Reducing Cosmic Background for the Elastic Compton Scattering at HIGS*

Protons and neutrons (nucleons), the key constituents of atomic nuclei, are made up of quarks and gluons. The COMPTON@HIGS collaboration is working to understand how nucleons respond to external electromagnetic fields due to their internal structure and the nature of the nuclear strong force. This is done by inducing Compton scattering between gamma ray (photons) with energies in the range of 60–100 MeV and different types of light nuclei (A = 1-6). Currently, a liquid 3He target is being used to study the neutron, and the experiment is running at the High Intensity Gamma Ray Source Facility of the Triangle Universities Nuclear Laboratory, located at Duke University in North Carolina.

To conduct such measurements successfully, background suppression especially from cosmic rays is important. In this experiment, a new veto paddle was developed to further reduce background events in data collected by the five NaI detectors (HINDAS). Two main challenges emerged with the current detector setup: the out-of-plane detectors were inefficient at rejecting cosmic rays, and there was an overlap between the energy deposited by cosmic rays and the energy from Compton scattered photons. This overlap is particularly problematic when using higher energy gamma rays, reducing the effectiveness of the shielding system. Therefore, we need to further suppress cosmic ray events to accurately detect Compton events. The paddles are made up of a thin scintillating sheet, a cylindrical light guide, and various photomultiplier tubes. After covering all the light leaks and finding the operating voltage of each of the 6 paddles, we designed a way to mount the paddles onto the HINDAS on supports that are stable and easy for mounting or dismounting so that we can accurately record the position of the paddles. The typical cosmic rate in a HINDA detector is ~72,000 counts per hour. With a series of preexisting methods, we could reduce the rate to nearly 50 counts per hour. With the paddles, we further reduced the background by a factor of two, bringing the signal-to-noise ratio close to 1:1.