Beta Counting System
The beta counting system is a series of silicon detectors used to characterize the beta particles emitted from captured radioactive beams. Many thousands of nuclei are predicted to exist, and most of these radioactive species should decay by emitting beta particles. However, only a few of these nuclei have been studied in the laboratory. The beta counting system can get information on lifetimes of nuclei that are very unstable and only exist for a tiny fraction of a second. This information is essential for understanding the synthesis of heavy elements in the universe.
Expanded Description
The central silicon implantation detector in the beta counting system is divided into 40 horizontal and 40 vertical strips, effectively providing 1600 independent silicon pixels. Each pixel is used to detect the incoming radioactive beam and subsequent beta radiations that occur when the nuclear isotopes in the beam decay. Beta decay properties that can be deduced using this device include half-lives, branching ratios, and decay energies.
Traditional beta decay studies involved the collection of a bulk sample, whose overall decay was monitored as a function of time. By using a highly segmented silicon implantation detector, direct correlations can be made between individual radioactive isotopes and their emitted beta particles. When a beam particle implants into a pixel of the segmented silicon detector, information is recorded on a computer that helps identify the particle by mass and nuclear charge. In addition, the absolute time of the event is recorded. After some delay, a second event, corresponding to the beta decay of this particle, is detected in the same pixel. The energy of the beta particle and the absolute time of the event are recorded. The time difference between implant can be used to extract the beta decay half-life of the nuclear species.
The beta counting system is optimized to measure the short half-lives expected with nuclei with extreme numbers of protons or neutrons. The high segmentation of the implant silicon detector also permits the measurement of half-lives for nuclei that are produced at rates of only a few per day.
The beta counting system can be supplemented with other detectors, for example, the MSU Segmented Germanium Array or the Neutron Emission Ratio Observer (NERO) to obtain additional information on the decay properties of radioactive nuclei.
The Importance of the Beta counting system
The beta counting system can be used to attain data on lifetimes of nuclei far from the line of stability.
The rate of radioactive decay is quantified by the decay half-life. The half-life is the average time it takes to reduce the initial number of nuclei by a factor of two. The half-life, in essence, determines how long a radioactive sample will exist. Lifetime data have been experimentally deduced for nearly all nuclei near the valley of stability. However, very far from stability, experimental lifetime data is very limited. This is due to the low production and short half-lives expected for these exotic species.
Beta decay lifetimes of nuclei with extreme neutron excesses are important to the understanding of the astrophysical rapid neutron capture process. This process is responsible for the production of more that 60 percent of the stable isotopes found in nature. Modeling the rapid neutron capture process can be markedly improved with improved experimental data on the beta decay of exotic, neutron-rich nuclei.
Nuclear shape changes can also be resolved based on beta decay lifetimes. Although most people think of the nucleus as a spherical body, it can actually take on other shapes. For example, nuclear shapes can resemble footballs (prolate shapes) or pancakes (oblate shapes). Transitions between these different shapes are expected to occur in nuclei. One signature of these transitions is the dramatic change in the beta decay lifetimes for neighboring nuclei.
Technical Information
The beta counting system (BCS) is built around a double-sided silicon strip detector with 1600 pixels (40 strips in each of the horizontal and vertical directions). Radioactive species produced by fast fragmentation are implanted in this detector. Implantation events are correlated with subsequent beta decays on a pixel-by-pixel basis, allowing the identification of the species observed to decay and a direct measurement of the decay time. A stack of Si detectors and a Ge planar detector can be placed downstream of the BCS implantation detector to measure the total energy of emitted beta particles. The BCS can be used with other detector systems (such as the segmented germanium array or the neutron ratio emission observer) to study beta-delayed radiations.
Status: Operational
Location: S2 vault, N2 vault
Contact person: Paul Mantica
Funding acknowledgement: Supported in part by the National Science Foundation grant PHY-0110253.
Reference:
J.I. Prisciandaro, A.C. Morton, and P.F. Mantica, Nucl. Instrum. Meth. A505 (2002) 140.
doi: 10.1016/S0168-9002(03)01037-4
Operation of NSCL as a national user facility is supported by the Experimental Nuclear Physics Program of the U.S. National Science Foundation.