Making Room For Everything

HIGS target room, called the gamma-vault, receives the collomated gamma-ray beam throuh a well-shielded wall which seperates it from the collimation hutch. The vault is approximately 30 feet deep along the beam direction. All major detector systems are housed inside the vault. As the experiments are switched, the active setups are moved along the wall and the new detectors are placed on line. A sophisticated system of alignment laser, survey marks and positioning targets makes placement of detector components along the beam path possible. The beam is profiled (flux and position) in the collimation hutch. The beam after transversing the targets and any detector components is trapped into the beam-beam located at the rear exit of the room. The detectors in the gamma-vault can be classified into two types, the beam profiling detectors, and the detectors associated with a measurement for an experiment. The beam profiling detectors remain the same from one experiment to the other.

 

Monitoring the Beam

The gamma-ray beam is monitored for energy, flux, and position. The energy is measured by either a HPGe or large NaI. If the flux is reasonable and not beyond the limits of the detectors, HPGe or the NaI is placed directly in the path of the beam. If the flux is higher than what the detectors can take, they are moved to a known angle relative to a thin Cu plate and the Compton scattered gamma-ray energy is monitored. Typically, the beam is tunned at low flux and detectors directly in the path of beam. Once the optimum tune is reached, the flux is increased and the detectors are moved to a pre-fixed angle for monitoring purposes. The flux is measured with the aid of dual-paddle system, shown here. This system is called anti-coincidence convertor monitor. A thin convertor foil is inserted between two similar scintillating paddles. The idea is to measure everything which originates from the foil and nothing from upstream of it. All events are vetoed by the signal in the upstream paddle and counted by the downstream paddle. The relative number of counts are then absolutely calibrated by comparing count in the beam energy measuring detectors with known efficiency. The position of the gamma-ray beam can be determined by two different ways. On method, well known technique at TUNL, of exposing a large X-ray film plate, or imaging the beam on a thin scintillator and capturing the image on a CCD camera. One such device system has beem developed by Univ. of Saskatchewan for HIGS and was successfully test in 2003. This device is now being implemented as a permanent beam profiling monitor.

The Detector Systems

The detector systems at HIGS can be grouped according to their use in experiments.For instance, the Blowfish array, featured here, along with Li-Glass detectors, and TUNL liquid scintillating detectors are primarily used for neutron detection at HIGS. The Blowfish array is being used for the GDH measurements on the Deuteron, the Li-Glass array and the TUNL liquid scintillationg array are being used for low-energy photodisintegration of the deuteron. Likewise, HPGe detectors, NaI, and Neutral Meson Spectrometer (NMS) are all part of the gamma-ray detection system. The HPGe are used for high energy resolution, low-energy (< 20 MeV) gamma-ray detection experiments such as the NRF measurements. The NaI detectors are used for low resolution, moderate energy (20 - 80 MeV) gamma-ray detection such as the Compton scattering sxperiments. The NMS is reserved for high-resolution, high energy (100 MeV and above) experiments such as photopion production.

Detector Systems @ HIGS

Detector	Quantity	Source	Detection
Blowfish	88-cell Liquid-Scint. Array	U Sask / U Va	Neutron
Liquid-Scint. Array	4-cell array	TUNL	Neutron
Li-Glass Array	4-cell array	LLNL, on loan	Neutron
HPGe	4 (~ 60 %) + 1 (123 %)	TUNL	Gammas
NaI	4-detector polarimeter	TUNL	Gammas
Pb-Glass	Igloo System	LANL	Gammas
NMS	120 CsI, BGOs, Wire Chambers	LANL, on loan	pions

 

 

 

 

 

The Target Systems

The HIGS facility also houses cryogenic hydrogen target system. The system is being test and has not been yet used at HIGS. However, the system has been successfully used at University of Saskatchewan . In addition, a frozen-spin deuteron target system is also being developed for use in the GDH measurements. TUNL and UVa are jointly involved in this development. The latest addition to target resources at TUNL is the development of optically pumped He-3 target. This target is being developed at TUNL under supervision of Dr Haiyan Gao.