The time between a start pulse and a stop pulse is measured by pulse-height analysis of a time-
to-amplitude converter (TAC) pulse. The data are measured as a function of applied eld-cage
voltage for various gas mixtures.
The drift velocity of the electrons in the chamber are found by taking the dierence between the
times measured by the apparatus with the source located at two locations above the chamber.
Start pulse:
From a plastic scintillator nger on the end of a PMT located under the chamber
directly beneath the source . This signal is discriminated and turned into 30 ns NIM pulse.
Stop pulse:
From a proportional-wire detector anode inside the drift chamber. The signal is ampli-
ed by charge-sensitive preamp and then discriminated and turned into 30 ns NIM pulse.
Setup and check-out procedure
1. Turn on air handling apparatus and set mixture to 90 cc/min Ar and 10 cc/min CO
2
. Let
gas ow in the chamber and look for steady bubbling of the bubbler.
2. While gas is owing, make sure chamber and lead collimator plate are aligned with the
1 sensitive part of the scintillator nger. Start with the part of the chamber that is farthest
from the detector (Position 1 in Fig. 1).
3. Obtain the Sr-90 source. Caution: these sources are much more active than the
gamma sources we use in other experiments.
(approximately 1 mCi activity
versus 0.1 mCi maximum for the gamma sources).
Handle them as little as
possible and point the hole at the base of the source away from yourself and
other people.
4. Place the source over the hole in the lead collimator plate, and align the score marks on the
source with the score marks on the plate.
5. Turn on HV supply to the PMT (750800 V), and look for signal from the scintillator on the
scope. You should see something like Fig. 2.
Figure 2: Typical signals from the scintillator nger PMT using the Sr-90 source.
6. Run this signal into the discriminator, and set the discriminator to give a reasonable response
above noise. A typical setting would be about 70 mV (-0.70 V on the test point)
7. On the bright-colored NIM-bin HV supplies, make sure all switches are down, and the
multi-turn potentiometer knobs are completely counterclockwise. Turn on the digital me-
ters (DMM), and set them to a 10V range with three decimal digits readout.
8. Connect the signal output from the chamber to one of the inputs of the charge-sensitive
preamp, and connect the associated output of the preamp to the scope, terminated with 50
ohms. Turn the preamp power on, and set the voltage to 10 volts.
9. Turn on the output (usually A) of the negative (bright ORANGE) HV supply; you should
see the associated LED light. Watch the DMM and set the voltage to about -500V (0.5 V on
the DMM). The value of this voltage is not criticalyou just need enough to pull electrons
toward the detector.
2 Figure 3: Signals from the PMT discriminator and the anode wire (via the preamp) for 93 cc/min
Ar and 7 cc/min CO
2
.
10. Turn on the output (usually A) of the positive (bright RED) HV supply. Watch the scope
signal as you turn the voltage up. At about +1.8 to +1.9 kV (1.8 to 1.9 volts as measured
on the DMM), you should see the negative-going signal of the wire detector.
11. If you connect the discriminator from the PMT signal to channel 1 of the scope and the
output of the charge-sensitive preamp to channel 2, and then trigger the scope on channel 1,
you should get a display similar to Fig. 3.
12. Now play with the negative HV voltage, and note how the time position of the wire-detector
Figure 4: Same signals as in Fig. 3 but with 100 V on the cage.
3 Figure 5: Discriminated signals from the PMT and anode detector. This pair of signals is fed to
the start and stop inputs of the TAC.
signal moves. You should see that the time between the start pulse (scintillator nger signal)
and the stop pulse (wire-detector signal) varies as you vary the negative high voltage. If you
dont see this ask for help now! Compare Fig. 4 (100 V) to Fig. 3 (500 V).
13. Run the output of the preamp into another discriminator channel, and set the discriminator
to a low value (typically 3070 mV). You will need to see most of the pulses created by the
detector, and these vary in height a lot, depending on the negative HV setting. Look at both
discriminator outputs on the scope; you should see something like Fig. 5.
14. Connect the discriminator outputs to the TAC inputs, and see the resulting positive TAC
pulse on the scope. (You will need to set the trigger to positive-going signals, and turn the
channel sensitivity down.) Note how the height varies as the TAC settings and negative HV
settings vary. A good setting to start with is 100ns with x100 multiplier.
15. Finally, run the TAC output to the input of the pulse height analyzer, and take a few test
runs to check count rates and TAC settings so that you can cover the widest range of useful
negative HV values.
A few tips on operating the experiment
Check the signal for the maximum and minimum negative HV settings that you plan to use.
Typically, the minimum value is near 200 V and the maximum near 6 kV. You may nd
that the signal will decrease at high voltages. This can be compensated for by a small increase
of the anode-wire voltage.
Plan your voltage increments of the HV supply in a way that will allow you to map out the
drift velocity curve in a sensible way: you will need more points in the range where there is a
lot of change in the drift time versus voltage and fewer where the time stays more constant.
4 When you run the apparatus with larger Ar/CO
2
ratios, the anode wire signal may become
very noisy or show signs of overload. If this happens, reduce the anode-wire voltage. You may
nd that you need to adjust the discriminator level and possibly the power-supply voltage to
the charge sensitive preamp.
With the source over the hole nearest the anode wire, the time intervals between the start
and stop pulses are very short. You can get better resolution of these pulses by reducing the
maximum voltage setting on the LabVIEW pulse-height analyzer program. By default, this
is set to 10.0V, but the 5.00V or 2.00V setting may work better.
Also, when measuring with the near hole (position 2 in Fig. 1), the time will not change
very much as you vary the HV supply, since most of the time is determined by the drift
within the proportional-wire detector cage. Thus, you can get by with a few measurements
with the source at this position and interpolate the values you need to make the velocity
calculation.
Do not forget to use a pulser to calibrate the TAC. The pulser will produce very uniform pulse
heights from the TAC. When these are measured with the pulse height analyzer, you will see
at most one or two bins come up on the screen. You need only measure a few widely-spaced
pulse settings to get a good calibration.
Prepared by D. Pengra
setup_notes.tex -- Updated 14 May 2007
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