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University of Leicester
PLUME
Ref: PLM-PAY-LaserPrePlan-037-1
Date: 25/01/2010
Preliminary PPB laser testing plan
Philip Peterson
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Updated Reference Number
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25/01/2010
PLM-PAY-LaserPrePlan-037-1
First draft
The first laser test using the NIM amplifiers produced very good results, an example pulse
height distribution is given in figure 1 below. The peak from the laser corresponds to an input
charge cloud of 3x106 electrons.
Figure 1: A pulse height distribution for the laser experiment using the NIM electronics; each
channel corresponds to around 3,750 electrons. Features from left to right: background
noise, noise from the laser pump’s power supply, laser signals.
The NIM electronics have a much better noise profile than the PPB, but even then they were
inadequate for detecting x-ray events. Typical peak-to-peak noise values for the PPB at the
moment are between 1V for low gain and 4V for high gain settings.
The PPB is designed to have a voltage gain range of 10 to 100 and a 1pF feedback
capacitance on the preamp. Calibration reveals the true gain range to be 13 to 90, so a
single electron would produce an output voltage between 2x10-6 and 1.4x10-5V. From this,
an input of 3x106 electrons to the PPB would produce outputs between 6.24V and 43.2V;
these voltages are beyond the 4.4V maximum output swing of the PPB and would saturate
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University of Leicester
PLUME
Ref: PLM-PAY-LaserPrePlan-037-1
Date: 25/01/2010
the amplifiers. This merits a reduction in the lower bound of the PPB’s gain range – this is a
simple operation that involves replacing a resistor to change the PPB’s designed minimum
gain from 10 to 5, say. At the low gain setting, this would give a laser experiment a voltage of
approximately 3V, which should provide sufficient resolution at a voltage level low enough
for the PPB to handle.
At this stage it seems plausible that with such a modification, the PPB could be used
successfully with the laser experiment. The theoretical signal to noise ratio of the PPB with
the laser is around 6 to 1, worse than the NIM experiment but still good enough that the laser
peak should be easily distinguishable.
Our immediate course of action is to modify the PPB to give lower gain and recalibrate it. A
brief test using a pulse generator to ensure that the saturation voltage of the amps is well
above the expected signal voltage would be sensible - the pulse generator’s test output
should be 480mV for this exercise. Before the test plan for the PPB laser experiment can be
written, the plate voltage for the first experiment must be determined as part of the next
experiment will involve varying the high voltage and seeing the effect this has on gain.
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