Document type:
Title:
Revision date:
Revision:
User's Manual (MUT)
Mod. N968 Spectroscopy Amplifier
14/06/2004
4
NPO:
Filename:
Number of pages:
Page:
00100/03:N968x.MUTx/04
N968_REV4.DOC
14
12
3. Functional description
3.1 Shaping determination
The proper shaping time constant must be seen as the better compromise in order to
match high count rates requirements (which lead to small time constant) and good signal-
to-noise ratios (which lead to big values). For scintillation counters the energy resolution
depends largely on the detector (scintillator and photomultiplier), therefore a shaping time
constant of about four times the decay time constant of the scintillator is a reasonable
value.
Gas proportional counters usually have a collection time in the 0.5 ÷ 5 µs range, so a ~2 µs
time constant could provide a good resolution.
Instead, for surface barrier semiconductors, a time constant in the 0.5 ÷ 2 µs range is
recommended.
3.2 Equipment facilities
In order to provide the User with an equipment suitable for almost any spectroscopy
application, the Mod. N968 includes:
−
a Pile-up Rejector/Live Time Corrector (PUR/LTC) circuitry which allows operation with
minimal dependence on system count rate. The PUR circuit interrogates incoming
pulses for pile up and generates a signal that prevents the ADC from converting the
piled up events. The LTC circuit then generates a system dead time that extends the
collection time to compensate for the events lost due to pileup rejection. The result is
lower background, better resolution and accurate live time information for the best
possible analysis results.
−
a gated Baseline Restorer (BLR), which includes a discriminator that operates the
sensing circuits that normally establish the baseline reference of the Multichannel
Analyzer (MCA). Performance of the spectrometer depends on the precision of the
setting of the BLR threshold. Such automatic threshold control typically gives as good
or better results than those the most experienced operator could achieve manually.
−
a Pole Zero (PZ) Visual Indicator: a LED adviser which leads to high precision
adjustment of the pole-zero cancellation, for exact compensation of the decay time of
the preamplifier signal. Two LEDs indicate either over-compensation or under-
compensation.
−
a Bipolar Output, which provides a zero-crossing pulse, suitable for precisely detecting
the radiation arrival time. In fact, the zero-crossing of the pulse occurs with a constant
delay with respect to the onset of the pulse, independently from the amplitude, thus
allowing to reduce time-walk effects.
−
an Inhibit Output, which provides a pulse when pile-up occurs. This pulse shall be
applied to the MCA’s anticoincidence input in order to prevent it from measuring and
storing a non-valid amplitude.