Coincidence Rates. QuarkNet. summer workshop June 24-28, 2013

Transcription

1 Coincidence Rates QuarkNet summer workshop June 24-28,

2 Example Pulse input Threshold level (-10 mv) Discriminator output Once you have a digital logic pulse, you can analyze it using digital electronics (a computer ). 2

3 Detector Electronics Measures the times of the leading and trailing edge of the discriminator pulses. The difference is called Time Over Threshold Larger pulses have a larger time-over-threshold We don t measure the pulse height directly The electronics has an internal clock that ticks every 1.25 ns This determines how precisely times can be measured Threshold voltage Input pulse Clock ticks 3

4 Coincidence Definition When the discriminator on one channel fires, it opens a gate for a fixed period of time. If the discriminator on another channel fires during that time, we have a 2-fold coincidence If two channels detect pulses during that time we have a 3-fold coincidence If all four channels have pulses within the time determined by the gate width then we have a 4-fold coincidence 4

5 Recorded Data Only two types of data can be recorded: 1. Number of counts in a given time interval 2. Times of leading and trailing edges of pulses The first produces a small amount of data Just the number of counts every few seconds or minutes The second can produce a large amount of data Times on several channels at a rate of Hz The coincidence trigger only reads out times on events that satisfy the trigger condition. 5

6 Pipeline Delay One complication: We detected a coincidence at time but the first pulse fired the discriminator at an earlier time, < We need to remember times of all leading edges just in case they subsequently fire a coincidence trigger. Solution: Put all the times into a circular pipeline Delay it by a specified amount of time Print out times in the delayed pipeline after a coincidence is detected 6

7 Pipeline Delay Gate width of 10 clock ticks (12.5 ns) 2-fold coincidence detected Pipeline delayed by 6 clock ticks (7.5 ns) Leading and trailing edges are recorded over this 20 ns time interval. 7

8 Coincidence Definition Trigger controls Selected channels Coincidence level Gate width Pipeline delay Discriminator thresholds 25 mv is a nice number Data output Enable it to get the leading/trailing edge data. 8

9 Coincidence Definition A good choice for today: Pipeline delay: 20 ns Gate width: 100 ns Accounts for different lengths of cables, different transit times in photomultiplier tubes. 9

10 Random noise pulses What is the probability that two random pulses accidentally satisfy the trigger condition? Suppose the random trigger rate is (Hz) Rate at which two channels randomly fire is 2 Gate width is (ns) Probability that the other channel fires while the gate is open: = Random coincidence rate is 2 10

11 Random noise pulses More complicated situation: Rate on channel 1 is Rate on channel 2 is Accidental rate when channel 1 fires first: Accidental rate when channel 2 fires first: Total accidental rate: 2 Three-fold accidental rate ~ Four-fold accidental rate ~ 11

12 Third Exercise The photomultiplier tubes need voltage to operate Higher voltage produces bigger pulses The rate of noise depends on the voltage applied to the photomultiplier tubes Higher voltage produces more random pulses 12

13 Third Exercise Set the gate width to 100 ns Pick two channels (for example, 1 and 2) Enable only those two channels, use 2-fold coincidence Turn up the voltage on one channel to give a rate of Hz Measure the rate on the other channel as a function of voltage on that channel Also measure the coincidence rate as a function of voltage 13

14 Third Exercise 14

15 Third Exercise Is the coincidence rate caused by random noise? (show your work) Assuming the voltages are all set so that the photomultiplier tubes are fully efficient, what is the rate of 3-fold and 4-fold coincicences? Can you describe the properties of the rays that pass through the detectors? 15

16 Third Exercise (part 2) Let s go to the basement how much lower is the rate of 4-fold coincidences? 16