M.C. Browne, N.P. Kherani,^* A.W.P. Poon, R.G.H. Robertson and C.E. Waltham^¤

We are developing a compact 20 MeV gamma-ray source for energy calibration at the Sudbury Neutrino Observatory (SNO). The gamma-rays are produced from the radiative capture reaction ³H(p,)⁴He. The design and the operational characteristics of the proton source are described in a previous report.¹

We have built a prototype gamma-ray source with a scandium deuteride (ScD₂) target. Scandium was chosen because of its good thermal stability². We have designed and built an ultra-high vacuum system for the target evaporation process. We chose a molybdenum substrate for the target. Prior to the target fabrication process, the substrate was cleaned and etched in different solvents and acids to ensure a clean adhesion surface to accept the scandium film. The substrate and the evaporation system were baked extensively to ensure cleanliness. A scandium film of thickness of 7000Å was evaporated onto the substrate in the latest run. During the evaporation process, the pressure of the evaporation chamber did not exceed 3x10^-7 torr despite the intense heat required to evaporate scandium. To ensure a good deuterium to scandium stoichiometric ratio, the scandium film was deuterated in situ. The substrate was heated up to 400°C by an internal heater installed in the evaporation system. Deuterium was let into the system and was pumped by the scandium film. Once the scandium film had been saturated by the deuterium, the temperature of the substrate was slowly brought down to anneal the film. Because of the extra care taken into ensuring a clean evaporation environment, the latest target has a scandium to deuteron atomic ratio of 1:2.2

The prototype source is being tested. A 12.7cm (Ø) x 15.2 cm NaI with an active annular cosmic-ray veto and lead shielding is used to detect the 5.5 MeV gamma-ray in the ²H(p,)³He reaction. A 12.7cm (Ø) x 5.1 cm liquid scintillator with active cosmic-ray veto is used to monitor the neutron production rate by pulse shape discrimination. The neutrons are produced from ²H(d,n)³He, where the incident deuterons come from the naturally occurring deuterium in the hydrogen discharge gas and the exchange between the target deuterium with the ambient discharge gas.

Preliminary analyses show an increase of neutron and gamma event rates above background when the proton beam is turned on. In the pulse shape discrimination analysis of the liquid scintillator data, we found a (24±2)% increase in neutron production above background, and a (2.5±0.1)% increase in gamma-ray production. It is difficult to detect the ²H(p,)³He 5.5 MeV photopeak in the NaI detector because of the (n,) background in this energy region. In order to accurately determine the energy spectrum when the proton beam is on, we measured the beam-off background extensively (28-day real time equivalent). The background subtracted energy spectrum shows an increase in gamma-rays at 5 MeV (single escape) and at 5.5 MeV. From the single escape peak and the full energy peak in this spectrum, we estimated the ²H(p,)³He reaction rate in our prototype source to be 1 s^-1 at a proton beam energy of 26 keV. The detection of the 20 MeV gamma-rays from the reaction ³H(p,)⁴He at the SNO detector will not be as difficult owing to the high primary gamma-ray energy, and the essentially unit efficiency for gamma-ray detection.

We are also in preparation of a second deuterium prototype run to familiarize ourselves with the target fabrication process. One improvement we plan to implement in this run is the use of isotopically pure hydrogen gas with less than 15 ppm HD. This will reduce the neutron production rate, and hence, the neutron capture background, in the energy region of interest. The final scandium tritide target will be fabricated in a similar evaporation system at the tritium facility at Ontario Hydro Technologies in Toronto, Canada. This tritiation run will be carried out in summer 1996.