Compton Suppression System
| Participants: | Kenan Ünlü, Professor of Mechanical and Nuclear Engineering |
| J. S. Brenizer, Prof. of Mechanical and Nuclear Engineering Department | |
| N. O. Çetiner, M.S. student, Mechanical and Nuclear Engineering Dept. | |
| Services Provided: | Radionuclear Application Laboratory |
| Sponsors: | DOE-INIE, DOE-NEER, RSEC |
Introduction
The three major interaction mechanisms of gamma-rays with matter are photo-electric absorption, Compton scattering and pair production. In all these interactions, gamma-ray photon energy is partially or completely transferred to electron energy. In the photoelectric absorption, photon interacts with an atom and the photon completely disappears. In the Compton scattering the gamma ray interacts with an electron, causing an increase in the electron's energy. A new gamma ray with a smaller energy is then emitted. The new gamma ray can escape from the matter or can be absorbed through the photoelectric effect. In the pair production high-energy gamma rays are absorbed and two particles are created (an electron and a positron) and share the energy of the gamma ray. The positron loses its energy through ionization or excitation. If it is stationary, the positron interacts with an electron creating two gamma rays with energies of 511 keV each (annihilation radiation). These two gamma rays can escape or interact with matter through the Compton scattering or Photoelectric effect. Pair production does not occur below 1.022 MeV. The Compton effect is the predominant effect at intermediate gamma energies (200 keV to several MeV).
Figure 1. Compton scattering diagram
The vast majority of the scattered photons in Compton Scattering escape the detector by causing background counts in the gamma spectrum. If all of the energy of the incident photon is not absorbed in the detector, then there is a continuous background in the energy spectrum, known as the Compton continuum. This continuum extends up to an energy corresponding to the maximum energy transfer, where there is a sharp cut-off point, known as Compton edge.
In order to reduce the contribution of scattered gamma-rays the detector can be surrounded by a guard detector. The two detectors are operated in anti-coincidence, which means that if an event occurs at the same time in both detectors, then the event is rejected. The guard detector catches the escape photons and the effect of those photons is subtracted from the background. Compton suppressors provide a tool to suppress the unwanted background. The combination of a 9?-diameter annulus NaI(TI) detector and a central germanium detector is called a Compton suppression spectrometer.
Coincidence/Anti-Coincidence
Coincidence and anti-coincidence are detection modes used to produce a simplified spectrum from certain types of detector systems. In a system of two detectors, each detector produces separate signals. In coincidence mode those signals are counted. In anti-coincidence the signals produced at the detectors cancel or veto each other, leaving the non coincident signals to be counted. The advantage of coincidence or anti-coincidence techniques is achieving greater accuracy in the determination of full energy peaks in the spectrum.
Compton Suppression System at PSU Facility
|
Compton Suppression System at the facility includes
|
| Figure 2. Compton Suppression System at PSU |
Canberra Model 3106D NIM high voltage power supply is used for operation with HPGe detector.
The HPGe detector properties:
| Reverse electrode closed-end coaxial Ge detector | |
| Realative efficiency | 50 % |
| Resolution | 2.2 keV(FWHM) at 1.33 MeV |
| Peak/Compton | 58:1 |
| Diameter | 64mm |
| Length | 71mm |
| Cryostat description | Vertical slimline dipstick cryostat having 2.5? endcap, 4? long remotedetector chamber, and ultra low background cryostat hardware. |
The suppression of Compton events can only be as good as the ability of the guard detector to detect the scattered photons. Canberra Model 3002D high voltage power supply is used with NaI(TI) guard detector. NaI(TI) detector is consist of annulus and plug detector. Addition of plug above the sample will greatly reduce the Compton edges.
 |
 |
|
| Figure 3. Inside view of the shield where plug detector and PM tubes can be seen | Figure 4. NIM Bin | |
|
||
 |
|
Genie 2000 Basic Spectroscopy Software is a comprehensive environment for data acquisition, display and analysis in personal computers. It provides independent support for multiple detectors, extensive networking capabilities, windowing interactive human interface and comprehensive batch procedure capabilities. |
Figure 5. Genie 2000 Basic Spectroscopy Software |
||
Testing Results
The 137 Cs source is counted in Compton Suppression System. Peak/Compton ratio, which is the ratio of the full energy peak to the Compton continuum, is calculated in order to see the performance of the system. Figure 6 shows the comparison of counting when suppression (SUP) is on with Pile Up Rejection (PUR) and when suppression is off (NOSUP) with Pile Up Rejection (PUR).
Figure 6. Spectra of 137 Cs counted with the Compton Suppression System with suppression enabled (red) and disabled (blue)
