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The energy signals from the HPGe detector and the lifetime signals
from both the phototube (Ps creation) and HPGe detector (Ps death)
are sent to a series of electronics to be fine-tuned.
The energy signals first go to an attentuating circuit (not pictured)
that adjusts the voltage of the energy signals to correct shifting
of the energy data over long periods of time. The attentuating
circuit is controlled by our Labview program The Interface (discussed
in the software section). After passing through the attenuating
circuit, the energy signals go to a spectroscopy amplifier that
amplifies and shapes the voltage spikes. The adjusted energy signal
is sent to an analog-to-digital converter that changes the voltage
spike to a digital signal. Finally, this signal is sent through
the coincidence circuit and read by the Labview software.
The Ps creation lifetimes from the phototube go to a constant
fraction discriminator that cleans up the voltage spikes and filters
out unwanted signals. The new signals are then sent on to a time-to-digital
converter that starts a clock to time the Ps lifetimes. Meanwhile,
the Ps death lifetimes from the HPGe detector are sent to a fast
filter amplifier that quickly amplifies the voltage spikes and
sends the adjusted signals onto a constant fraction discriminator.
Like the creation lifetime signals, the constant fraction discriminator
cleans up and filters the death lifetime signals and sends them
on to the time-to-digital converter. Now with a creation lifetime
and a death lifetime, the time-to-digital converter can calculate
a lifetime for the Ps atoms. This total lifetime of the Ps atoms
is made into a digital signal that is sent on to the coincidence
circuit.
We can determine if an energy and lifetime signal are from the
same positronium annihilation by use of a circuit we have designed
called the coincidence circuit. The energy and lifetime signals
arrive at the circuit one signal pair at a time. We can set a
time frame in which the arrival of an energy and a lifetime signal
will be considered to be simultaneous. The circuit is setup so
that if the energy and lifetime signals arrive simultaneously
(i.e., there is a coincidence), then the energy-lifetime pair
of signals are given an identification number and are marked as
being a coincidence. If there is not a coincidence, the energy
and lifetime signals pass through the circuit without getting
marked.
The circuit consists of five dual monostables, a 4-bit up counter,
and two quad multiplexers. The monostables add delay to the energy
and lifetime signals so that they can be organized the tagged
properly. The counter provides the identification number for coincidences.
One multiplexer determines if the signals are to pass through
the circuit or not before being sent on to the software. The other
multiplexer selects whether or not the data should be tagged as
a coincidence.
After the data has passed through the coincidence circuit, it
is sent to a digital I/O board from which our software takes in
the data.
What does the software
do? --->
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