Seminars in 2014:

Roshan Sellahewa (University of Surrey)

The Gogny force: Isovector properties and superfluidity in neutron stars

Thursday 27th November 2014

J. N. Wilson (Institut de Physique Nucléaire d’Orsay, France)

Spectroscopy of the neutron-rich fission fragments produced in the 238U(n,f) reaction

Of the potential ~7000 atomic nuclei that can possibly exist, fewer than 3000 have been fabricated in

the laboratory. Currently, nothing is known about the rest of these nuclei and their structure remains

a deep mystery. The experimental study of exotic nuclei is particularly challenging since we are at

the mercy of the limited production mechanisms available. 

Presented in this talk is a new technique for the production and study of exotic nuclei via the cold

fission of 238U by fast neutrons with an average energy of 1.5 MeV. It relies on the recently-developed

inverse kinematics neutron source, LICORNE, which exploits the intense 7Li beams available at the

Tandem accelerator of the IPN Orsay to produce directional beams of neutrons using the p(7Li,n)

inverse reaction. These neutrons are used to fission a sample of 238U placed in the centre of the

MINIBALL gamma ray spectrometer which detects the prompt and delayed coincident gamma rays

emitted from the excited, neutron-rich fission fragments.

 Since the LICORNE neutron beams can be pulsed at 2.5 MHz, the exploitation of time correlations

can be used to provide a very high selectivity for certain nuclei and their neutron rich fragment partners

by detecting isomeric states with half-lives from a few ns to a few µs. If the isomer selected is known

and close to stability, its fragment partners will be guaranteed to be exotic from the conservation of

mass and charge. The 239U* compound nucleus produced with this method is the most neutron-rich

achievable using a massive target and since the neutron energy is just above the fission threshold,

the resulting exotic, excited fission fragments will not evaporate excessive numbers of neutrons.

Since several hundred neutron-rich isotopes are populated, several physics cases are simultaneously

addressed, such as the evolution of the Z=50 shell gap for Sn isotopes and neighbouring elements,

nuclear shape co-existence, the population of nuclei in the r-process path, particularly near N=82,

etc. First results and future plans will be presented.

Tuesday 25th November 2014

Recent progress in radioactive-ion beam experiments at Argonne National Laboratory

The Argonne Tandem Linac Accelerator System (ATLAS) user facility at Argonne National

Laboratory, USA has been at the forefront of nuclear physics research since its first

accelerated beams were delivered in the 1970’s. The CAlifornium Rare Isotope Breeder

Upgrade (CARIBU) provides a unique opportunity for expanding research in nuclear structure,

astrophysics and applications with neutron-rich exotic beams. CARIBU utilizes the spontaneous

fission of 252Cf in generating such beams. Fission fragments are thermalized in a gas catcher,

whereby a combination of high-purity He gas flow and DC/RF electric fields act to form a

low-emittance beam, before purification in an isobar separator.

Major steps have been taken in developing experiments that utilise low-energy CARIBU

beams. A new decay-spectroscopy station optimized for performing β-γ-t and β-γ-γ coincidence

measurements has recently been commissioned for use with low-energy CARIBU beams.

The decay station consists of the “X-Array”, a highly efficient array of five HPGe clover detectors

for detection of γ rays, and the “SATURN” system of plastic scintillators and moving tape

collector, which offers significant removal of long-lived radiation that would otherwise

contaminate the data. The modularity of the X-Array allows for the addition of dedicated neutron

detectors to be utilized in β-delayed neutron measurements. This is in addition to the dedicated

Beta-decay Paul Trap (BPT) set-up, which is undergoing significant upgrades. A Total Absorption

Gamma-ray Spectroscopy (TAGS) detector is also being re-commissioned to perform such

measurements in 2015. Major upgrades to ATLAS have been completed, with measures

implemented to improve both efficiency and beam intensity. In addition to infrastructure upgrades,

a new 1.7-Ci 252Cf has been acquired which will greatly improve CARIBU beam intensities,

particularly with re-accelerated beams. Re-accelerated radioactive beams made available via

the in-flight production technique are also being expanded with the Argonne In-flight Radioactive

Ion Separator (AIRIS), which is currently under development. 

Tuesday 18th November 2014

Paul Davies (University of York)

β-decay study of 16+ and 9+ spin-gap isomers in 96Cd and 98In

The self-conjugate nuclides 96Cd and 98In have identified isomeric states of spin-parity

16+ [1] and 9+[2], respectively. A recent investigation of the 16+ spin-gap isomer in 96Cd

suggests it exists as a consequence of the T=0 n-p interaction [1], the 9+ isomer in 98In is

expected to arise from the same interaction. The states populated by the β-decay of these

isomers, along with the B(GT) strengths and beta-delayed proton branching ratios, will

provide a sensitive probe of shell model (SM) calculations in the 100Sn region. 

Large-scale SM (LSSM) calculations [1], performed for 96Ag in the sdg model space predict

the existence of ‘resonance-like’ states of spin-parity 15+, 16+ and 17+, which sit above the

proton separation energy. These calculations indicate that 30% of the B(GT) strength from the

decay of the 16+ isomer in 96Cd should populate these ‘resonance-like’ states in 96Ag, which

are predicted to proton decay to excited states in 95Pd. This is in contrast to the shell model

calculations performed in the more restricted pg model space, with the Gross-Frenkel

interaction, which predict that the 16+ isomer in 96Cd will only decay to the 15+ isomer in

96Ag with a B(GT) value of 0.14 . 

Previous studies of 98In [2] measured both a fast (32 ms) and a slow (1.02 s) component to

the β-decay. It has been suggested the decay of the 0+ T=1 ground state should be fast [3],

hence, the slow component may correspond to the Gamow Teller (GT) decay of the 9+ isomer.

SM calculations [2,4] predict the decay of the isomer will populate the 8+, 9+, and 10+ states in

98Cd, of which the 9+ has yet to be experimentally observed. Furthermore, calculations suggest

that the isomer may have a β-delayed proton decay branch. 

This presentation will report on results from a recent experiment performed at RIKEN using RIBF

and SIMBA. The first evidence of a beta delayed proton decay from the 16+ isomer in 96Cd will

be presented. The beta delayed proton branching ratio has been measured, along with upper

and lower limits for the B(GT) strength of the decay from the 16+ isomer to the 15+ isomer in

96Ag and the ‘resonance-like’ states, respectively. These results will be compared to predictions

of the latest SM calculations.  The β delayed g-ray spectroscopy of the 9+ isomer in 98In will also

be discussed and the measured B(GT) strength will be presented. The latest status of the data

analysis will be presented, including the half life of the N<Z nuclide 95Cd.  

[1] B. S. Nara Singh, et al. Phys Rev Lett, 107(17), 172502, (2011)

[2] P. Kienle et al., Prog. Part. Nucl. Phys. 46, 73 (2001)


[3] H. Herndl and B.A. Brown, Nuclear Physics A 627, 35-52 (1997)

[4] T. Faestermann et al. Eur. Phys. J. A 15, 185 (2002)

Ab initio many-body methods have been developed over the past ten years to address

closed-shell nuclei up to mass A~130 on the basis of realistic two- and three-nucleon

interactions. A current frontier relates to the extension of those many-body methods

to the description of open-shell nuclei. Several routes are currently under investigation

to do so among which one relies on the powerful concept of spontaneous symmetry

breaking. Singly open-shell nuclei can be efficiently described via the breaking of U(1)

gauge symmetry associated with particle-number conservation, as a way to account

for their superfluid character. Doubly open-shell nuclei can be addressed by further 

breaking SU(2) symmetry associated with angular momentum conservation. Still, the

description of finite quantum systems eventually requires the exact restoration of

symmetry quantum numbers.
 
In this context, we will first discuss two on-going research developments based on the

breaking of U(1) gauge symmetry, i.e. the Gorkov implementation of self-consistent

Green's function theory and the Bogoliubov extension of single-reference coupled

cluster theory. First realistic applications of these two innovative many-body theories

to the computation of semi-magic nuclei will be presented. Second, we will present

the recent extension of coupled-cluster theory that allows for the exact restoration of

the broken symmetry, i.e.  SU(2) and/or U(1), at any truncation order. The formalism, 

which encompasses both single-reference coupled cluster theory and projected

Hartree-Fock-Bogoliubov theory as particular cases, permits the computation of usual

sets of connected diagrams while consistently incorporating static correlations through

the highly non-perturbative restoration of the symmetry.

Tuesday 17th June 2014

Mike Bentley (Department of Physics, University of York)
Breaking of Isospin Symmetry: Knockout Measurements and Shell-Model Analysis
The approximate charge symmetry and charge independence of the nuclear force results

in striking symmetries in nuclear behaviour between isobaric analogue states (IAS). The

very small differences in excitation energy between the IAS can be interpreted in terms

of Coulomb, and other isospin-non-conserving, effects. The analysis of these energy

differences has been shown to be a remarkably sensitive probe of nuclear structure

effects as well as providing stringent tests of state-of-the-art shell-model calculations.

New experimental techniques have been applied in the last few years to access excited

states in isobaric multiplets of larger isospin through spectroscopic studies of proton-rich

nuclei heading towards the proton drip-line. One of the most effective recent techniques

is to use "mirrored" direct reactions to populate analogue states in "analogue" reactions. 

In this work, performed at the NSCL facility, we have populated isobaric analogue states

in isospin multiplets through “mirrored” knockout reactions. This has allowed examination

of excited states in the heaviest studied Tz = -3/2 and -2 nuclei, allowing studies of mirror

pairs of high isospin (T) in the fp-shell, and the examination of isospin-breaking effects.

State-of-the-art predictions of the knockout mechanism, led by the Surrey group, have also

added significantly to our ability to interpret the results.
In this seminar, some basic concepts of isospin symmetry, and the mechanisms that break

it, will be presented. Following presentation of the newest data, an attempt is made to bring

together what we have learned to try to draw conclusions on the influence of isospin-non

conserving interactions in this region.

Tuesday 20th May 2014

rules of electromagnetic and neutral-current response functions. The longitudinal elastic

form factor and the electromagnetic sum rules are found to be in satisfactory agreement

with available experimental data. The transverse electromagnetic and neutral current

sum rules receive large contributions from the two-body currents. In the Electromagnetic

case they are needed for a better agreement with experimental data; this fact may have

implications for the  anomaly observed in recent neutrino quasi-elastic charge-changing

scattering data off 12-Carbon. The role played by nuclear correlations is discussed in

both 12-Carbon and neutron matter case. In particular, I will show how the neutrino mean

free path in cold neutron matter  turns out to be strongly affected by both short and long

range correlations, leading to a sizable increase with respect to the prediction of the

Fermi gas model.

Tuesday 13th May 2014

Study of the 44Ti(a,p)47V reaction - Using radioactive waste to explore core

collapse supernovae

The underlying physics triggering core collapse supernovae is not fully understood but

observations of material ejected during such events helps to solve this puzzle. In particular,

several satellite based gamma-ray observations of the isotope 44Ti have been reported

in recent times, giving rise to discrepancy with model prediction, but also, and very

recently, highlighting the asymmetry of the explosion.

The amount of this isotope in stellar ejecta is thought to depend critically on the explosion

mechanism. The most influential reaction to the amount of 44Ti in supernovae is 44Ti(a, p)47V.

A direct study of this reaction was conducted at REX-ISOLDE, CERN in December 2012

and the findings, recently published, will be presented.

The experiment was performed with a 44Ti beam at Elab=2.16 MeV/u, corresponding

to an energy distribution, for reacting -particles, centred on Ecm=4.15 MeV with a 1 width

of 0.23 MeV. This is, for the first time, well within the Gamow window for core collapse

supernovae. The material from which the 44Ti beam was extracted originates from highly

irradiated components of the SINQ spallation neutron source of the Paul Scherrer Institute.

Such a use of accelerator waste represents a novel technique. No yield above background

was observed, enabling an upper limit for the rate of this reaction to be determined. This

result is below expectation, suggesting that the 44Ti(a, p)47V reaction proceeds more

slowly than previously thought and implications for core collapse supernovae will be

discussed.

Tuesday 6th May 2014

Breakup of C, Be and O isotopes from a proton target

One neutron and one proton knockout from C, Be and O isotopes due to the collisions

with the proton target will be discussed. Reaction dynamics and pair interaction effects

will be addressed. Particular attention will be paid to the description of the reaction

formalism and to the accuracy of the structure information extracted from comparing

of the calculated observables with the experimental data.

Monday 28th April 2014

Where are the neutrons?

Heavy nuclei have are more neutrons than protons due to the Coulomb repulsion between

protons. The proton distribution in the nucleus is well known from precise electron

scattering measurements. The location of the neutrons is much less well known. Knowledge

of the neutron distribution nuclei is important for precise calculations of atomic parity violation

and for the determination of the neutron equation of state used to calculate the mass and

radii of neutron stars. I will describe recent theoretical work that determines the position of

the neutrons based on properties of doubly-magic nuclei and ab-initio calculations of

Monday 14th April 2014

Experimental yields of in-flight fission products from Ni to Pd measured following

U-238 fragmentation at NSCL

In a recent experiment conducted at the National Superconducting Cyclotron Laboratory

(NSCL) at Michigan State University a cocktail beam of radioactive nuclei was produced

in the projectile fragmentation and in-flight fission of 80 MeV/A U-238 ions impinging

upon a 33.5 mg/cm2–thick diamond target.

The target was positioned at the pivot point of the S800 magnetic spectrograph and

within the GRETINA gamma-ray tracking array. Reaction products were identified on

an event-by-event basis (Z, A) by the S800 spectrograph and correlated with gamma-

rays detected by GRETINA.

A study by Suzuki et al. [1] using a higher U-238 beam energy (345 MeV/A) shows a

good agreement between measured and expected production yields calculated using

Abrasion- and Coulomb-fission models [2] for neutron-rich fission fragments with

20 ≤ Z ≤ 50.

In the current work over 100 fission fragments with 28 ≤ Z ≤ 46 have been identified.

Production yields shall serve as input into models of abrasion-fission at intermediate

energies and may be used to plan future experiments. In-flight fission remains a

valuable tool for nuclear spectroscopy in the medium-mass region.

Monday 7th April - Wednesday 9th April 2014

Tuesday 25th March 2014

Natalia Timofeyuk (University of Surrey)

New developments in understanding the theory of (d,p) reactions

The progress in development of radioactive beam facilities and the methods of

particle detections made it possible to measure cross sections of transfer reactions

A(d,p)B in inverse kinematics. Such reactions are at the frontier of the research into

evolution of the single-particle structure of nuclei with changing neutron to proton ratio.

The conclusions about this structure are often deduced from the comparison between

the measured (d,p) cross sections and the predictions from reaction theory. At present,

many experimental groups use distorted wave Born approximation (DWBA) developed

in 1960s. In this approximation many parts of exact scattering wave function are

neglected. For (d,p) reactions these neglected parts include the components in which

the wave function of d+target is represented by the p+n+A continuum. The simplest way

to deal with it is to use adiabatic approximation developed by Johnson and Soper in

early 1970s. Such an approximation allows the same DWBA codes to be used. What

has not been realised until very recently is that the scheme employed by the adiabatic

approach has to be modified because of non-locality of the n-A and p-A interactions

in the p+n+A components. In this talk I will present the breakthrough in the development

of the theory of (d,p) reactions made at Surrey over the last two years that accounts for

fundamental property of quantum systems, their non-locality. I will show that non-locality

can change the interpretation of the data in terms of both absolute and relative

spectroscopic factors. A very useful feature of new method is that it can still employ the

old DWBA codes that nuclear experimentalists have got used to.

Tuesday 18th March 2014

Adam Godbear (University of Surrey)

Quantum measurement and thermally assisted proton tunnelling

Despite compelling evidence to the contrary in recent years, the view still persists

that quantum effects cannot survive for very long periods within warm, noisy and

complex environments, such as a living cell, and are washed out at timescales

far too short for any chemically or biochemically interesting processes. It is also

assumed that as the temperature of the surrounding environment increases, so

the efficiency of processes such as quantum tunnelling drops. One way of viewing

this has been to invoke the quantum Zeno effect: that the watched pot never boils.

This theoretical work shows that, over a range of temperatures, the opposite is true.

For a quite general open quantum system, a proton in an asymmetric double-well

potential, the action of the environment is to enhance the tunnelling rate (an anti-Zeno

effect). Two simple mathematical models are compared to show that, over a specific

temperature range, thermally enhanced quantum tunnelling is equivalent to increasing

the frequency of a von Neumann-type measurement by the environment on the system.

Tuesday 11th March 2014

Nigel Orr (LPC-Caen, France)

Continuum states of light neutron-rich nuclei via transfer and knockout:

The light nuclei have long provided a testing ground for our understanding of many

aspects of nuclear structure and the development of models. In recent years it has

become possible experimentally to access light nuclei lying beyond the neutron

dripline using radioactive beams. In parallel theoretical developments have been

made, ranging from ab initio approaches to the shell model in the continuum, which

are beginning to reach the level of sophistication necessary to provide realistic

descriptions of such systems.

In this seminar I would like to discuss two different reaction probes which enable

the structure of unbound nuclei to be probed using energetic radioactive beams –

nucleon transfer and high-energy breakup or "knockout". To do so I will use the

example of the most neutron-rich N=7 isotones 9He and 10Li. I will attempt to

compare and contrast the two methods and discuss the rather rudimentary nature

of the reaction modelling used to interpret the experiments. A discussion of the

results and the inferences that may be drawn regarding the low-lying level structure

of 9He will also be presented.

If time permits I will also touch on some very recent results concerning the structure

of 13Be and preliminary results for the most neutron-rich unbound B isotopes from

our RIKEN based programme.

Tuesday 4th March 2014

Ed Simpson (University of York)

The no-recoil approximation in two-nucleon removal reactions

Calculations for two-nucleon knockout typically make the no-recoil approximation,

assuming the core of the projectile is sufficiently massive to be coincident with the

projectile centre-of-mass. Core recoil in single-nucleon diffraction is known to reduce

the cross section by ~20-30%, so quantifying these effects is crucial for precision

studies of two-nucleon removal in light systems and future investigations of the reaction

mechanism. Here we will discuss the impact of core-recoil on the diffraction-stripping

component of two-nucleon knockout. We find the effect of core recoil to be less significant

than in single-nucleon diffraction, due to enhanced core-diffraction-type events. There

is a weak state dependence arising from the different spatial correlations that result

from different pair total angular momenta J. When compared to the no-recoil limit, larger

J give larger diffraction stripping cross sections, though the absolute change is case

specific. These results give improved agreement with data on the relative strengths

of elastic and inelastic removal mechanisms for 28Mg(−2p), albeit within the large

experimental uncertainties.

Tuesday 18th February 2014

Exploring the ground state properties of very neutron-rich isotopes

The last decades have seen a steady progress in the range of isotopes that can

be produced at radioactive ion beam laboratories worldwide. This has been

accompanied by developments of novel and improved experimental techniques

that push the reach of experiments to more exotic nuclei. I will discuss two sensitive

techniques that, applied at fast fragment beam facilities, can provide some of the

first details about the evolution of nuclear structure away from beta-stability: time-

of-flight mass measurements, and studies of matter distribution through reaction

cross-section measurements. In particular, I will show results from the program of

time-of-flight mass measurements at the NSCL and discuss the status of time-of

-flight programs at other laboratories, indicating their relevance to nuclear

astrophysics applications. In the case of reaction cross section experiments,

I will present recent results from a measurement of charge changing cross

sections at GSI, and its application to the study of the proton distribution of

light halo nuclei.

Tuesday 4th February 2014

pattern of QCD is exploited to constrain the dynamics of nucleons at low energy scales. 

I will review recent work aimed at improving our understanding of neutron-rich systems 

-- from exotic isotopes to neutron stars -- with the application of chiral effective field 

theory techniques. Current challenges include the description of medium-mass nuclei 

and the implementation of consistent next-to-next-to-next-to-leading order (N3LO) 

nuclear many-body forces. I will describe ab initio many-body methods currently under 

development that may help to address these challenges and also allow for direct 

connections to lattice QCD simulations.

Tuesday 14th January 2014

The EURICA project at RIKEN after one year of operation

The Radioactive Isotope Beam Factory (RIBF) is a state-of-the-art radioactive ion

(Euroball-RIKEN Cluster Array), consisting of twelve high-purity germanium cluster

detectors from the Euroball IV array, is one of the highest-efficiency gamma-ray

detectors existing, and coupled to the world's most intense in-flight radioisotope

beams it creates a unique opportunity for the worldwide nuclear physics community.

In this seminar we will discuss the measurements in the first year which includes,

but are not limited to: the beta-decay properties of key r-process nuclei like 78Ni

and 128Pd, a study of the possible N = 64 subshell closure around 110Zr, search

for long lived isomers in a large part of the nuclear chart, shape evolution in very

neutron-rich nuclei, the structure of 100Sn and N<Z in the region, as well as

proton-separation energy measurements relevant for the rp-process.