabilities of the crystal, as well as the temporal and spatial correlation of the events, allow a sensitive measurement of intrinsic radio-purity of 235U, 238U and 232Th series. Their concentration gradients with the crystal growth axis, and uniformity among di erent crystal samples, are discovered. This analysis not only provides in the measurement of the detector performance parameters, such as spatial resolution and quenching factors, but also reduces the background due to the contamination with 0.0085 kg 1day 1 at 3 5 MeV.
A preliminary background level of 0.412 kg 1day 1 at 3 8 MeV, were achieved based on 5592 and 2079 kg-day of reactor ON and OFF data in Period II 1. After subtracting ON and OFF data, we got the e event rate level of 0:0320 0:0167 kg 1day 1 at 3 8 MeV. The best t of sin2 w,0:37+- 0:13 at 3 8 MeV, 0:30+- 0:22 at 3.5 8 MeV respectively have been carried out.
The possibilities of reducing background further can be the PSD as thefunction of 2 related to r/e reference pulses, global- t method, more data accumulated which already has another 27000 kg-day 2 on hand and the coming Period IV 3 On the other hand, background understanding corresponding to the 2-hit and 3-hit analysis, including the pair production e ect, 208Tl contamination and ine cient cosmic ray, the preliminary data has been able to explain the dominating background. The expect result of the interaction of e-e Scattering cross section can achieve better than 5 measurement, <8% accuracy of sin2 w, and 4 measurement of destructive interference
term.

並列關鍵字

Reactor neutrino ； neutrino and electron scattering ； Weinberg anglel ； radiopurity U and Th ； CsI crystal

參考文獻

[2] M. Z. Wang, et al., Phys. Lett. B, (2002)

[3] W. P. Lai, et al., Nucl. Instr. and Meth. A 465, 550 (2001)

[5] Y. Liu, et al., Nucl. Instr. and Meth. A 482, 125 (2002)

[6] Q. Yue, et al., Nucl. Instr. and Meth. A 511, 408 (2003)

[7] H. T. Wong, et al., Astropart. Phys. 14, 141 (2000)