UWS NUCLEAR PHYSICS RESEARCH GROUP

ABOUT OUR RESEARCH

The UWS Nuclear Physics Research Group has been established for almost 30 years. Our research has traditionally been focussed on the study of the structure and properties of exotic atomic nuclei using gamma-ray and charged-particle spectroscopy. Our research is typically conducted at overseas nuclear-physics laboratories such as JYFL in Finland, Argonne National Laboratory in the USA, ISOLDE at CERN, and TRIUMF in Vancouver. We are leading programmes in a number of different areas of nuclear physics at these laboratories. We are members of SUPA, the Scottish Universities Physics Alliance, within which we are leading nuclear physics experiments at the new SCAPA facility – the Scottish Centre for the Application of Plasma-based accelerators. Our research has traditionally been fundamental in nature, studying the structure and properties of atomic nuclei, but we are developing themes of research in areas of applied nuclear physics, with the intention of developing impact to society, industry and the economy. Our research has been continuously funded by research councils (EPSRC and STFC) for almost 30 years. We are presently funded by an STFC Consolidated Grant and an STFC Project Grant (AGATA).

The nuclear dipole response

The nuclear dipole response has an electric and a magnetic part. Both contributors exhibit a variety of features with a multitude of implications, such as the violation of fundamental symmetries, extreme astrophysical objects such as neutron stars, the synthesis of heavy elements, or the dynamics of supernova type-II explosions.

The electric dipole response (E1R) can be classified in low-lying quadrupole-octupole coupled excitations, which are particular interesting if the nucleus has quadrupole but also (so far established in less than a handful of cases) octupole correlations in its ground state. The E1R is dominated by the Giant Dipole Resonance (GDR) at comparably high excitation energy beyond the particle thresholds. On the GDR’s low-energy tail near the proton and neutron evaporation threshold a less strong additional enhancement of E1 strength is found. This, so called Pygmy Dipole Resonance (PDR) is often visualized as an out-of-phase motion of a neutron skin formed by excess neutrons and an isospin saturated core composed of an almost equal number of protons and neutrons. This mode provides an additional wide-open doorway in particle-capture reactions in astrophysical scenarios as the slow and rapid neutron capture processes. Furthermore, the skin picture of ‘pure’ neutron matter suggests connections via the symmetry energy, which enters the nuclear Equation-of-State, to neutron stars.

The magnetic dipole response splits into the scissors resonance near 3 MeV and the Gamow-Teller Resonances near the particle threshold. The latter has strong implications for beta decay probabilities and even more so neutrino-nucleus scattering cross sections. The latter are deemed to be the mechanism driving the explosion of type-II Supernova.

Our group has become interested to benchmark these excitations in nuclei with equal proton and neutron number. For example, in the case of the PDR, this allows to disentangle shell effects (in our nuclei) from neutron-skin+shell effects in more neutron rich nuclei. We have an ongoing experimental program for the lightest stable Ni isotopes 58 and 60 and aim to investigate 24-Mg and 28-Si. As experimental tools we use the scattering of real photons, bremsstrahlung for the excitation strength and fully-polarized laser-Compton backscattered photons to assign the E1 or M1 character. Furthermore, we employ inelastic alpha-particle scattering experiments to assign an isospin character to the observed E1 excited levels. Recently, our group has proposed to use high-Q value beta decay of mothers with a low ground state spin to gain an alternative insight in the levels of the PDR. In a first campaign we were able demonstrate that the PDR plays even a role in reactor physics.

Measurements of lifetimes of nuclear states

many-body system made of tens or hundreds of protons and neutrons, is an excellent laboratory with which to test quantum physics. A consistent description of atomic nuclei, from first principles, across the entire periodic table is still unavailable. There exists, however, a large number of successful models which can be employed to describe specific behaviour in nuclei such as collective or non-collective motion. Experimental nuclear physics looks to measure certain key observables with which to test these models with the aim of improving the models which will ultimately lead to a more consistent description of all observed behaviour.

One such key observable is the excitation energy of the quantised excited states of the nucleus. These are typically obtained from measurements of gamma radiation emitted as the excited nucleus decays towards the ground state. Another, perhaps more stringent, test of nuclear models is obtained by measuring the quantum mechanical transition matrix elements which reveal the overlap in the wavefunctions of the nuclear states. The absolute transition matrix elements are obtained through a measurement of the mean lifetime of the nuclear state. A number of techniques have been developed to measure the mean lifetime of a nuclear level and which technique is employed depends on the timescale over which the nuclear level is expected to decay. The timescales typically range from femtoseconds (10^-15 s) to milliseconds (10^-3 s).

Members of the nuclear physics group at the University of the West of Scotland (UWS) have been leading efforts to measure nuclear lifetimes using a number of different techniques. Gamma radiation emitted from fast-moving atomic nuclei produced in nuclear reactions is subject to the Doppler effect which can be utilised to measure the lifetimes of some of the shortest-lived nuclear states. These experiments are conducted at large international laboratories around the world.

For many nuclear states, it is not possible to measure the mean lifetime using Doppler-based techniques and more direct, electronic techniques are used. These techniques utilise radiation detectors, which have very fast electronic response to the incident radiation, and state-of-the-art electronics to directly measure the time which elapses between the creation and decay of an excited state. Such experiments are conducted both in the detector development laboratory at UWS and at international laboratories across the world.

AGATA – the Advanced Gamma Tracking Array

The UWS group is a member of the AGATA collaboration. AGATA is the Advanced Gamma Tracking Array and is the world’s most powerful and sensitive gamma-ray spectrometers for use in research of the study of nuclear structure and nuclear research. The AGATA collaboration consists of over 40 different institutions in around 12 different countries. The construction of AGATA is a large-scale scientific and engineering endeavour which is being realized in phases over a number of years. When complete, AGATA will consist of 180 high-volume high-purity germanium (HPGe) detectors. Unlike previous gamma-ray spectrometers, AGATA will forego the use of Compton-suppression shields in favour of the use of gamma-ray tracking. Instead of vetoing (throwing away) any gamma rays that are scattered out of an HPGe detector and therefore partially detected, any scattered gamma rays in AGATA are scattered into another HPGe detector, and the gamma ray is tracked and reconstructed. All of the 180 HPGe detectors of AGATA are packed closely together to cover as much of the complete solid angle as possible. The science reach of AGATA is wide-ranging covering nuclear structure, nuclear reactions, and nuclear astrophysics. To further broaden the scope of the science that can be achieved, AGATA is operated in campaigns at different laboratories in Europe. The initial demonstration phase of AGATA too place at the Legnaro National Laboratory in Italy. It has since been operated in campaigns at GSI (Germany) and at GANIL (France) where it is currently operational. In late 2021, AGATA will be moved back to the Legnaro National Laboratory in Italy, where it will be operated at first with stable beams from the Tandem-ALPI accelerators, before being used for experiments with the new SPES radioactive-beam facility. The construction of AGATA is about 50% complete. A UK collaboration consisting of the University of Liverpool, the University of York, Daresbury Laboratory, and UWS was recently awarded an STFC Project Grant to fund the next UK contribution of the AGATA array. The UWS group was one of the most active groups in the previous AGATA campaign at Legnaro, leading two experiments, and we anticipate leading a number of experiments with AGATA in the next Legnaro campaign beginning in 2022.

_{The AGATA Demonstrato at Legnaro National Laboratory, front and side view.}

Nuclear astrophysics

This is a new area of research at UWS led by Dr Nara Singh Bondili. The reactions of interest occur at very high temperatures in astrophysical environments and govern the chemical evolution of the universe. Currently, our research is focused on reactions involving light nuclei, and we will extend this to medium-mass nuclei.

One of our interests is based on experiments relevant to one of the unsolved open problems - the ⁷Li abundance puzzle which describes a disagreement between predictions and observations by a factor of three. Solving the lithium puzzle is a high priority task in nuclear astrophysics that requires accurate knowledge of the ³He(a,g)⁷Be reaction rate. We are planning a programme of experiments on this reaction and similar reactions such as ³H(a,g)⁷Li. Our experiments will be conducted at small-scale facilities such as tandem accelerator in Madrid, Spain through the detection of g rays and also at larger-scale facilities such as the DRAGON facility at TRIUMF (Vancouver) through the counting of ⁷Be ions.

Our research will also be relevant to neutrinos produced in the sun and has connections to the standard model. Hydrogen is turned into helium in the sun at temperatures ~6000K. Several other fusion reactions follow, producing energy that is required to maintain life on earth. The reactions also produce neutrinos that reach earth, which are detected to study physics beyond the standard model of particle physics. The ³He(a,g)⁷Be reaction rate is required to calculate the number of neutrinos that can be detected on earth using the standard solar model and compare it with the observations.

Experiments to measure the cross sections for astrophysical reactions in the hot-pp chain involving light nuclei, especially, carbon nuclei are currently being planned through a high-energy beta detection method that is under development.

Image 1: A chamber used for the fusion reaction in Madrid . A ⁴He beam passing through ³He gas target produces visible light.
Image 2: The DRAGON separator that is used to identify and count ⁷Be after the fusion reaction at TRIUMF.

Detector development

Throughout the history of nuclear physics, the techniques and apparatus employed to enhance the understanding of the atomic nucleus have found application in diverse fields ranging from medical imaging and border security to the provenance of antiquities. Members of the nuclear physics group at the University of the West of Scotland have been working successfully with external funding agencies to realise the application of some of the detector technology which has been used as part of their fundamental nuclear physics research programmes. In recent years, the Royal Society and Medical Research Scotland have both funded the development of detector technology which could be employed in future medical devices. The Department of Transport has recently funded the construction of a prototype Compton camera system which could be employed at transportation hubs to monitor the movement of radioactive materials.