PROBING THE 11 Li HALO STRUCTURE BY TWO NEUTRON INTERFEROMETRY EXPERIMENTS

1. PROBING THE 11Li HALO STRUCTURE BY TWO NEUTRON INTERFEROMETRY EXPERIMENTS IFIN-HH, ROMANIA • M.Petrascu • A.Constantinescu • I.Cruceru • M.Giurgiu • A.Isbasescu • H.Petrascu • C.Bordeanu RIKEN, JAPAN • I.Tanihata • T.Motobayashi • T.Kobayashi • K.Katori • K.Morimoto • A.Ozawa • T.Suda • K.Yoshida • S.Nishimura • M.Chiba M. PETRASCU

2. Overview • Short introduction • Description of the recent experiment on neutron pair pre-emission • True coincidences and cross-talk • The neutron-neutron correlation function • An experimental evidence for residual correlation of pre-emitted halo neutrons • Preliminary results on halo structure • Next steps M. PETRASCU

3. Short Introduction The neutron halo nuclei were discovered by :I.Tanihata, et al., Phys.Lett.160B 380(1985) The designation of “halo” for the low density matter around the core was first introduced by: P.G. Hansen, B. Jonson, Europhys. Lett. 4 409 (1987) Pre-Emission : Short cut for Pre-Equilibrium Emission. First Pre-Emission paper: M. Petrascu et al., Phys.Lett. B405, 224.(1997) M. PETRASCU

4. Pre-emission of halo neutrons in a fusion process It was predicted M.Petrascu, et al.Balkan Physics Letters 3(4), 214. (1995) that, due to the very large dimension of 11Li, one may expect that in a fusion process on a light target, the valence neutrons may not be absorbed together with the 9Li core, but may be emitted in the early stage of the reaction. Indeed, the experimental investigations of neutron pre-emission in the fusion of 11Li halo nuclei with Si targets have confirmed this view. M. PETRASCU

5. Pre-emission of halo neutrons It was found: • In about 40 % of fusion processes 1 or 2 halo neutrons from 11Li are pre-emitted. Why are not 100% of the halo neutrons pre-emitted? This is shown in the previous slide. • Besides a distribution of the pre-emitted neutrons corresponding to the transverse momentum of the halo neutrons predicted by COSMA model, M.V.Zhukov et al. Phys.Rep. 231, 151 (1993), a very narrow peak in forward direction was found . • The preliminary measurements indicate the presence of neutron pairs within the narrow peak. It was assumed that the narrow peak is due to n-n final state interaction. It was decided to perform a new experiment by using an array detector. M. PETRASCU

6. Description of the recent experiment on neutron pair pre-emission: The experimental setup

7. The MUSIC-type Detector The detector is designed to "see" ionizing particles starting with Z=1, which have large specific ionization, so we need a very accurate dependence between the particle's energy and the number of electrons which appear in the particles trace. Any other type of gas detector (especially the proportional counters) loose this information, having a variable gas multiplication factor with Z (proportional counters are good for one defined type or a very small range of particles). H.Petrascu et al., Rom. Journal of Physics –vol.44, no. 1-2, Supplement, 105.(1999). M. PETRASCU

8. The Neutron Array Detector Consists of 81 detectors, made of 4x4x12 cm3 BC-400 crystals, mounted on XP2972 phototubes. This detector, placed in forward direction at 138 cm from the target, was used for the neutron energy determi-nation by time of fight technique and for neutron position determination. The distance between adjacent detectors was 0.8 cm. The array components were aligned to a threshold of 0.3 MeVEE, by using the cosmic ray peak at 12 MeV (8 MeVEE). M. PETRASCU

9. Results: The forward neutron peak It was found that the forward neutron peak consists predominantly of neutron pairs.

10. The aperture of the forward n peak The FWHM of this spec-trum is ~ 13 cm and corresponds to an aper-ture of ~ 5.5 deg of the cone enveloping this forward neutron peak (solid angle ~9 msr). This angle is ~15 times smaller than the angle correspon-ding to internal momen-tum of halo neutrons.

11. True n-n coincidences and cross-talk Cross-talk is a spurious effect in which the same neutron is registered by two or more detectors. A coincidence between two detectors is rejected whenever E1 >Emin. The left formula was validated in R.Ghetti et al., NIM A421 542 (1999). For the first rejection we took dmin equal to the distance dcc, between the detector centers. For example, by applying this criterion to the first order coincidences(dcc=4.8 cm), a number of 118 coincidences were found. For further rejection we consider that it is more appropriate to use the dmin parameter instead of time of flight, because the distance between the adjacent detector centers is close to the detector dimension. By taking dmin=1.8 cm (for adjacent detectors), the number of first order true coincidences is reduced from 118 to 46. Pierre Desesquelles, author of MENATE, adapted for us a code for c.t. rejection calculation in the case when dmin is used as rejection parameter. We have found that in the case dmin=1.8 cm, only 1 neutron of 3000 c.t. can pass as a true coincidence.

12. True n-n coincidences and cross-talk. Overall simulation of the array performances by aid of MENATE The significance of the obtained data was supplementary tested through a complete simulation of the array detector performances by using Monte-Carlo program Menate. We have investigated in this way the c.t. distribution as a function of t=t2-t1 for different coincidence 1st to 4th orders. The simulation was performed by firing the central #1 detector by neutrons of given energy and by extracting the cross-talk events corresponding to #2-#9 detectors (first order), to #10-#25 detectors (second order), and so on. For each event, the space and time coordinates and also the light output were available. In these simulations was found the notable fact that, in the case of a cross-talk, ~ 87% of the events are concentrated in the first half of detector #1, and only ~13% of the events are distributed within the second half of this detector. Vice-versa, are the events in #2 detector (here by #2 is understood whatever c.t. partner of #1 det.).

13. True n-n coincidences and cross-talk. Overall simulation of the array performances by aid of MENATE program First order coincidences Second order coincidences

14. True n-n coincidences and cross-talk. Overall simulation of the array performances by aid of MENATE program In the t2-t1=0-1 ns only γ rays can be present C.T trajectory length as a function of t2-t1

15. Possible application of c.t. effect Resolution in the case of 2nd order c.t. Resolution in the case of single neutron detection

16. The neutron-neutron correlation function The correlation function comes from astrophysics: R. Handbury Brown and R.Q. Twiss: Phil. Mag. 45 (663) 1954. M. I. Podgoretsky of Dubna was among first to draw attention about possibility of c.f. application in nuclear and particle physics to get information about the size of the zone from which particles are emitted. M.I.P raised around him a school of which we quote. G.I.Kopylov, Phys.Lett. 50b 472. (1974) who gave a general formulation of c.f. for nuclear physics. R.Lednicky, L. Lyuboshits Sov. J. Nucl. Phys. 35 (1982) 35 , who calculated c.f. in the case of n-n final state interaction. M. PETRASCU

17. Determination by Michelson interferometry of the radio source size

18. The principle of correlation interferometry S1(t) S2(t) S1(t)x S2(t-τ) S1(t) S1(t)S2(t-τ) S2(t)

19. The correlation function for neutron interferometry The two-neutron correlation function is given by: . (2) in which Nc(q) represents the yieldof n-n coincidence events and Nnc(q) the yield of uncorrelated events. The variable q is the relative momentum q=1/2 | p1-p2| in which p1, p2, are the momenta of the two neutrons in L.S.The normalization constant k is obtained from the condition that C(q)=1 at large relative momenta. A crucial problem for getting C(q) is the construction of the denominator in formula (2) and therefore the main emphasis of my talk in the following is related to this problem. In general two different approaches are commonly used. In the event-mixing technique the denominator is generated by randomly mixing neutrons from the coincidence sample. This method ensures that the uncorrelated distribution includes the same class of collision and kinematic constraints as the numerator, but has the disadvantage that it may distort the correlations one wishes to measure since the event-mixing technique may not completely succeed in decorrelating the events. In the single product technique the denominator is constructed from the product of single distributions. This method was preferred previously, considering that the background yield is truly uncorrelated, but this is not valid for halo neutrons, due to the residual correlation of single neutrons. F. Marques et al Phys. Lett B 476 219 (2000)

20. Experimental evidence for residual correlation of halo neutrons Fig.I: The denominator B, (open circles) and the event mixing denominator (crosses) are represented together, in steps of 0.5 MeV/c. Fig.II: The same denomina-tors are represented as a function of q, in 2 MeV/c steps; Fig.III: The denominators A, (solid squares with error bars), B (open circles), are repre-sented as a function of q, in 0.5 MeV/c steps;

21. Preliminary results on 11Lihalo structure By using denominator A, we obtained a correlation function corresponding to the red line of the figure. By using denominator B, we obtained a correlation function re- presented by the experimental points with error bars. The blue curve represents the correlation function in agreement with COSMAmodel. Our results favor COSMAI. The value of rrms predicted by COSMAII is too far to be ac-counted by the small anomalies seen in denominator B.

22. Next steps • A definite answer to question raised here will be an experiment aiming to determine the intrinsic correlation function by using 11Li and 11Be halo nuclei. The nucleus 11Be will be an ideal uncorrelated background source, since it contains only one halo neutron. This new experiment should be done by using 12C instead Si target. A sharp cut-off estimation has indicated that the n-n correlation peak will be about 2 times higher in the case of 12C than in the case of Si target. • The experimentally observed signatures of residual correlation could be of use in the identification of new Borromean halo nuclei. • We consider to be of interest for applications to further investigate the connection between c.t. and resolution. M. PETRASCU