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?.

i ii iii iv

A significant portion of the nuclei overlap Compression, heating,

and exchange phaseExpansion 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 4He, 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 ≈ rt + rp)

• The projectile is the source of fragments and does not have a complete compression phase in the fragmentation process2 Kaufmann et al, Proc. 2nd Conf. Reactions Between Comples Nuclei

Projectile Fragmentation

.b

rp

rt

Projectile

Target

p

● When b ≈ rt + rp, 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 34P

– Example: GEMINI

34P

32P

12B 17O

13C 19F

n

n

n

pp

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 3He + natAg reaction

•<E4He

> < <E3He

>

•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 FragmentationModels 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

– <E> ≈ 3/2 kT + 2 AmN<v2> + <Ecoul

> – 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

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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 11C, 12C fragments from the 112Sn+112Sn reaction

• In this model, 11C is emitted preferentially prior to 12C (figure 1) and both isotopes tend to have a greater mean energy when emitted earlier (figure 2), leading to 11C having a greater mean energy than 12C

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 11C, 12C 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 EESEnergy spectra for 11C, 12C from data and EES

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What reactions are we studying?

Peripheral reactions resulting from isobaric projectiles

• Isobaric beams (same A), 20F, 20Ne, 20Na, on 197Au– 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=Zbeam,

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, 20Ne beam

Lab angles

Yie

ld /

Ste

radi

an (

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its)

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Experimental: MARS(Momentum Achromat Recoil Spectrometer)

• Radioactive (secondary) beams 20F and 20Na produced in the reactions 19F + d → 20F + p and 20Ne + p → 20Na + 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: 19F or 20Ne

Primary target Secondary beam: 20F, 20Ne, or 20Na

Secondary target (197Au) and FAUST

Q1Q3

Q2D1

D2Q4, Q5

Velocity filter

MARS and FAUST pictures from Dr Doug Rowland

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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 Rowland11

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

C

BeB

EZ 2 N Z

E

12

0 20 40 60 80 100 120 140 160 180 200

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Mean energies for 14N spectra

20F

20Ne

20Na

center of mass angle

<E

> (

MeV

)

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<θqp<126, and forward/backward angles, 0<θqp<36 and 126<θqp< 180

Normalized energy spectra for 12C from 20Ne beam at 10 cm angles

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Energy (MeV)0 20 40 60 80 100 120 140 160 180 200

0

1

2

3

4

5

6Mean energies for 12C spectra

20F

20Ne

20Na

center of mass angle

<E

> (

MeV

)

}}mid

angles

forward/backwardangles

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Procedure x 211C12C13C14C

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Energy (MeV/A)

Normalized Energy Spectra for C Isotopes with 20F beam

13N14N15N16N

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Energy (MeV/A)

Normalized Energy Spectra for N Isotopes with 20F beam

11C12C13C14C

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Energy (MeV/A)

Normalized Energy Spectra for C Isotopes with 20Ne beam

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Energy (MeV/A)

Normalized Energy Spectra for N Isotopes with 20Ne beam

11C12C13C14C

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Energy (MeV/A)

Normalized Energy Spectra for C Isotopes with 20Na beam

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Energy (MeV/A)

Normalized Energy Spectra for N Isotopes with 20Na beam

• The difference between the mid and forward/backward angles for all three beam can be seen

• However, for 20Na beam, the spectra will be for all angles since statistics are low

Low Statistics and Sad Times for Sodium

mid angles mid anglesmid angles

forward/backward angles forward/backward

angles

forward/backward angles

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13N14N15N16N

13N14N15N16N

11C12C13C

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Energy (MeV/A)

Normalized Energy Spectra for C Isotopes with 20Na beam

11C12C13C14C

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Energy (MeV/A)

Results

Normalized Energy Spectra for C Isotopes with 20F beam

11C12C13C14C

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Energy (MeV/A)

Normalized Energy Spectra for C Isotopes with 20Ne beam

11C: <E> = 3.92 12C: <E> = 3.2913C: <E> = 2.8614C: <E> = 2.31

11C: <E> = 4.3812C: <E> = 3.4713C: <E> = 2.5414C: <E> = 2.02

11C: <E> = 4.28 12C: <E> = 3.4713C: <E> = 3.1914C: <E> = 2.90

• Spectra for middle cm angles for F, Ne and all cm angles for Na

• 14C data not shown where statistics are low

• For all three beams, heavier isotopes correspond to smaller mean kinetic energies

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13N14N15N

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Energy (MeV/A)

Normalized Energy Spectra for N Isotopes with 20Na beam

13N14N15N16N

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Energy (MeV/A)

Results

Normalized Energy Spectra for N Isotopes with 20F beam

13N14N15N

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its)

Energy (MeV/A)

Normalized Energy Spectra for N Isotopes with 20Ne beam

• Spectra for middle cm angles for F, Ne and all cm angles for Na

• 16N 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

13N: <E> = 3.1414N: <E> = 2.0815N: <E> = 1.6316N: <E> = 1.40

13N: <E> = 3.06 14N: <E> = 1.5415N: <E> = 1.2816N: <E> = 1.45

13N: <E> = 3.21 14N: <E> = 2.6815N: <E> = 1.9316N: <E> = 1.90

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Y(14

C)/

Y(to

t C)

Energy (MeV/A)

Relative Yield for 14C Isotopes

Y(11

C)/

Y(to

t C)

Energy (MeV/A)

Relative Yield for 11C Isotopes

Y(12

C)/

Y(to

t C)

Energy (MeV/A)

Relative Yield for 12C Isotopes

Y(13

C)/

Y(to

t C)

Energy (MeV/A)

Relative Yield for 13C Isotopes

More Results

• Relative Yields for middle angles (even for 20Na)

• Plots show the yield of carbon isotopes divided by the yield of all carbon isotopes as a function of energy

• The relative yield of 12C and 14C is significantly different for the Fluorine than for the other beams

20F beam20Ne beam

20Na beam

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Y(16

N)/

Y(to

t N)

Energy (MeV/A)

Relative Yield for 16N Isotopes

Y(13

N)/

Y(to

t N)

Energy (MeV/A)

Relative Yield for 13N Isotopes

Y(14

N)/

Y(to

t N)

Energy (MeV/A)

Relative Yield for 14N Isotopes

Y(15

N)/

Y(to

t N)

Energy (MeV/A)

Relative Yield for 15N Isotopes

More Results

20F beam20Ne beam

20Na beam

• Relative Yields for middle angles (even for 20Na)

• The behaviour of the relative yield for 15N is significantly different for the Fluorine beam than the other beams

• Is this behaviour reproducable with theoretical multi-fragmentation codes such as DIT/SMM?

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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 20F 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

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Thanks to:

Cyclotron Institute at Texas A&M University

Sherry Yennello and the SJY Group

Department of Energy

National Science Foundation

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