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BESIII 亮度监测器的运行结果

薛镇

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Outline

• 亮度监测器原理和结构• 亮度监测器相对亮度的测量的精度• 亮度监测器的 bunch by bunch 测量• 利用亮度监测器对束团尺寸的测量

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亮度监测器的原理和结构

单轫致辐射过程

• e+ e- e+ e- 通过探测 来确定亮度

• 截面大，可以实现快速更新 ～ 161 mb

when kt=50MeV

• 出射的 沿着束流方向且张开角度很小，0.5 mrad 以内的光子占 66%以上 @1.89GeV

“ 零角度亮度探测器”

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• 切伦科夫介质采用 熔融石英 (fused silica)非常高的抗辐照性能 (>109 rad/y)n=1.46

• 采用钨作转换体。• 仅测计数，所占空间小。• 主要用于相对测量

亮度监测器的原理和结构

5surrounded by tungsten shield

Light pipe

钨fused silica

PMT

钨转换体厚为 1.2cm 。

熔融石英的几何尺寸为 4.5 x 4.0 x 6.6 cm3

亮度监测器的原理和结构

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According to Fig. (a) and (b) the ratio between the counting rates with separated beams and colliding beams are lower than 3.5%.

Fig. (a) Counting rate with colliding beams normalized by the current of e- and e+ bunches vs. the FEE threshold. Fig. (b) Counting rate with separated beams normalized by the current of e- and e+ bunches vs. the FEE threshold.

相对亮度测量的精度

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相对亮度测量的精度

According to the table above the error of relative luminosity with threshold of 400mV and the pair of bunches currents of 5×5mA2 is about 0.9%.

the counting rates and their errors due to different thresholds in SBBS (I+=5.11mA,I-=5.00mA ）

Thr.(mV) 100 200 300 400 500 600 800 1000 1200 1400

25991 19946 16245 13535 11281 9226 6036 3305 1509 649

25992 19946 16245 13535 11281 9226 6036 3305 1509 649

161 141 127 116 106 96 78 57 39 25

857 575 420 306 250 192 120 58 17.5 0

29 24 20 17 16 14 11 8 4 0

31775 24413 19914 16628 13853 11335 7414 4064 1866 811

204 179 161 147 134 121 98 72 49 25

0.0081 0.0092 0.0102 0.0111 0.0121 0.0134 0.0166 0.0222 0.0326 0.0081

tn

n

n

bn

bn

SBn

SB

SB

SB

n

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相对亮度测量的精度

由上图可见 , 东西零角度亮度监测器给出的相对亮度的趋势和量能器端盖的亮度值的趋势接近

9零角度亮度监测器能够给出各个束团的相对亮度 ,从而使调束人员对各个束团的注入和对撞情况有了清楚的了解 .

亮度监测器的 bunch by bunch 测量

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从纵向扫描看 , 前 5 个束团明显比后 5 个束团注入质量更高 ;从横向扫描看 , 这一点不明显 , 因为横向的束团尺寸较大 .

亮度监测器的 bunch by bunch 测量

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By steering the electron beam while observing the counting rate changes of the luminosity monitor, the horizontal and vertical sizes of the bunch spots can be estimated as shown in the figure. There are 70 e+ bunches and 10 e- bunches stored in the storage rings. The total e+ current is 321.660 mA, the total e- current is 54.758 mA. The data of the figure is of the fifth e+ and e- bunch pair. Fitted by gaussian function, the horizontal standard deviation σx=0.50±0.01mm, the vertical standard deviation is σy

=0.0146±0.0003mm.

mmss xexe 01.035.0 mmss yeye 0002.00103.0

,

Supposing σe+ = σe- in both horizontal and vertical direction, then

利用亮度监测器对束团尺寸的测量

12σx=0.636mm, σy=0.015mm which is close to the luminosity monitor resultσx=0.50±0.01mm,σy=0.0146±0.0003mm.

利用亮度监测器对束团尺寸的测量