Choosing a target material is a compromise between our desire for a pure proton target, and the practical necessities of materials which perform well under DNP and heavy radiation damage. Ammonia and deuterated ammonia (14NH3 and 14ND3) have emerged as the most attractive materials for our uses. When doped with paramagnetic centers to provide free electrons for the spin-spin coupling, ammonia can achieve greater than 90% proton polarization.
To dope the ammonia with free electrons, it is irradiated in a smaller accelerator before it comes to JLab. This irradiation produces radicals such as NH2 from the NH3 in what is called a "warm dose". In the beam at JLab, temperatures are much lower, and different radicals, such as atomic H, are produced under this "cold dose".
Each target insert holds two cups with ammonia material samples. The cups are cylindrical, and roughly 1 inch in diameter and length. For g2p, the same microwave guide will be used for both top and bottom cups; the photo to the right shows the old insert cups from a previous experiment.
The Target Material is a frozen solid material with the necessary characteristics for nucleon polarization.