A Dynamically Polarized Proton Target PDF Download

I have understood my assignment as a review of some of the work done in high-energy physics with polarized proton targets and a description of some of the special problems connected with polarized targets. Most of my report will be based on the polarized target that I am most familiar with--that constructed by Jeffries, Schultz, Shapiro, and myself. This target is no longer unique; in fact, it is now somewhat old-fashioned in some respects. Other polarized proton targets are in operation at CERN, Saclay, the Rutherford Laboratory, Argonne National Laboratory, the Soviet Union, and there is a target newly in operation at the Brookhaven Laboratory. Other targets are in operation or are in the process of design or construction at a number of other places. Unfortunately, none of these targets consists of pure hydrogen. The target material most often used is made of lanthanum magnesium nitrate, LMN. About a quarter of the weight of this crystal is water; it is the protons within the water molecules that are polarized. Hydrogen constitutes only 3 percent of the weight of the crystal. This means that scattering processes on hydrogen must be distinguished kinematically from scattering processes involving the heavy elements of the target if the target is to be used efficiently in high-energy scattering experiments. In fact, some of the experiments one would very much like to do appear to be very difficult. In LMN the protons are polarized by an indirect process known as dynamic polarization. Neodymium ions are added to the crystal when it is grown from a water solution. The neodymium ions are substituted for lanthanum to the extent of one percent or less. The neodymium ion has an odd number of electrons; it has a doublet ground state, called a Kramers doublet, that acts very much like a single free electron as far as its spin is concerned, but anchored in space to a particular lattice site. When the crystal is properly oriented in a magnetic field it has a g factor that is about 1.3 times as great as that of a free electron. I will refer to these neodymium ions as 'electrons'. The crystal is placed in a magnetic field, 18 kilogauss in our case, and is held at low temperature by a bath of liquid helium constantly being pumped on to maintain a temperature of about one degree Kelvin. Because of the low temperature and the high magnetic field the electrons are highly polarized, as may be calculated using the Boltzmann factor. In our target the electrons are polarized to the extent of 88 percent. Because the magnetic moment of the proton is so small, the protons are polarized only to the extent of 0.15 percent--too little to be useful. However, the protons can be polarized to an extent comparable to that of the electrons if we use a trick developed by Jeffries and by Abragam, sometimes referred to as the solid effect. The trick consists in irradiating the sample with microwaves of a frequency chosen to cause a particular transition. One starts with a Boltzman distribution of states in the crystal but then selectivity disturbs this Boltzman distribution to accomplish the desired result.