J.F. Amsbaugh, J.H. Gundlach, M.P. Kelly C.E. Linder, K.A. Snover, D.W. Storm and J.P.S. van Schagen

An upgrade of the '10-inch' beamline designed to enhance our capability for high energy gamma ray experiments is nearing completion. Two large NaI spectrometers have been obtained on long-term loan, one from Ohio State University and the other from the University of Illinois. These spectrometers are similar in design and performance to the Seattle NaI spectrometer, with high-quality NaI scintillators and both active and passive shielding. All three spectrometers will be located in Cave 2 on a refurbished beamline previously used with the Seattle spectrometer.

In order to accommodate the extra weight and surface area requirement of the three spectrometers, a new 13' by 14' platform has been constructed consisting of six reinforced concrete slabs supported by a flat framework of steel I-beams sitting on 18 steel support posts which rest on the 12" thick concrete basement floor. After some unsuccessful experimentation with fast-setting concrete-type products, the requirement of a superflat floor, flat to 20 mils, was achieved by pouring a thin epoxy layer over the slabs with the use of a leveling bar.

The NaI spectrometers, each weighing 2.5 to 3.5 tons, will be mounted on carts with air pads, and positioned on radially mounted rails, one for each detector, which rotate about the platform center on a pivot which also serves as the support for the target chamber. The NaI gains will be stabilized using LED's and computer controlled feedback via a programmable HV supply.

The vacuum system is being upgraded as well. The old oil diffusion pump has been removed and replaced by two 360 liters/sec turbomolecular pumps, one located at the collimator box just upstream of the target chamber and the other located at a new station on the beam dump, which has been enlarged from 4" to 8" diameter. Liquid nitrogen cold traps will be employed at the target chamber and upstream of the collimator box.