Positron Research Group -|-|- St Olaf College, Northfield, MN, USA


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Positron Research Group
Science Center 155
1520 St. Olaf Avenue
Northfield, MN 55057

Dr. Jason Engbrecht
507-646-3968 FAX


Anna Legard '08

Dan Endean '09

David Green '09





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.

Flow of energy and lifetime data through electronics on its way to the coincidence circuit.

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.

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? --->