Study of the  -decay of 12 B Proposal to INTC 25th February 2002 SpokespersonH.O.U. Fynbo ContactpersonU.C. Bergmann

Why is that interesting ? Isn’t that all known ?

Region probed by  -decay What we know of 12 C Latest evaluation 1990 Region probed by delayed  -emission

Recent interest Shell-model, cluster model, Group theory ……… What do they predict?

Model Predictions -“Exact” 12-body calculations coming in 1-2 years time - Improved shell-model on its way

The triple-  reation rate The NACRE compilation Nucl. Phys. A656 (1999) 3.

Previous Experiments Why it isn’t all known

12 B  -decay Studies In beam -> energyloss Evidence for energetic  particles

The 10.3 MeV state E = 10.1(2) MeV,  = 2.5 MeV E=10.3(3) MeV,  = 3.0(7) MeV Singles spectra

The  -decay of 12 N Unpublished data from Heidelberg from 1978 (Schwalm) Main experimental problems are Energy loss in target Summing Evidence for new state near 13MeV

Reaction studies ? Eyrich et al. PRC36 (1987) 416. Lewis et al. Nucl. Phys. A474 (1987) 499. Youngblood et al. PRC57 (1998) Overlapping positive and negative parity states -> 0 + strength present

Why we can do better Now Illustrated with results from 12 N experiment at JYFL

Main experimental Improvements –Modern segmented Si-Detectors Large Solid Angle –detect all particles High Segmentation –no summing –ISOL beams Low energy isotope separated beam Can be stopped in a thin collection foil (carbon) Well-defined source Little energyloss of emitted particles Previous studies using this setup 31 Ar 2p Nucl. Phys. A677 (2000) 38 9 C 2  p Nucl. Phys. A692 (2001) N 3   Under analysis R.R.Betts IL NOUVO CIMENTO 110A (1997) 975

12 N results from JYFL experiment - Resolution - Statistics

12 N Results : “The 10.3MeV state” Select 8 Be 0 + branches by gating on diagonal Use Monte-Carlo to correct for detection efficiency Fit

12 N Results : The 12.71MeV state –Motivation for previous proposal to INTC –Break-up mechanism ? Sequential of simultaneous What is the role of 8 Be 2 + – 12 N Data : Simultaneous gives best description of the data E1E1 E2E2 E3E3 Dalitz plot

Proposed experiment Dedicated measurement of the  -decay of 12 B for the question of 0 + and 2 + states in 12 C

5 cm 10 cm Setup and Production –Detectors : 2 DSSSDs New DSSSD with reduced deadlayer is ordered. ISOLDE ball of single detectors is under construction, and will be included if ready. Dedicated thin detector for singles New improved electronics –Production 12 B can be produced with high yield indirectly via 12 Be. –Analysis Reduced contribution of 12.71MeV Measure  -angular distribution Measure singles Decays via 8 Be 2 + is applicable

Summary and beam-time Request Strong indications for –both 0 + and 2 + states at 8-11 MeV –new state near 13 MeV New experimental possibilities –ISOL beams (reduced energy loss) –Segmented Si-detectors We ask for 15 shifts