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Accidental Coincidence RatesPowerPoint Presentation

Accidental Coincidence Rates

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June 24, 2008

Sci A

Probability of an open window

= Rate A • Window length

Noise Hit

Sci B

Coincidence

ChanceCoincidence

Window from yesterday = 100 ns

Let rate of Sci A = 1 MHz

Ex) Probability of an opening = 10-7 • 106 = .1 = 10%

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Rate of an ACCIDENTAL Coincidence= Rate of Sci B • Probability of an Opening

= RB • RA • Window

Set window from yesterday = 100 ns

Rate of scintillator A = 40 Hz

Rate of a ACCIDENTAL

Coincidence

= RB • RA • Window

Rate of scintillator B seen from yesterday= 100 Hz

Rate of Accidental Coincidence = 100 Hz • 40 Hz • 10-7 = .04 Hz

A fluctuation that small is obviously not seen on this graph

Instead of a coincidence rate changing by .04 Hz,

is it possible for the rate to fluctuate by 5 Hz?

AC = RB • RA • Window

RB = 5 Hz / (40 Hz • 10-7) ≈ 12 • 105 Hz

1.2 MHz is a much larger rate for B than was tested yesterday, but if it were to be tested, then one would see that the line for coincidence would deviate from the “plateau” by 5 Hz

RA • RB = 5 • 107 Hz2

Why does the coincidence rate rise?

- Thermal Noise due to higher voltages
- Efficiency = # coincidences / # scintillator hits
- Noise in the scintillator, would cause an under-estimate of efficiency
- Noise in the coincidence counter, would cause a false increase in efficiency

- After-pulse can overlap with a single event, causing a false coincidence

http://www.physics.uiowa.edu/~uakgun/FILES/Akgun_Nov04_HCAL_Meeting.pdf

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