1 Beta Counting System Li XiangQing, Jiang DongXing, Hua Hui, Wang EnHong Peking University

2 outline light neutron-rich nuclei  decay experiment setup and our group work  detector problem Beta Counting System Summary Nuclear Landscape protons neutrons

3  light neutron-rich nuclei  decay AZNAZN A Z+1 N-1 QQ EE  -decay SnSn S 2n 11 Li : Q  =20.6 MeV, 11 Be: S n =0.5 MeV P n ~92% N >>Z : Q , S n EE A-1 Z+1 N-2  -n decay A-1 Z+1 N-3 E neutrons  -decay is often characterized by the large decay energies(10~20MeV), which will lead to the population of excited states with a wide excitation range, in particular particle unbound states, in the daughter nuclei. As a result, delayed neutrons or other particles may be emitted following the emission of  -rays. Such a complex decay scheme yields a great deal of information on  -decay properties of the neutron-rich mother nucleus and nuclear structures of the daughter nucleus, which provides a stringent test of the validity of structure models, such as the shell model, understands astrophysical rapid neutron capture process and nuclear shape changes.

4  experiment setup  decay of neutron-rich unstable nuclide often results in delayed neutron and  emissions from the excited daughter and granddaughter nuclei. Therefore, the coincident measurement of  - n-  particles is generally required to unambiguously assign the quantum-state poroperties of the related nuclei.

5  Our group work ? ? 16 C 17 C 18 C 19 C 17 N 18 N 19 N 20 N 22 N 23 O 17 B 21 N 14 Be 11 Li 9 Li 8 He 15 B Studied at NSCL Studied at GANIL Studied at RIKEN Studied at many Labs Studied at PKU Z.H.Li et al., PRC72,064327(2005) J.L.Lou et al., PRC75,057302(2007) Z.H.Li et al., PRC80,054315(2009)

6  Our group experiment setup Traditional beta decay studies involved the collection of a bulk sample, whose overall decay was monitored as a function of time.

7  problem Please specify how the decay was fit; was a standard decay code used? and what the chi square per degree of freedom was for the fit. Was the error bar increased to obtain a chi square per degree of freedom fit of 1? I also suggest that they include the growth and decay of all known contaminants and their daughters/grand-daughters, so that the impact on the half-life determination (and its uncertainty) is quantified. Another test of this would be to exclude the first 100 ms of the decay data (since they may be polluted with 19C or other short-lived species) and quantify how the half-life fit changes. Perhaps the background is not flat? Did they collect additional data that extends out to longer times? If so, include this data and/or mentioned the additional information learned about the variation of the background with time. Do they know what isotopes dominate this background? Is it due to the build of daughter activities, such as 20O(13.5 s) or 21O(3.4 s)/21F(4.16 s), over time? I suggest that they refit the background with the growth and decay of dominant activities that are stopped in the beta detector (as mentioned above). This various comparisons will give the reader more confidence in the extracted half-life and quoted error bar (which you will most likely need to increase because of these impurity considerations). The Radioactive Ion Beam Intensity will always be an issue. Cocktail beams. Many nuclei implant at detectors. High selectivity even with mixed (“cocktail”) beams, relevant particle properties can be detected (TOF, energy losses …) Tag products – remove beam-decay background (separator, etc..) The background was very large! Isopopes and their daughter activities dominate the backgroud! Z.H.Li et al., PRC80,054315(2009)

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9  Beta Counting System (BCS) Si PIN DSSD (  Implantation DSSD: x-y position (pixel), time Decay DSSD: x-y position (pixel), time Si PIN Veto light 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 or near 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.

10 Detector setup for  half lives measurement Implantation-decay correlations with large background (half lifes similar to the implantation rate): implantation-decay time correlation: active catcher implantation-decay position correlation: granularity implantation of several ions: thickness and area energy of the implanted ion and the emitted  Active catcher for implantation-decay correlations By using a highly segmented silicon implantation detector, direct correlations can be made between individual radioactive isotopes and their emitted beta particles.  Si BCS

11 Implantation station: The Segmented Germanium Array (SeGA)  -delayed gamma spectroscopy of daughter

12 Implantation station: The Beta Counting System (BCS) Beta decay properties that can be deduced using this device include half-lives, branching ratios, and decay energies. 16 SeGA detectors around the BCS Efficiency ~7.5% at 1 MeV W.Mueller et al., NIMA 466, 492 (2001) mother daughter 105 Zr Fit (mother, daughter, granddaughter, background)  T 1/2

13 Reach for future experiments with new facilities (ISF, FAIR, RIBF…) Future Facility Reach (here ISF) Known before NSCL Experiments done NSCL reach Almost all  -decay half-lives of r-process nuclei at N=82 and N=126 will be reachable with ISF

14 Neutron rich heavy nuclei (N/Z → 2) Large neutron skins (r -r  → 1fm) New coherent excitation modes Shell quenching 132+x Sn Nuclei at the neutron drip line (Z → 25) Very large proton-neutron asymmetries Resonant excitation modes Neutron Decay Nuclear shapes Exotic shapes and isomers Coexistence and transitions Shell structure in nuclei Structure of doubly magic nuclei Changes in the (effective) interactions 48 Ni 100 Sn 78 Ni Proton drip line and N=Z nuclei Spectroscopy beyond the drip line Proton-neutron pairing Isospin symmetry Transfermium nuclei Shape coexistence New challenges in Nuclear Structure

15 summary Beta-decay properties of exotic nuclei turned out to be an effective probe for nuclear structure studies of exotic nuclei. The beta counting system is optimized to measure the short half-lives expected with nuclei with extreme numbers of protons or neutrons. future Beta decay lifetimes of nuclei with extreme neutron excesses are important to the understanding of the astrophysical rapid neutron capture process. Nuclear shape changes can also be resolved based on beta decay lifetimes.

16 THANK YOU

17 Cluster Detector Setup for Fast Beam Germanium Campaign 15 * 7 Germanium - Cluster Detectors optimized geometrically for efficiency and resolution

18 RISING from above

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