1. New proposal for the BRIKEN campaign
« Search for the two-neutron radioactivity »
S. Grévy, P. Ascher, M. Aouadi, B. Blank, M. Gerbaux, J. Giovinazzo, T. Goigoux, C. Magron, T. Kurtukian-Neto
CENBG Bordeaux-Gradignan, France ….
P. Ascher, BRIKEN Workshop, Valencia, July

2. From classical radioactivities... to exotic radioactivities
b- / b+ radioactivities
Fission, alpha emission
(2002)
2p radioactivity
Découverte en 1981
Radioactivité 1p
1p radioactivity
(1981) Z
drip-line neutron N
Découverte en 1981
Découverte en 1981
Découverte en 1981
Découverte en 1981
Découverte en 1981
Découverte en 1981
Découverte en 1981
 What about neutron radioactivity ?

3. strong nuclear interaction
2n radioactivity
2n radioactivity is defined as a spontaneous and simultaneous emission of 2n from the ground state
S2n < 0 AND S1n > → even N and at dripline
centrifugal barrier → high 
free neutrons
paired neutrons
strong nuclear interaction
barrier
Frag. exp.: T1/2 > 100ns
→ Q2n < 15 keV (orbital d)
→ Q2n < 150 keV (orbital f)

4. What about 1n radioactivity?
L. Grigorenko et al., PRC 84 (2011)
Implantation experiment (T1/2 > 100 ns): Half-lives too short
In-flight decay experiment (few ps < T1/2 < 100 ns): probable only for high 
→ therefore we will focus in the following on 2n radioactivity

5. What do we learn ? wave function composition barrier penetrability
pairing/correlations
masses of nuclei at the drip-line
When nuclear binding energy decreases, concept of separation between well localized bound states from the continuum scattering states is not valid anymore
 difficulty to describe halo states, clustering…
 new spectroscopic information on very exotic nuclei and complex decay modes is essential for a unified description of the nuclear structure and the nuclear reactions (SMEC…)

6. 31F and 34Ne: 2 good candidates
S2n < 0 AND S1n > 0: near/at the drip line
centrifugal barrier: high 
SPEG/Gaudefroy et al., PRL 2012 pf
SM calculations using the SDPF-U interaction (A. Poves et al.) for 31F
f7/2² : 40%
p3/2² : 27% dominance of f²
f7/2p3/2 : 22%
31F and 34Ne already observed in impl. experiments → T1/2 > 100 ns
Is it because they are bound or is it because of the centrifugal barrier??

7. Experimental Method
Study the b-decay after implantation  if 2n radioactivity : b-decay of the A-2X daughter nucleus b bn
b2n
no 2n
- T1/2(AX) = T1/2(A-2X) : strong indication
- Decay scheme for firm conclusion
27Ne
28Ne
29Ne
30Ne
31Ne b bn
b2n 2n
29F
31F b bn
b2n
no 2n
30Ne
31Ne
32Ne
33Na
34Na b bn
b2n 2n
32Ne
34Ne
In this experiment, we propose :
measure T1/2 of 31F : if T1/2(31F) = T1/2(29F)  very strong indication of 2n radioactivity
measure T1/2 of 34Ne : if T1/2(34Ne) = T1/2(32Ne)  very strong indication of 2n radioactivity
to study the b-decay of 29F and 32Ne (T1/2, Pn, …)
 background for proving the 2n radioactivity in 31F and 34Ne
 spectroscopic data for exotic nuclei around the Island of Inversion
-i- implantation in DSSSD Si detector
-ii- Ge detectors
-iii- He counters array

8. Counting rates and implantation in detectors
use of 48Ca 345 MeV/u, 500 pnA, Be target
2 different settings, 1 for the very exotic isotopes, 1 for the less exotic ones
Setting on 34Ne (Br1 = Tm )
with F mm -> implantation profile ~ 5 mm (OK if 5 DSSSD)
→ 34Ne ~ 3.0 e-4 pps
→ 31F ~ 7.1 e-4 pps
Same spectro setting but 2 different Al degrader (67 mm for 34Ne, 74 mm for 31F)
Setting on 32Ne (Br1 = Tm)
→ 32Ne ~ 1.85 e-1 pps
→ 29F ~ 5.09 e-1 pps
Same as 34Ne and 31F for the implantation

9. Beam time Thank you! Exotic setting
34Ne ~ 3.0 e-4 pps → ~ 26 counts/day
31F ~ 7.3 e-4 pps → ~ 63 counts/day
100 counts 34Ne and 31F  able to separate T1/2(31F) and T1/2(29F) with CL >99%
(considering T1/2 from Gross theory)
4 days for 34Ne, 2 days for 31F
Less exotic setting
32Ne ~ 1.85 e-1pps → ~ 600/hour
29F ~ 5.09 e-1 pps → ~ 1800/hour
10000 counts of 29F and 32Ne  g transitions with BR ≥ 5% (eg~3% )
 Pn/2n values
1 day for 32Ne and 29F
Thank you!

10. Implantation in detectors and counting rates
To produce both 31F and 34Ne :
 use of 48Ca 345 MeV/u, 500 pnA
- target = Be mm
- Br1 = Tm (Setting on 34Ne)
- F1=+/- 65 mm
→ 34Ne: ~ 9.5e-4 pps
→ 31F: ~ 2.2e-3 pps
32Ne and 29F also produced in this setting
29F not implanted in the DSSD
32Ne : 5.2e-2 /
with 62 mm Al degrader
5x1mm DSSSD
We could close the F1 slits but we would have to have 2 different settings for 31F and 34Ne
By leaving the slits +/-65 mm, we lose but can have both in the same setting
→ 34Ne: ~ 3.0 e-4 pps
→ 31F: ~ 7.3e-4 pps
With F2+-15 : 110 cont./s (mostly not implanted)

11. Recent related activites
 Search for 4n cluster
 Observation of 2n emission from 16B