Positron_Workshop_2009 : March 25-27, 2009

Foreword

Below we would like to write up an introduction to the JPOS09 proceedings.  
We need 1 paragraph on the nuclear physics using GeV positrons and one paragraph 
on thermal positron uses.  A paragraph reviewing other positron sources and 
their capabilities.  A paragraph identifying the more promising methods for 
positron production with CEBAF and an estimate of the beam properties.

Wed 3/25/09

2 [math]\gamma[/math] exchange

looking for positron currents of 1 [math]\mu[/math] A in Hall C or 100 nA in Hall B.

Positron production

Energy options

At > 50 GeV incident electron energies, an undulator can be used to generate MeV energy photons which pair produce in a target. You get about 200 photons for each incident electron?

At between 1 and 50 GeV, back scattered compton photons may be the optimal choice for producing positrons.

The conventional method of using a converter target my be the best choice for incident electron energies less than 1 GeV.

The undulator and compton techniques create polarized photons and then polarized positrons. For the conventional method you could use off axis bremmstrahlung to create linear photons or polarize the incident electrons and have the polarization transfer.

Thermal energy production

Sergey Chemerisov was able to capture 100 more transmitted positrons (positrons emitted on the same side of the target as the incoming electron beam) than reflected positrons at 20 MeV incident electron energies.

Thermal positron rates of [math]10^{11}[/math] e+/sec

Converter target choice

Alexander Mikhailichenko suggested using liquid Bismuth as the converter target and a high current incident electron beam ( mA). The target would need to handle 50 MW of power. You may get one positron for 500 incident electrons.

Target is 55.51% Bi and 44.49 % Pb by mass and is liquid at 159.5 C.

The loop needs to be Ti (melting point 1668 C).

Polarimetry

P. Schuller (DESY) has the iron block used in E-166 at SLAC for the compton transmission polarimeter. The incident photon Energy went up to 8 MeV.

We could measure positron polarization by measuring the polarization of photons from the positron annihilation. Compton scattering depends on the polarization of the struck atomic electrons. An external B-field will polarize the FE block. You can either flip the direction of the B-file or the direction of the incident photon polarization to measure the photon polarization.

The IAC could make a compton transmission polarimeter to use on the photo-fission experiment as well as a proto-type for JLab.

Thermal positrons

How big is the user thermal positron user community?

Need to organize the community, perhaps write a 5 year long range plan, then present plan to JLab management and then DOE.

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JPOS09

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