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

Dr. Jason Engbrecht
engbrech@stolaf.edu
507-646-3849
507-646-3968 FAX

  [-web-]

Anna Legard '08
legard@stolaf.edu

Dan Endean '09
endean@stolaf.edu

David Green '09
greend@stolaf.edu

 

 

Analysis

 

Gas scattering apparatus: The radioactive source is glued inside the light-shielded gas chamber between a light guide and a plastic scintillator. When positrons are emitted, they pass through the scintillator, which emits light in response. The light travels through the guides, made of optical plastic, through total internal reflection and is counted by the phototube. This phototube signal provides the start pulse for the lifetime measurement. Once the positron is through the scintillator, it forms positronium and begins to scatter off of gas atoms/molecules in the sealed chamber. As the Ps atoms lose energy and begin to annihilate, the Ge detector measures the energy of the gamma rays and provides a stop signal for the lifetime measurement. The energy and lifetime measurement signals are compared via our custom electronics. Afterwards, statistical analysis is performed in LabView. Different gases can be pumped in and removed through the plumbing system.

Gas chamber mounted in magnet coils.

The heart of the gas scattering apparatus is the gas chamber, where the positronium forms and annihilates. It is machined out of 1” diameter aluminum tubing and approximately 20” long, designed to fit tightly through the aperture in the circular magnet coils. The chamber contains the source, scintillator, and light guides. A threaded aluminum plug is mounted on one end of the tube to create a vacuum tight seal to the vacuum system, made of ¼” copper tubing. The tubing provides a regulated one-way supply of gases from a gas canister into the chamber. The tubing also allows us to purge the system via a vacuum pump.

The magnetic field produced from the magnet is about 2.5 kilogauss in strength and oriented along the axis of the tube in order to confine the positrons within the chamber. The magnetic field also has the effect of mixing the quantum states of the Ps to ensure only two gamma rays are emitted from the Ps annihilation.

We power the magnet with a 2 kW power supply and cool it with filtered tap water to keep coil temperature low. An interlock circuit was built to monitor coil temperature and force a manual reset of the power supply if the temperature exceeded limits. The coil temperature is obtained as a voltage from thermocouples mounted within the magnet coils.

Temperature interlock circuit Temperature interlock circuit diagram
The temperature interlock controls the power supply and monitors magnet coil temperature. Circuit diagram for temperature interlock (click for detailed explanation).

Our Na-22 source is mounted within the gas chamber between a plastic scintillator and light guides made of optical plastic. Positrons emitted from the source pass through the scintillator, which then emits a photon. We need to count these photons for our lifetime measurement, which is easily accomplished with a phototube. However, the phototube is sensitive to magnetic fields, so we use light guides to send the photon outside the magnet, where the magnetic field dies quickly. The light guides are simply very clear plastic that transmit light via total internal reflection. The photon counting system is made light-tight with electrical tape and o-rings.

The HPGe detector, positioned closely to the gas chamber.

The HPGe detector measures precisely the energies of emitted gamma rays. It is positioned closely to the chamber via its stand in order to improve data rate. Its output is processed with our electronic circuitry to give energy spectra and lifetime measurements (in conjunction with the phototube output).

 

 

 

 

 

 

 

How about those electronics? --->