Laser Spectroscopy

Atomic physics and optical techniques have played a crucial role to study the behaviour of nuclear matter at low excitation energy. Using these techniques, nuclear properties such as the spin, magnetic dipole moment and electric quadrupole moment, can be extracted through the observation of the hyperfine structure in optical spectra. The changes in nuclear charge radii can also be accessed through the measurement of the isotope shifts  along an isotopic chain. For 40 years, lasers were used at radioactive beam facilities to measure these quantities in a nuclear-model-independent fashion, guiding our understanding of nuclear structure and providing stringent tests for nuclear models.

S3 Low Energy Branch

The S3-LEB (Low Energy Branch) is installed at the final focal plane of the S3 (Super Separator Spectrometer), designed to support a variety of low-energy nuclear measurement techniques, including laser spectroscopy, decay spectroscopy, and mass spectrometry. The underpinning working principle of S3-LEB is the in-gas-jet laser ionization and spectrosopy technique technique, which enables medium resolution and highly sensitive measurements using a narrow-bandwidth, high-power, pulsed laser system. This setup also ensures element-selective and efficient ionization of atoms of interest for other experiments downstream. S3-LEB will be used  to study nuclei along the N=Z line as well as in the heavy and super heavy mass regions. Additionally, it can produce pure radioactive ion beams at low energy to be delivered to the DESIR facility.

GISELE

In order to conduct experiments on radioactive ions, knowledge of laser ionization schemes for each element is required. When performing laser spectroscopy, added information about the sensitivity of the ionization scheme to the underlying nuclear observables is also needed. This requires developing and testing ionization schemes on stable isotope(s) of the element of interest, which can be performed at the GISELE laser laboratory in GANIL. GISELE has the same laser system as will be available at S3, and in addition an Atomic Beam Unit (ABU), in which material can be evaporated for subsequent laser ionization tests. The purpose of the GISELE laboratory is to perform offline laser ionization and spectroscopy experiments with the elements of interest for the S3 -LEB facility.

LUMIERE

DESIR is a new experimental hall in GANIL dedicated to low-energy nuclear physics experiments, that will be equipped with several beam preparation devices providing purified radioactive ion beams from both S3 and SPIRAL1 to various experimental setups. Experiments utilizing laser spectroscopy techniques in DESIR are grouped together under the name LUMIERE. LASAGN is the collinear laser spectroscopy setup of LUMIERE, dedicated to laser spectroscopy experiments for nuclear structure studies. In its first phase, LASAGN will consist in the integration of the LINO beam line (constructed and commissioned at ALTO) in DESIR to perform high-resolution collinear laser spectroscopy measurements with fluorescence detection. In its second phase, LASAGN will be upgraded to enable collinear resonance ionization spectroscopy in order to improve the sensitivity of the setup while conserving its high-resolution capabilities. The laser-ion beam produced at LASAGN would also be delivered to other trap or decay setups to perform laser-assisted experiments or trap/decay assisted laser spectroscopy.

Trap-based mass spectrometers

Ion traps are key components of ISOL facilities, acting both as means to prepare the radioactive ion beams (cooling, purification, charge breeding) and as tools for precision mass spectrometry. They can also confine the beams for other types of experiments which require long observation or interaction time, such as in-trap decay or optical pumping. In-trap decay can also be used to produce secondary beams of isotopes difficult to extract by the ISOL technique. Finally, traps can be coupled to other experimental setups and provide either mass-tagged detection, or high resolution mass separation for trap-assisted experiments (such as decay spectroscopy). Mass measurements allow determining the trends of nuclear binding energies or the excitation energies of long-lived isomeric states, both sensitive to different nuclear structure phenomena such as shell closures, nuclear deformation and pairing. They are also input data to simulations of various nucleosynthesis processes (such as the r- and rp- processes).  High-precision mass measurements and in-trap experiments are also relevant in weak interaction and neutrino studies (see also MORA). 

The ISOL-France collaboration is currently developing ion-trap setups for the French radioactive ion beam facilities SPIRAL2-GANIL and ALTO.

The set-up Pièges de Penning pour les RAdionucléides à DESIR (PIPERADE) is a double Penning-trap designed as a high-throughput and high-resolution beam purification and mass measurement apparatus. Together with the High Resolution Separator (HRS) and the General Purpose Ion Buncher (GPIB), it will provide the beam purification, cooling and bunching capabilities for the DESIR facility, as well as contribute to the scientific program with high-precision mass measurements.

Another Penning-trap system being developed in France is called MLLTRAP. Currently installed at the ALTO facility, it will be commissioned there and perform mass measurements of neutron-rich isotopes produced by the ALTO photofission source. A future version of the setup will use a Penning trap with electrodes converted to silicon detectors. This version will allow performing in-trap decay spectroscopy and mass measurements within a single apparatus. It will be installed at DESIR to perform unique experiments with heavy and very heavy nuclei produced by the SPIRAL2 facility.

The setup called Piège à Ions Linéaire du GANIL pour la Résolution des Isobares et la mesure de Masse (PILGRIM) is a multi-reflection time-of-flight mass spectrometer (MR-TOF MS) coupled to the S³ Low Energy Branch at SPIRAL2-S³. It will be used to perform mass-tagged ion counting for gas-jet laser spectroscopy at S³-LEB, or provide mass-separated beams to different decay setups downstream. It will also perform TOF mass spectrometry on neutron-deficient isotopes produced by S³.

The ISOL-France collaboration is also involved in the scientific programs of different ion-trap experiments at international facilities, notably ISOLTRAP at ISOLDE, JYFLTRAP at JYFL-ACCLAB and SHIPTRAP at GSI-FAIR and TITAN at TRIUMF-ISAC.

Decay spectroscopy

Decay spectroscopy is an essential tool for probing the nuclear structure of exotic nuclei. The ISOL and ion-catcher techniques can generate low-energy beams of neutron- or proton-rich nuclei with considerable purity. The beams produced this way can be implanted in a thin band or foil (part of a movable tape or wheel system), allowing decay particles (β, conversion electron, proton, neutron, alpha, fission fragments) to escape and reach the detectors with a minimum of scattering and energy loss. These experimental set-ups, known as decay stations, are highly versatile and include a range of detectors arranged around the implantation point. They are often conceived as travelling detectors which can profit of radioactive beams from different facilities, but can also be quasi-fixed setups attached to a specific beamline.

The detector choice and configuration is always optimized for the type of radiation which is to be detected and the type of measurement to be performed. High-resolution decay spectroscopy uses combinations of High Purity Germanium detectors (HPGe) with tagging on the decay particle, such as the BEDO decay station at ALTO or the SEASON decay station at S³-LEB. Higher efficiency over a wide energy range can be achieved by arrays of scintillators, such as the PARIS array, while scintillator-based calorimeter setups can be used to perform total absorption spectroscopy (TAGS) and obtain pandemonium-free measurements. Plastic-scintillators arranged in different geometries can be used for neutron counting (TETRA ) or energy determination (MONSTER), while magnetic-lens setups like COeCO  can be used to perform dedicated conversion-electron spectroscopy.

Nowadays decay-spectroscopy stations are not only standalone setups, but are often coupled to ion-trap mass spectrometer or laser ion sources for trap-assisted decay spectroscopy or decay-based ion counting for laser spectroscopy (such as the SEASON decay station).

Tests of fundamental interactions and symmetries

Nuclear beta decay has played a major role in establishing the key features of the weak interaction which are now included in the Standard Model of particle physics. It allows us to push the boundaries of knowledge in the weak sector to this day. High precision measurements of nuclear beta decay observables bring stringent constraints on Physics Beyond the Standard Model, which are competitive and complementary to  direct searches pursued at ultra-high energy at the Large Hadron Collider (LHC).

These studies are possible thanks to the high statistics available on a broad range of radioactive nuclei produced as pure beams at ISOL-type facilities, such as SPIRAL1 (and SPIRAL2 in the future), ISOLDE and JYFL.

The ISOL-France community is involved in several projects aiming at testing the existence of exotic weak currents, of couplings with non-standard helicity, of a hypothetical 4th generation of quarks or searching for Time Reversal symmetry violation in beta decay.

Unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix

A long-standing effort over the past three decades has led to a continuous increase in precision on measured nuclear half-lives, branching ratios and Q-values (masses) of pure Fermi 0⁺⇾0⁺ super-allowed beta decays. More than 220 independent high precision measurements are now used to extract the value of the 1st element of the CKM quark mixing matrix, V_ud, at a precision level of 3×10^-4. They are also used to test the CKM matrix unitarity, or probe the existence of exotic scalar currents in the weak interaction. Efforts are ongoing to reduce both experimental and theoretical uncertainties for a set of key super-allowed Fermi decays, or for a set of mirror decays which provide an independent test of the CKM matrix unitarity. New independent measurements are regularly performed to improve on previous results.

ASGARD (Assembly of Super-Conducting Arrays for Radiation Detection)

The ASGARD experiment aims at using quantum sensors, with novel aluminium based superconducting tunnel junctions (STJs), to perform high precision spectroscopy measurements of the nuclear recoil from beta decay. Such sensors have recently been used for this type of experiments  reaching precisions down to a few eV. The nuclear recoil in beta decay is directly sensitive to the beta-neutrino angular correlation, which is a key observable in mirror beta decays to constrain the CKM matrix unitarity, complementary to the one obtained from 0⁺⇾0⁺ super-allowed pure Fermi transitions. In addition, ASGARD will perform novel measurements of the ratio between electron capture and β⁺ decay rates, to test the existence of exotic scalar and tensor currents. Such a counting measurement greatly enhances the sensitivity to exotic currents as several competing systematic effects cancel out.

bSTILED (b:Search for Tensor Interactions in nucLear bEta Decay)

The bSTILED experiment is a two-phase project dedicated to the high precision measurement of the beta spectrum shape in a pure Gamow-Teller transition to extract the Fierz interference term, b, that is highly sensitive to the existence of left-handed exotic tensor currents. Phase I consists of two experiments based on a 4pi calorimetry technique  with 6He nuclei produced at two different beam energies at GANIL. Each measurement tackles different systematic effects. The objective of Phase I is to achieve a relative precision of 0.4% on b which corresponds to the present limits from all measurements in nuclear and neutron decay. Phase II will then allow to build upon the most promising configuration and eventually reach a 0.1% relative precision, which is the level at which the constraints on New Physics are competitive with LHC direct searches.

Detection setup for the low energy beam measurement in the implantation (left) and data-taking (right) configurations. Both the movable and fixed detectors are composed of YAP and PVT scintillators in a phoswich configuration coupled to a PMT. Typical measurement cycles comprise a 3 second period for beam implantation followed by a 15 second  period for the 6He decay and constant background measurements.

MORA (Matter’s Origins from RadioActivity)

MORA is an experiment which combines a transparent Paul trap with a laser polarisation technique to measure the triple correlation between the nuclear spin and the momenta of the beta particle and the neutrino of beta decaying ions: the so-called D correlation violates the Time reversal symmetry and is thus a probe of CP violation. CP violation is one of the 3 key ingredients to explain the baryogenesis process, leading to the matter/antimatter asymmetry in the Universe. The D correlation originates from an interference between V and A interactions, and can thus only appear in mixed Fermi and Gamow-Teller transitions. MORA is now taking data at Jyväskylä for demonstrating the innovative polarisation technique, and for attaining a sensitivity of the order of ~10^-4 on a non-zero D correlation, i.e. competitive with the present constraint obtained a decade ago in neutron decay by the emiT experiment. At DESIR MORA is aiming at a precision of ~10^-5,  to complement the constraints on New Physics Models from Electric Dipole Moment. At this level, MORA will also be sensitive to Final State Interactions, which are recoil-order corrections compliant with the Standard Model.

WISArD (Weak Interaction Studies with ³²Ar Decay)

The WISArD experiment is probing the existence of exotic currents via two types of high precision measurements: a beta-neutrino angular correlation measurement and beta spectrum shape measurements. The beta-neutrino angular correlation is measured in the decay of a beta-delayed proton emitter delivered by ISOLDE . The approach of WISArD to reach ultra-high precision is two-fold: instead of performing direct nuclear recoil spectroscopy, the magnitude of the nuclear recoil is deduced from the kinematic effect on the energy of the beta-delayed protons emitted by the daughter nucleus. The protons are detected in coincidence with the beta particles, which greatly enhances sensitivity. High precision beta spectrum shape measurements are also performed withthe same set-up, taking advantage of the high magnetic field to confine beta particles between two symmetrical detectors, thus creating an effective 4π detector. 

International activities

The members of the ISOL-France collaboration are involved in several international projects and networks relevant to low energy nuclear physics, mainly in Europe, but also overseas. One of the world-leading laboratories in (low energy) nuclear physics is ISOLDE at CERN, where ISOL-France members are involved among other in mass measurements with ISOLTRAP, laser spectroscopy with CRIS and weak interaction studies with WISArD. There is also a strong collaboration with GSI and its German partner institutes, for example on the topic of target development for radioactive ion beam production. A collaboration network IN2P3-GSI on the topic of (in-gas-jet) laser spectroscopy also exists. ISOL-France members are additionally involved at the Accelerator laboratory of the University of Jyväskylä in Finland, with activities mainly focused in the IGISOL hall. Several set-ups developed by French institutes have been/are/will be tested and commissioned there, e.g. MORA and SEASON, and ISOL-France researchers actively participate in many experimental campaigns of mass measurements, decay spectroscopy in the medium and heavy-mass regions, and laser spectroscopy.