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Detailed introduction
The Compton Suppression System reduces the Compton platform height for low background counting systems, thereby increasing the minimum detectable activity and overall spectral quality - particularly useful for small volume samples such as air filters and Petri dishes.
Compton suppression system basics
In a typical low background application system, great efforts are made to reduce the inherent radioactivity of the counting system components (detector, lead shield, and air in the shield chamber). In high-purity germanium spectroscopy systems, these low-background measurement systems tend to have less cosmic ray background and less natural environment background.
Compton suppression systems are designed to reduce plateau counts in these typical counting systems. While low-background systems remove most of the sources of increased background, they do not resolve the contribution of the continuum below the peak: Compton scattering events. Compton scattering occurs when the entire energy of an incident photon is not absorbed by the HPGe detector, while the portion of the energy leaving the detector cannot be calculated. In the gamma ray spectrum, this partial count appears below the peak of the Compton continuum plateau and is a random event.
The ratio of the peak height to the average height of the Compton continuum is called the peak-to-Compton ratio (P/C). In standard HPGe detectors, the peak-to-combustion ratio of Co60 at 1.33 MeV is usually between 40:1 and 60:1, and the peak-to-combustion ratio of larger detectors can be close to 100:1.
Because the escaping energy is emitted as photons, it is possible to collect this ray with another detector. This is usually achieved using a large crystal made of a relatively cheap material (such as NaI), which becomes a coincidence detector. By correlating the count events in the HPGe and coincidence detectors through time electronics, the count time in the coincidence detector can be used to discriminate between simultaneous count events in the HPGe detector and discard them. The result was the suppression of Compton's platform. In a Compton suppression system, a peak-to-scan ratio of over 1300:1 can be obtained using an N-type detector with 60% relative efficiency. This will result in a reduction of about 10 times in platform height and more than 3 times in MDA value.
Selection Factors for HPGe Detectors in Compton Suppression Systems
The effectiveness of the Compton suppression system depends on the ability to collect photons scattered by the HPGe detector. Because a photon has the potential to interact with every material it encounters, as little material as possible must be used between the effective volume of the HPGe crystal and the detector crystal. Relevant materials include:
Users familiar with low background detectors will note that magnesium end caps, copper end caps, and copper crystal support cups are commonly used with low background detectors. Although conventional thinking holds that Compton suppression systems are the solution for ultra-low activity sample measurements, this is not the case. By using these materials in the detector structure, the overall effect is to reduce the performance of the Compton suppression system (i.e., reduce the peak-to-complexity ratio). So in Compton suppression systems, trade-offs are very important.
Other important components in the Compton suppression system
In order to be able to simultaneously detect the Compton scattered rays scattered from the high-purity germanium detector into the NaI ring or plug detector and minimize the Compton platform height, the correct electronic setup is very important. Historically, ORTEC has provided a simulation system based on NIM plug-in electronics. With the rapid development of digital technology, ORTEC has launched a digital dual-channel MCA. The digital version of the Compton suppression system is significantly less complex and time-consuming to commission.
The digital version of CSS significantly reduces the requirements for electronic equipment, using fewer components and a more compact space design. Large NIM racks are no longer required. In order to further improve space utilization without using any NIM electronics, ORTEC replaced the original box-type shielding body with a more integral integrated cast cylindrical shielding body. The shield is 4 inches (10 cm) thick and has an interior height that is 4 inches (10 cm) higher than traditional shields. The weight is only half of the original shielding body, and the floor space is also half of the original shielding body. In addition, the top sliding door design of the cylindrical shield allows the compliance detector to be installed in the top shield. This innovative approach greatly simplifies the loading and unloading of samples, requiring only one simple step, saving time. At the same time, it also reduces possible damage to the detector each time the sample is loaded and unloaded. Removing the plug detector from the NaI ring can greatly increase the sample measurement space without sacrificing the 4-π coverage geometry and Compton suppression performance.
In a typical low background application system, great efforts are made to reduce the inherent radioactivity of the counting system components (detector, lead shield, and air in the shield chamber). In high-purity germanium spectroscopy systems, these low-background measurement systems tend to have less cosmic ray background and less natural environment background.
Compton suppression systems are designed to reduce plateau counts in these typical counting systems. While low-background systems remove most of the sources of increased background, they do not resolve the contribution of the continuum below the peak: Compton scattering events. Compton scattering occurs when the entire energy of an incident photon is not absorbed by the HPGe detector, while the portion of the energy leaving the detector cannot be calculated. In the gamma ray spectrum, this partial count appears below the peak of the Compton continuum plateau and is a random event.
The ratio of the peak height to the average height of the Compton continuum is called the peak-to-Compton ratio (P/C). In standard HPGe detectors, the peak-to-combustion ratio of Co60 at 1.33 MeV is usually between 40:1 and 60:1, and the peak-to-combustion ratio of larger detectors can be close to 100:1.
Because the escaping energy is emitted as photons, it is possible to collect this ray with another detector. This is usually achieved using a large crystal made of a relatively cheap material (such as NaI), which becomes a coincidence detector. By correlating the count events in the HPGe and coincidence detectors through time electronics, the count time in the coincidence detector can be used to discriminate between simultaneous count events in the HPGe detector and discard them. The result was the suppression of Compton's platform. In a Compton suppression system, a peak-to-scan ratio of over 1300:1 can be obtained using an N-type detector with 60% relative efficiency. This will result in a reduction of about 10 times in platform height and more than 3 times in MDA value.
Selection Factors for HPGe Detectors in Compton Suppression Systems
The effectiveness of the Compton suppression system depends on the ability to collect photons scattered by the HPGe detector. Because a photon has the potential to interact with every material it encounters, as little material as possible must be used between the effective volume of the HPGe crystal and the detector crystal. Relevant materials include:
| Features that should be selected | Features that should not be selected | |
| HPGe external contact grade | Ultra-thin external contact grade:Using an ORTEC Gamma-X (GMX) detector, N-type crystal, the outer contact stage is only 0.3 microns boron thick. | Standard P-type probe:P-type detectors, such as the GEM detector and the optimized GEM-M detector, have a thicker external contact stage (~600 micron lithium implant layer). This contact stage is three times thicker than that of the Gamma-X N-type detector. This results in a greatly increased probability that scattered rays will be blocked. Energy extended P-type high-purity germanium detector:Energy-extended P-type detectors, such as the optimized GEM-C, only have an ultra-thin entrance window at the front of the detector. But the sides are surrounded by a thicker lithium ion implantation layer, so standard P-type detectors should not be used. |
| High purity germanium crystal support cup | Low Density Support Cup:ORTEC uses a 0.5mm thick, low-background aluminum support cup. In standard detector manufacturing. | Copper material support cup:The use of copper as a crystal support cup should be avoided due to its high material density and mass absorption coefficient. This greatly reduces the probability of scattered photons entering the detector. |
| Detector end cap | Low Density End Caps: 1.5mm thick carbon fiber end caps or low background aluminum end caps with carbon fiber windows. | Magnesium end cap:Magnesium material has a higher mass absorption coefficient than aluminum material, which greatly increases the probability of interaction between rays and matter. This reduces the probability of scattered rays entering the detector. Copper end cap: As with copper support cups, copper end caps should be avoided in low-background counting systems. |
| Air between high purity germanium detector and shield, other materials | Largest possible HPGe crystal diameter:In a typical 83mm diameter end cap, a 70mm diameter detector crystal can be placed. This is equivalent to placing an N-type detector with a relative efficiency of 70%. For custom Compton suppression systems, larger HPGe crystals and end caps can be used for improved performance. | |
| Fits detector size | The HPGe detector end cap diameter should match the inner diameter of the annular NaI: When ordering, you need to order a NaI ring detector that matches the size of the detector end cap. |
Users familiar with low background detectors will note that magnesium end caps, copper end caps, and copper crystal support cups are commonly used with low background detectors. Although conventional thinking holds that Compton suppression systems are the solution for ultra-low activity sample measurements, this is not the case. By using these materials in the detector structure, the overall effect is to reduce the performance of the Compton suppression system (i.e., reduce the peak-to-complexity ratio). So in Compton suppression systems, trade-offs are very important.
Other important components in the Compton suppression system
In order to be able to simultaneously detect the Compton scattered rays scattered from the high-purity germanium detector into the NaI ring or plug detector and minimize the Compton platform height, the correct electronic setup is very important. Historically, ORTEC has provided a simulation system based on NIM plug-in electronics. With the rapid development of digital technology, ORTEC has launched a digital dual-channel MCA. The digital version of the Compton suppression system is significantly less complex and time-consuming to commission.
The digital version of CSS significantly reduces the requirements for electronic equipment, using fewer components and a more compact space design. Large NIM racks are no longer required. In order to further improve space utilization without using any NIM electronics, ORTEC replaced the original box-type shielding body with a more integral integrated cast cylindrical shielding body. The shield is 4 inches (10 cm) thick and has an interior height that is 4 inches (10 cm) higher than traditional shields. The weight is only half of the original shielding body, and the floor space is also half of the original shielding body. In addition, the top sliding door design of the cylindrical shield allows the compliance detector to be installed in the top shield. This innovative approach greatly simplifies the loading and unloading of samples, requiring only one simple step, saving time. At the same time, it also reduces possible damage to the detector each time the sample is loaded and unloaded. Removing the plug detector from the NaI ring can greatly increase the sample measurement space without sacrificing the 4-π coverage geometry and Compton suppression performance.
