1. Kurchatov Institute, Moscow
Recent and prospective studies of three-body decays and related phenomena at GSI
θp1 -14O (mrad)
θ p2 -14O (mrad)
16Ne
L. Grigorenko
FLNR, JINR, Dubna
GSI, Darmstadt
Kurchatov Institute, Moscow
19Mg
pictures by
I. Mukha, J. Marganiec
17Ne

2. Two-proton radioactivity
Predicted by V.I. Goldansky in 1960
Discovered at GSI in 2002
Classical case: one particle emission is always possible
Quantum mechanical case: it could be that both particles should be emitted simultaneously
6He  6Be
9Be  9B
12Be  12O
True 2p decay has different energy systematic compared to 1p
Specific correlations among decay products
True two-proton radioactivity is a "substitute" for Borromean halo nuclei when we
move from neutron to proton dripline:

3. Related phenomena Two-proton radioactivity and three-body decay
True two-neutron decay exists. What about two-neutron radioactivity?
Efimov states?
Three-body virtual states?
Reactions with few particles in the final states
Exotic excitation modes. Soft dipole mode of the GDR
Decays into few particles
True five-body decay (4-neutron/4-proton emission)
Dynamics of the processes can not be reduced to the two-body dynamics
and studies should be done using methods of the few-body theory
Theoretical dreams. What about reality?

4. What is known beyond the driplines?
Red – true 2p/2n emitters studied
Green – halo systems
Black – nothing is known
7H and 8C are four neutron emitters. The candidates
are 28O and 29Si.
Two-neutron g.s. emission in 10He and 13Li studied
recently at GSI
True 2p emitters: 6Be, 12O, 16Ne, 19Mg, 45Fe, 54Zn.
Discovery of 19Mg and most recent data on 16Ne
are from GSI
Every second nucleus beyond the proton dripline is
true 2p emitter
Pretending to understand asymmetric nuclear matter EOS one need to extend our knowledge as far as possible beyond the driplines.
It is probable that in the first instance the 2p decays
of 26S, 30Ar and maybe 34Ca will be studied

5. Instrument: three-body cluster model
L.V. Grigorenko, R.C. Johnson, I.G. Mukha, I.J. Thompson, M.V. Zhukov, PRL 85 (2000) 22.
Schoedinger equation with complex energy
Hyperspherical Harmonic method
Approximate boundary conditions of the three-body Coulomb problem
“Natural” definition of width
Adopted to radioactive decay studies
Coulomb three-body problem is not tractable in general case: the exact analytical boundary conditions are not known
Typical precision: stable
solution for G/ET > 10-30

6. News: 48Ni and 54Zn
True 2p character of decay is directly confirmed for 54Zn
2p radioactivity is discovered in 48Ni
Predictions: Grigorenko et al. (2003)

7. 19Mg: decay in flight experiment
Idea of decay-in-flight experiment (GSI S271):
I. Mukha and G. Schrieder, NPA 690 (2001) 280c.
Structure and decays of 19Mg:
L. Grigorenko, I. Mukha, M. Zhukov, NPA 713 (2003) 372.
Dependence of the predicted lifetime on the structure.
“Belt” of posible lifetimes defined by calculations with pure configuration

8. 19Mg: vertex distribution and angular correlations
Analogy tracking approach in high energy physics. However.
Charges of decay products are too different: very specific setups

9. 19Mg: vertex distribution and angular correlations
FRS, fragmentation of the secondary 20Mg beam at S2
Microstrip detectors tracking system
Very thick target – efficient experiments for low intensity exotic beams

10. 19Mg: angular distribution of fragments and decay energy
Predicted p-17Ne correlations from 19Mg,
L.V.Grigorenko, I.G.Mukha, M.V.Zhukov, Nucl.Phys. A 713 (2003)
Kinematical enhancement
close to a maximum angle:
precise energy measurements
without invariant mass reconstruction

11. Angular p-17Ne correlations
from 17Ne+p+p events
selected inside the target
Angular p-17Ne correlations
from 17Ne+p+p events
selected after target
Angular p-17Ne correlations
from 17Ne+p+p events
selected before the target

12. What about doing the same with neutrons?
Continuation?
Experiment S388 “Two-proton decay of 30Ar and one-proton decays of 69Br, 73Rb” is preliminary scheduled for January 2012
Actual plans include studies of
New ideas?
What about doing the same with neutrons?
High granularity neutron detector:
Resolution aim: ~1 mrad

13. Two- and four-neutron radioactivity search prospects
Two-proton radioactivity is the long awaited and the most recently found mode of
the radioactive decay. Can neutron radioactivity exist?
Estimates: one-neutron radioactivity is highly unlikely.
There are additional effective few-body “centrifugal” barriers making few-body
emission relatively slower.
Long-living Two-neutron decay and moreover four-neutron decay states are
reasonably probable.
It is proposed at GSI to develop neutron tracking technique (in analogy with microstrip proton tracking) having in mind narrow neutron resonances and 2n radioactivity search.

14. Two- and four-neutron decay studies with HGND
Prediction: distinctive correlation patterns allows to work with 2n emission down to very low energies. For 4n emission they allow to distinguish different decay modes .
Angular distributions between heavy fragment and one of the neutrons for true 2n decay
of 26O (decay energy 20 and 300 keV).
True 4n decay of 28O (decay energy 300 keV) and sequential decay via 26O g.s. (the
latter has decay energy 20 keV).
Features of possible experiments to be understood prior the construction.

15. Modes of two-proton radiative capture
rp-process at high temperature and density.
Could be important in the regions of nuclear chart, where there are “ridges”
connected with paring and intermediate system is not nuclear stable. Analogy with
true 2p emission.
15O, 18Ne, 38Ca : J.Gorres, M.Wiescher, and F.-K.Thielemann, PRC 51 (1995) 392.
68Se, 72Kr, … ,96Cd : H.Schatz et al., Phys. Rep. 294 (1998) 167.
Famous a+a+a →12C+g capture process belongs to this class also.

16. “Soft E1” mode in proton dripline nuclei
Soft dipole mode: fragmentation of the GDR in systems with spatially extended density (haloes, skins).
Existence of “soft E1” mode now established in neutron-rich nuclei (“pigmy” dipole resonance in heavy systems).
L.V.Grigorenko, K.Langanke, N.B.Shul’gina, M.V.Zhukov, Phys. Lett. B641 (2006) 254.
Possibility of “soft E1” in proton-dripline nuclei: predict a strong and narrow E1 peak in 17Ne.
The 2p capture rate is dominated by the nonresonant E1 capture connected with SDM for T < GK and T > GK.

17. Competition between a and 2p capture
15O(2p,g)17Ne versus 15O(a,g)19Ne
Densities (in g/ccm) at which the production rate of 17Ne by 2p-capture on 15O equals the production rate of 19Ne by α capture as function of temperature and α-particle mass abundance Xa = 4 Ya.

18. Status of 17Ne S318 experiment
“Study of the Borromean dripline nucleus 17Ne”.
17Ne beam, Pb/C/CH2 target.
Very pronounced and narrow E1 peak in 17Ne.

19. Data below 2.5 MeV E2 Coulex contribution dominated.
Strong deep at CM angle 11 mrad.
Different angular correlations for CM angle larger and smaller 11 mrad – different reaction mechanisms.
Classical interpretation: 11 mrad = 16 fm of impact parameter. Touching distance for 17Ne on Pb target is ~10 fm.
To be understood.

20. Data between 2.5 and 7 MeV E1 Coulex contribution dominated.
NEW cluster sum rule exhausted by MeV.
Nice agreement with theoretical predictions.
First observation of the soft dipole mode in 2p halo nucleus.
Predicted for 2p halo in 17Ne.
Implementations for astrophysics.

21. Data above 7 MeV 2.5 < E < 7 7 < E cos(qpp) distribution
Broad angular distribution – different reaction mechanism.
Strange events: protons emitted together or back to back in a narrow cone (~30 dgr)
No theoretical explanation so far.
cos(qpp)
distribution
Red: cos(qpp) <
Blue: cos(qpp) < 0.75

22. Role of theory in these studies
Extremely detailed and high quality data. Qualitative simplistic treatment “not possible”.
Quantitative results not possible without complete MC studies.
Four different theoretical problems to consider: coulomb/nuclear interference, coulex E2, coulex E1, unusual “cones emission” events.
Example of recent 6Be decay data from Dubna: 6Be(p,n) 6Be reaction at 30 A MeV.
E (MeV)

23. Active tasks and prospects
Findings: two-proton halo in 17Ne seem to exist.
Findings: soft dipole mode in 17Ne seem to exist.
Findings: unusual correlation mode “cones emission” found in 17Ne excitation spectrum above 7 MeV.
Practical task: preparation of S388 experiment (microstrip tracking, new 1p and 2p emitters).
Practical task: MC “event generator” development for reaction studies of three-body systems (for 17Ne S318 experiment data treatment).
Practical task: theoretical modeling of the reaction mechanisms (input for MC “event generator”)
Last two points to be seen in poster by I. Egorova.
Prospective studies: substantiation of plans the 2n-4n emission studies.
Prospective studies: applicability of high granularity neutron detector for research with the neutron dripline nuclei.
Rapidly developing field. Changing situation.
Changes in the working plans.