# speed of light week two

verify all setting are the same as last week

## timewalk

as the voltage output of the PMT changes so will the time delay measured by the TAC, even though the actual time delay does not change

here, with the time delay in the delay box and the distance between the PMT and LED held constant, find some way of changing the voltage amplitude output from the PMT. Perhaps messing with the high voltage source? At various values for voltage, what is the centroid data for the MCA?

delaybox set to:56 ns

LED position:7cm

PMT Voltage Output Centroid FWHM centroid error time delay time error
12.6 V 693.0 29.976 70.5 64.16 s 12.90 s
12 V 698.8 31.152 73.4 64.71 s 13.20 s
11.4 V 706.1 32.487 76.5 65.40 s 13.54 s
10.8 V 715.0 33.959 79.9 66.25 s 13.93 s
10.2 V 724.1 36.262 85.5 67.11 s 14.50 s
9.6 V 734.9 40.08 94.4 68.14 s 15.42 s
9.04 V 744.3 46.74 110.1 69.03 s 16.97 s

The plot shows that a reliable trendline can not be placed, due to the very short domain and large(relatively) error on the range. Not enough data.

$\displaystyle{ time \, delay=(-1.3688 \pm 4.6220 \tfrac{s}{V})(Peak \, voltage) -( 81.1802 \pm 51.1251)s }$

There does seem to be a decline in delay time as the peak voltage rises (as I predicted), but the data's too noisy and too tightly clustered to confirm or deny it. Oh well, the least we know from this data is that if the pulses coming out of the PMT have peak voltages that don't vary more than 4 V, timewalk won't add more than random noise.

## Travel Time vs distance measurements

time for some actaul data

figure a good pmt output voltage to set all the measurements to.

withthe led and the pmt so far apart that the meter stick that I am pushing it in with is in the tube 5cm, the max output voltage I can get is 12.5 V with the HV to the PMT at 1500+496=1996 V and the polarizers aluigned as best I can. I'll shoot for 11.8 V in all my mmeasurements.

measurement at increments of 10 cm starting at 5 cm.

LED stick position centroid FWHM time delay time delay error
5 cm 702.0 33.521 65.01 13.75
15 cm 698.7 32.005 64.70
25 cm 697.9 30.245
35 cm 697.2 29.266
45 cm 696.1 27.601
55 cm 695.2 26.134
65 cm 693.2 24.448
77 cm 692.2 22.688
85 cm 691.7 21.600
95 cm 690.5 20.339
102 cm 689.1 18.540

ANOTHER RUN, THIS TIME AT INTERVALS ~25CM

LED stick position centroid FWHM
4 cm 702.5 33.274
29 cm 697.9 29.673
54 cm 694.2 26.293
79 cm 692.0 22.648