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Beta Counting System

Li XiangQing, Jiang DongXing, Hua Hui, Wang EnHong

Peking University

20100726@Chifeng

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outline• light neutron-rich nuclei decay • experiment setup and our group work detector problem• Beta Counting System• Summary

Nuclear Nuclear LandscapeLandscape

pro

ton

s

neutrons

82

50

28

28

50

2082

28

20

82

126

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light neutron-rich nuclei decay

AZN

AZ+1N-1

Q

E

-deca

y

Sn

S2n

11Li: Q=20.6 MeV , 11Be: Sn=0.5 MeV Pn~92%

N >>Z : Q, Sn

E

A-1Z+1N-2

-n decay

A-1Z+1N-3

Eneutrons

-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.

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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.

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Our group work

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16C 17C 18C 19C

17N 18N 19N 20N 22N

23O

17B

21N

14Be

11Li9Li

8He

15B

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)

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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.

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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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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.

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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 Si Si

BCS

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Implantation station: The Segmented Germanium Array (SeGA)

-delayed gamma spectroscopy of daughter

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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)

motherdaughter

105Zr

Fit (mother, daughter, granddaughter, background)

T1/2

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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

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Neutron rich heavy nuclei (N/Z → 2)• Large neutron skins (r-r→ 1fm)• New coherent excitation modes• Shell quenching

132+xSn

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

48Ni100Sn

78Ni

Proton drip line and N=Z nuclei• Spectroscopy beyond the drip line• Proton-neutron pairing• Isospin symmetry

Transfermium nucleiShape coexistence

New challenges in Nuclear Structure

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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.

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THANK YOU

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Cluster Detector Setupfor Fast Beam Germanium Campaign

15 * 7 Germanium - Cluster Detectorsoptimized geometrically for efficiency and resolution

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RISING from above

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