N/Z Dependence of Isotopic Yield Ratios as a Function of Fragment Kinetic Energy Carl Schreck Mentor: Sherry Yennello 8/5/2005 J. P. Bondorf et al. Nucl. Phys. A443 (1985) 321

Outline of Presentation ● Multifragmentation Reactions ● Motivation ● Experiment ● Results ● Acknowledgements

What is Multifragmentation? iii iii iv A significant portion of the nuclei overlap Compression, heating, and exchange phase Expansion and cooling phase Fragmentation phase ● A process by which an excited nucleus expands, cools, and breaks up into multiple pieces ● For example, when a projectile collides head on with a target, the projectile and the target fuse to form a composite nucleus (CN), which then expands and fragments 1

Characterizing Nuclear Reactions b Light Ion Collisions: usually induced by 4 He, protons, and pions ● Interaction causes the nucleus to expand and then fragment Heavy Ion Collisions: induced by elements with Z > 2 ● Interaction causes the nucleus to compress, expand, then fragment Central Collisions (very small b) ● Formation of compound nucleus compress, expand, and fragment Peripheral collisions (b ≈ r t + r p ) ● The projectile is the source of fragments and does not have a complete compression phase in the fragmentation process 2 Kaufmann et al, Proc. 2 nd Conf. Reactions Between Comples Nuclei

Projectile Fragmentation b rprp rtrt ProjectileTarget ● When b ≈ r t + r p, the projectile and target do not fuse, and fragmentation of the projectile can occur via grazing collisions or coulomb excitation ● Little to no nuclei are exchanged in this process and the fragmenting 'projectile like fragment' (PLF) is very close in nuclear composition to the projectile ● Projectile fragmentation reactions do not require a full 4π detector, and are much less expensive to study 3

Theoretical Considerations Once a particle is excited and assumed to be in equilibrium, a number of statistical theoretical models can be applied ● Bulk fragmentation models – Assume that the fragmentation proceed at some point in time after the excited system has ceased to expand – Example: SMM ● Sequential decay models – Assumes that the system decays in discrete steps via branching probabilities – Decay scheme to the right illustrates one possibility for an excited 34 P – Example: GEMINI 34 P 32 P 12 B 17 O 13 C 19 F n n n p p 4 Moretto et al., Ann. Phys. Rev. 379 (1993).

Motivation for Project It has been experimentally observed that neutron poor isotopes tend to have energy spectra that have greater mean energies, peak at higher energies, and have higher tails Spectra taken from a 3 He + nat Ag reaction Also seen with Li, Be, B, C, and N isotopes* * PRC, Vol. 44, Pg 44 (1991)Viola et al., Phys. Rev. C 59, 2260 (1999) 5

Difficulty of Bulk Fragmentation Models to Explain the Data Current bulk multifragmentation models cannot explain these results ● The mean energy for isotopes from an equilibrated fragmenting source in bulk models is predicted to increase with increasing mass – ≈ 3/2 kT + 2 Am N + – The thermal and (3/2 kT) coulomb terms are the same for all isotope of an element – The bulk motion term predicts that increasing mass will correspond to increasing mean energies 6

A Possible Explanation One explanation: Pre-equilibrium emission ● Researcher at Michigan State University have recently employed the Expanding Emitting Source (EES) model, in which fragments are statistically emitted prior to equilibrium, to explain this trend for 11 C, 12 C fragments from the 112 Sn+ 112 Sn reaction ● In this model, 11 C is emitted preferentially prior to 12 C (figure 1) and both isotopes tend to have a greater mean energy when emitted earlier (figure 2), leading to 11 C having a greater mean energy than 12 C figure 1 figure 2 Lynch, private communication (NSRC preprint, Liu et al) 7

A Possible Explanation ● Model agrees well with mean energies for energy spectra of 11 C, 12 C and the mean energies of He, Li, Be, B, C, N and O ● Questions with explanation – Does this trend occur in systems too light to for pre-equilibrium C emission? – Is this trend is a relic of the beam's N/Z? Mean energies for data and EES Energy spectra for 11 C, 12 C from data and EES 8

What reactions are we studying? Peripheral reactions resulting from isobaric projectiles ● Isobaric beams (same A), 20 F, 20 Ne, 20 Na, on 197 Au – Using isobaric projectiles allows us to gauge the impact of the beam's N/Z on the energy spectra of the fragments ● PLFs from peripheral reactions with little nuclei exchange – Only events with Z=Z beam, 18 ≤ A ≤22, mult ≥ 2 – Fragments are ejected mainly in the forward lab angle, while with central collisions fragments are ejected in the forward and backward angles Angular spectrum for Carbon, 20 Ne beam Lab angles Yield / Steradian (arbitrary units) 9

Experimental: MARS (Momentum Achromat Recoil Spectrometer) ● Radioactive (secondary) beams 20 F and 20 Na produced in the reactions 19 F + d → 20 F + p and 20 Ne + p → 20 Na + n ● D1,D2,D3: allow tuning to a range in charge/momentum ● Velocity filter: allow tuning to a range of velocities ● Q1,Q2,Q3,Q4,Q5,D3: focus beam Primary beam: 19 F or 20 Ne Primary target Secondary beam: 20 F, 20 Ne, or 20 Na Secondary target ( 197 Au) and FAUST Q1 Q3 Q2 D1 D2 Q4, Q5 Velocity filter MARS and FAUST pictures from Dr Doug Rowland 10

Experimental: FAUST (Forward Array Using Silicon Technology) ● Forward Array: Covers lab angles from 1.6 to 44.8 degrees ● Isotopic identification up to Z = 7 Five rings of detectors and 13 different angles Looking down FAUST Cross section of FAUST Real live FAUST Pictures of MARS from Dr Doug Rowland 11

FAUST Detection Detectors: Silicon (∆E) and CsI (E) ● Allows resolution of nuclei up to detector limit of Z = 7 Si Light guide CsI E ∆E∆E C Be B 12

Procedure ● All data analyzed using ROOT ● A 'quasi-projectile' is reconstructed from the fragments' kinetic energy and lab trajectory and then the fragment angles and energy are translated to the quasi-projectile center of mass frame ● Focused on energy spectra for isotopes of carbon and nitrogen ● Data grouped for middle angles, 36<θ qp <126, and forward/backward angles, 0<θ qp <36 and 126<θ qp < 180 Normalized energy spectra for 12C from 20Ne beam at 10 cm angles Yield (arbitrary units) Energy (MeV) } } mid angles forward/ backward angles 13

Procedure x 2 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam 13 N 14 N 15 N 16 N Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for N Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 Ne beam Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for N Isotopes with 20 Ne beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 Na beam Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for N Isotopes with 20 Na beam ● The difference between the mid and forward/backward angles for all three beam can be seen ● However, for 20 Na beam, the spectra will be for all angles since statistics are low 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 F beam Low Statistics and Sad Times for Sodium mid angles forward/backward angles forward/backward angles forward/backward angles N 14 N 15 N 16 N 13 N 14 N 15 N 16 N

11 C 12 C 13 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 Na beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Results Normalized Energy Spectra for C Isotopes with 20 F beam 11 C 12 C 13 C 14 C Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for C Isotopes with 20 Ne beam 11 C: = C: = C: = C: = C: = C: = C: = C: = C: = C: = C: = C: = 2.90 Spectra for middle cm angles for F, Ne and all cm angles for Na 14 C data not shown where statistics are low For all three beams, heavier isotopes correspond to smaller mean kinetic energies 15

13 N 14 N 15 N Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for N Isotopes with 20 Na beam 13 N 14 N 15 N 16 N Yield (arbitrary units) Energy (MeV/A) Results Normalized Energy Spectra for N Isotopes with 20 F beam 13 N 14 N 15 N Yield (arbitrary units) Energy (MeV/A) Normalized Energy Spectra for N Isotopes with 20 Ne beam Spectra for middle cm angles for F, Ne and all cm angles for Na 16 N data not shown where statistics are low Since it is very unlikely that a system this light (20 nucleons) would emit C or N pre-equilibrium, this trend most likely is a product of a different process 13 N: = N: = N: = N: = N: = N: = N: = N: = N: = N: = N: = N: =

Y( 14 C)/Y( tot C) Energy (MeV/A) Relative Yield for 14 C Isotopes Y( 11 C)/Y( tot C) Energy (MeV/A) Relative Yield for 11 C Isotopes Y( 12 C)/Y( tot C) Energy (MeV/A) Relative Yield for 12 C Isotopes Y( 13 C)/Y( tot C) Energy (MeV/A) Relative Yield for 13 C Isotopes More Results ● Relative Yields for middle angles (even for 20 Na) ● Plots show the yield of carbon isotopes divided by the yield of all carbon isotopes as a function of energy ● The relative yield of 12 C and 14 C is significantly different for the Fluorine than for the other beams 20 F beam 20 Ne beam 20 Na beam 17

Y( 16 N)/Y( tot N) Energy (MeV/A) Relative Yield for 16 N Isotopes Y( 13 N)/Y( tot N) Energy (MeV/A) Relative Yield for 13 N Isotopes Y( 14 N)/Y( tot N) Energy (MeV/A) Relative Yield for 14 N Isotopes Y( 15 N)/Y( tot N) Energy (MeV/A) Relative Yield for 15 N Isotopes More Results 20 F beam 20 Ne beam 20 Na beam ● Relative Yields for middle angles (even for 20 Na) ● The behaviour of the relative yield for 15 N is significantly different for the Fluorine beam than the other beams ● Is this behaviour reproducable with theoretical multi- fragmentation codes such as DIT/SMM? 18

Conclusion ● As expected, the spectra for Be, C, and N fragments exhibit a greater mean energy for the lightest isotopes ● The relative yield plots show a difference between the 20 F beam and the other beams, signifying a dependence of isotope yield ratios on the N/Z of the beam ● Future work – Compare data to DIT/SMM, DIT/GEMINI, EES theoretical codes – Look at fragment yield dependence on excitation energy 19

Thanks to: Cyclotron Institute at Texas A&M University Sherry Yennello and the SJY Group Department of Energy National Science Foundation 20