Proximity Decay and Tidal Effects

Transcription

1 Proximity Decay and Tidal Effects A. B. McIntosh,S. Hudan, C.J. Metelko, N. Peters, J. Black, RdS Dept of Chemistry and IUCF, Indiana University July : Comet P/Shoemaker-Levy 9 collided with Jupiter resulting in at least 21 discernable fragments with diameters estimated at up to 2 km. When the field gradient on the dimension of the comet is comparable to the binding energy of the comet

2 When a hot nucleus decays Resonance Spectroscopy 1 + R ( q ) = 1 Y Y ( p C 12 ( p ). Y ( p , 2 p 2 ) 2 ) Γ 3.5 MeV MeV MeV Γ=1.51 MeV Γ=6.8 ev 3.03 MeV gr. st MeV 8 93 kev Be α + α Relative Energy Determined by Quantum State J. Pochodzalla et al., PRC 35, 1695 (1987) Inclusive analysis! 22 h = MeV. s h 23 t = Γ = MeV t = s = 130 fm / c Γ Clusters probe the low density environment (e.g. the nuclear surface)

3 Coulomb interaction Tidal effects in nuclear decay Z residue Cluster V ( r ) 1 r Z residue Z residue Decay into two identical particles Longitudinal decays have lower relative energy Maximum yield (evaporative emission) small initial KE for cluster Decay angle dependence of the probability Transverse Higher E rel Same acceleration after decay (same Z/A) Change of the relative velocity Transverse decays have higher relative energy Higher probability to decay transverse to the emission direction P V T Longitudinal Lower E rel ( E ) e and V = f ( β ) P ( E, β )

4 What can we learn from Proximity Decay and Tidal Effects? What do light clusters such as 8 Be look like? FMD calculations T. Neff and H. Feldmeier GSI A significant amount of alpha clustering is necessary in FMD to understand the BE of the ground and excited states in 8 Be. How is the proximity of the emitting nucleus on the 8 Be manifested?

5 Previous observation of Tidal Effects 60 Ni Mo at E/A=11 MeV Emission of clusters is R.J. Charity et al., PRC63, (2001) Following fusion, in coincidence with evaporation residues one observes the emission of stable and unstable clusters. 3 MeV state Maxwellian (evaporative) 8 Be spectrum of g.s. reconstructed from α α coincidence matches behavior of other Be isotopes Longitudinal Transverse Longitudinal Transverse Spectrum of 3.03 MeV state exhibits tidal effect (transverse decays have larger relative energy compared to longitudinal decays). Limited statistics, kinematical coverage Radioactive beams?

6 Measured PLF in the RC: 15 Z 46 Particles measured in LASSA 114 Cd + 92 Mo at 50 A.MeV LASSA : Θ 0.8 Mass resolution up to Z=9 7 θ lab 58 Si- E Si-E CsI(Tl) pixel Beam Ring Counter : Detection of Si (300 µm) CsI(Tl) (2cm) charged particles 2.1 θ lab 4.2 in 4π 1 unit Z resolution Mass deduced : Modified EPAX K. Sümmerer et al., PRC 42, 2546 (1990) 48 Projectile

7 AMD 114 Cd + 92 Mo at 50 MeV/nucleon Sample b = 0 13 fm events accumulated Mass, charge, energy exchange Binary nature of the collision Transiently deformed nuclei Early cluster production, t 90 fm/c S. Hudan, R.T. de Souza and A. Ono, PRC (2006)

8 AMD: PLF * and TLF * properties PLF * = biggest frag. forward of C.M.; TLF * = biggest frag. backward of C.M. Smooth decrease of Z PLF*, v PLF* with b V PLF* is a Good b selector Increase of the excitation energy ( T) with increasing centrality followed by saturation for b<6fm t clust = 300 fm/c Vary clusterization time Rapid cooling

9 Tidal effect: data selection 114 Cd + 92 Mo at 50 MeV/nucleon R. Yanez et al., PRC68, (R) (2003) Isotropic emission forward of PLF * Data selection: 15 Z PLF θ PLF 4.2

10 E * = M + M + Q clusters n.ek cluster n.ek cluster PLF * : velocity damping With increasing damping: More emitted particles Larger slope parameter Linear increase of E * /A L. Beaulieu et al., PRL 84, 5971 (2000). damping R. Yanez et al., PRC68, (R) (2003) Rapid de excitation significant Final State Interaction (FSI).

11 Tidal effect: correlation function Γ 3.5 MeV MeV Data selection: 15 Z PLF θ PLF V PLF 9.5 <E * /A >= 2 4 MeV 2 α particles forward of PLF* (θ 100 ) Γ=1.51 MeV 3.03 MeV Γ=6.8 ev g.s. 114 Cd + 92 Mo 50 MeV α α 8 Be Background 3 MeV Peak at 3 MeV sitting on broad bump No Ground State Peak (detector acceptance) Background primarily due to sequential emission of alphas

12 Construct Mixed event Background α1 α1 α2 α2 Event #1 Event #2 Normalization region: 14 MeV E rel 50 MeV reasonable first order description of relative energy spectrum over prediction of yield at small E rel What is the sensitivity of the background subtraction?

13 Difference in longitudinal velocity for PLF* for two different events (Similar velocity damping) Resonant behavior observed independent of additional velocity restriction in background. Changes in PLF* between the two α emissions impacts the background yield distribution P ( E rel ) e Accounts for: detector acceptance Tidal effect ( E rel ) 2 2 ( )

14 Z source Tidal effect: angle dependence β Z source Z source longitudinal decay Lower <E rel > transverse decay Higher <E rel > Additional velocity restriction yields semi quantitatively comparable result Consistent with tidal model ~20% effect

15 Monte Carlo simulations Two main components: 1. Statistical phase space ( uncorrelated α particles) Final state Interaction (FSI may be significant due to the high T of emitting source) 2. Resonant emission of 8 Be Ingredients of the MC FSI model : Sample experimental energy and angular distributions for α particles Z, velocity, and angular distributions of reconstructed PLF* P(t) = exp( t/τ); τ = mean time between successive emissions. intervening emission between two α s simulated by anisotropic emission of a pseudo particle (results relatively insensitive). Filter for experimental acceptance (geometry, finite angular resolution, etc.)

16 Monte Carlo FSI: Influence of FSI and Detector Acceptance (Successive emission of two alpha particles from a PLF*) As expected, for short τ suppression of yield at small E rel is observed. We observe a sensitivity to the FSI despite the detector acceptance

17 Monte Carlo FSI: Influence of FSI and Detector Acceptance (Successive emission of two alpha particles from a PLF*) As expected, for short τ suppression of yield at small E rel is observed. The average timescale between successive α emission is long τ > 500 fm/c Average Timescale indicates T 2 MeV substantial cooling has occurred Only part of the 3 MeV peak in E rel can be described by statistical α emission (with FSI) resonant emission

18 # detected by LASSA 15 Z PLF v PLF 9.5 cm/ns θ PLF 100 Y( 7 Be) = 5337 Y( 9 Be) = 6727 Y( 10 Be)=4559 Yield comparison Assume Y( 8 Be) = 6000 all in ground or first excited state If T = 2 MeV: Y ( 3 MeV ) ( 2 J + 1 ) 3 MeV 3 / 2 = e = 5 ( ) Y ( g. s ) ( 2 J + 1 ) g. s. Therefore, ~ Be in 3 MeV state. From Monte Carlo simulation ε(3mev)/ε( 9 Be) = 0.25 T=2 Expect to detect Be T=4 Expect to detect Be These expected yields are comparable to the integrals of the difference spectrum ( Be in 3 MeV peak).

19 MC RES: Understanding the Resonant Decay Emit a 8 Be isotropically from a PLF* PLF*: sample Z,A,E, θ from experimental data; φ, isotropic P(t) = exp( t/τ); τ = fm/c (lifetime of 3.03 MeV state) P(KE Be 8 )=exp( E/T); T=7 MeV P(E rel ) gaussian with <E rel >=3.03 MeV; σ=1.51 MeV/2.35 Presence of external field acts to: Longitudinal Z source Decrease E rel Transverse Z source Increase E rel Proximity decay of 3 MeV state is clearly manifested by a broader E rel distribution particularly at large E rel.

20 Tidal effect clearly predicted by MC RES For the most transverse decays detector acceptance shifts and narrows the E rel distribution MC RES predicts a much stronger tidal effect than is experimentally observed

21 Tidal effect and nuclear proximity ln P(t) 8 Be near surface of emitting nucleus is influenced by proximity effect (nuclear attraction of surface) additional binding/reduced decay probability of 8 Be? (analog to 11 Li Borromean system?) t delay t (fm/c) Delay of t=200 fm/c (d ~ 2 4 fm from surface) results in reasonable agreement with data This proximity effect would be largest for weakly bound nuclei which are short lived (i.e. a large fraction of the resonant clusters decay near the emitting nucleus).

22 Conclusions The Coulomb tidal effect is clearly observed in the decay of 8 Be emitted from the PLF* following mid peripheral collisions of near symmetric heavy ions at intermediate energies. The magnitude of the measured tidal effect is significantly less than that predicted by a Monte Carlo simulation (MC RES). The observed magnitude of the tidal effect is consistent with the scenario that the proximity of the emitting nucleus stabilizes weakly bound short lived states. Thanks also to (for all those shifts on the experiment): R.J. Charity, L.G. Sobotka Washington University T.X. Liu, X.D. Liu, W.G. Lynch, R. Shomin, W.P. Tan, M.B. Tsang, A. VanderMolen, A. Wagner, H.F. Xi NSCL-MSU