Miniball experiment

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Isotope Separator On Line Device
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COLLAPS, CRIS, EC-SLI, IDS, ISS, ISOLTRAP, LUCRECIA, Miniball, MIRACLS, SEC, VITO, WISArD
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Miniball experimental setup at the ISOLDE facility (CERN) Miniball.jpg
Miniball experimental setup at the ISOLDE facility (CERN)

The Miniballexperiment is a gamma-ray spectroscopy setup regularly located in the ISOLDE facility at CERN, along with other locations including GSI, Cologne, PSI and RIKEN (HiCARI). [1] [2] [3] [4] Miniball is a high-resolution germanium detector array, specifically designed to work with low-intensity radioactive ion beams (RIB) post-accelerated by HIE-ISOLDE (High Intensity and Energy-ISOLDE), to analyse the decays of short-lived nuclei with the capability of Doppler correction. The array has been used for successful Coulomb-excitation and transfer-reaction experiments with exotic RIBs. Results from Miniball experiments have been used to determine and probe nuclear structure. [5]

Contents

Miniball has been operational at the REX-ISOLDE (Radioactive ion beam EXperiment-ISOLDE) post accelerator at CERN since 2001. [6] In 2015, it became part of the HIE-ISOLDE project, connected via the XT01 beamline. [7] It was the first fully operational spectrometer to determine gamma-ray position using germanium detectors and pulse shape analysis. [8]

Background

The two techniques used for experiments using the Miniball setup at ISOLDE are Coulomb excitation and transfer-reactions.

Coulomb excitation is a technique used to probe the electromagnetic (EM) aspect of nuclear structure. A nucleus is excited by an inelastic collision with another nucleus at a "safe" scattering angle, to ensure its via purely EM interaction. The nucleus then decays to a lower state, emitting a gamma-ray which can be detected using gamma detectors. [9] This method is useful for investigating collectivity in nuclei (motion of individual nucleons are correlated), as collective excitations are often connected by electric quadrupole transitions. [10]

Transfer reaction of projectile and target nucleus Transfer-reactions.png
Transfer reaction of projectile and target nucleus

During transfer reactions, one (or a cluster) of nucleons are exchanged between the target nucleus and the projectile, resulting in a different final state nucleus. [11] Measurements of the emission angle and energy for use in two-body kinematic calculations can give the excitation energy of the populated states in the final state nucleus. Additionally, the measured angular distributions are compared to theory to deduce the transferred orbital angular momentum in the reaction. For single-nucleon transfer, this indicates the orbital that the nucleon has been transferred into. Studying transfer reactions is useful in nuclear astrophysics as it replicates stellar evolution and can test theoretical models. [12]

Experimental setup

Overhead view of the Miniball set up Miniball overhead.jpg
Overhead view of the Miniball set up

The Miniball detector array consists of 24 high-purity germanium crystals which have a tapered front end. [8] They have a six-fold segmentation, differing them from other detectors at the time (e.g. EUROBALL), which are coupled to preamplifiers. [13] The crystals are sealed in a aluminium can, allowing access of the cold electronics without the use of a cleanroom, as the can did not need to be broken in to. [6] [5]

Each capsule is used to pack together clusters of three six-fold detectors. Housing each cluster are cryostats using liquid nitrogen, which give clusters a common vacuum chamber, cryostat and dewar. Configurations of the clusters can be arranged to provide solid angle coverage in a compact configuration, but this requires flexibility in the setup. [14] At ISOLDE, Miniball achieves this by mounting cryostats on half-circular, rotatable arms with the ability for continuous motion along the arms. [5] At other locations, differing amounts of clusters are arranged in different configurations, in order to achieve different results. [15]

CD detector used at Miniball at CERN Miniball CD.jpg
CD detector used at Miniball at CERN

The T-REX (Transfer at REX) setup is designed for measuring transfer reactions at the Miniball detector. The setup consists of a silicon barrel with forward and backward CD detectors, covering a solid angle of 66% of 4π. The T-REX measures the angular distribution of the light reaction products. [11]

Miniball uses digital pulse processing by using real-time digital filter algorithms to produce results for energy and time. The data acquisition and analysis system consists of a front-end system for data readout and transport, and a back-end system for control and data analysis. [1]

Results

In 2013, results from the Miniball experiment at ISOLDE was named in the Institute of Physics (IoP) "top 10 breakthroughs in physics". [16] The research found evidence of pear-shaped heavy nuclei, in particular in radon-220 and radium-224. [17] The breakthrough was also featured as the cover of Nature 2013. [18]

The main experimental technique used with Miniball is low energy Coulomb excitation. Results from using this technique have resulted in numerical values for the electric-dipole, quadrupole and octupole moments of the transitions. [19] The technique of transfer-reactions is also used in Miniball experiments. [11] The "island of inversion" nucleus 32Mg was of particular interest for experiments, which could be studied using transfer-reactions (as well as with Coulomb excitation). [20] [6]

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The ISOLDE Radioactive Ion Beam Facility, is an on-line isotope separator facility located at the centre of the CERN accelerator complex on the Franco-Swiss border. Created in 1964, the ISOLDE facility started delivering radioactive ion beams (RIBs) to users in 1967. Originally located at the Synchro-Cyclotron (SC) accelerator, the facility has been upgraded several times most notably in 1992 when the whole facility was moved to be connected to CERN's ProtonSynchroton Booster (PSB). ISOLDE is currently the longest-running facility in operation at CERN, with continuous developments of the facility and its experiments keeping ISOLDE at the forefront of science with RIBs. ISOLDE benefits a wide range of physics communities with applications covering nuclear, atomic, molecular and solid-state physics, but also biophysics and astrophysics, as well as high-precision experiments looking for physics beyond the Standard Model. The facility is operated by the ISOLDE Collaboration, comprising CERN and sixteen (mostly) European countries. As of 2019, close to 1,000 experimentalists around the world are coming to ISOLDE to perform typically 50 different experiments per year.

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References

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  11. 1 2 3 T-REX Collaboration; Bildstein, Vinzenz; Gernhäuser, Roman; Kröll, Thorsten; Krücken, Reiner; Wimmer, Kathrin; Van Duppen, Piet; Huyse, Mark; Patronis, Nikolas; Raabe, Riccardo (June 2012). "T-REX: A new setup for transfer experiments at REX-ISOLDE". The European Physical Journal A. 48 (6): 85. Bibcode:2012EPJA...48...85B. doi:10.1140/epja/i2012-12085-6. ISSN   1434-6001. S2CID   119716833.
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  14. ↑ Butler, P A; Cederkall, J; Reiter, P (2017-04-01). "Nuclear-structure studies of exotic nuclei with MINIBALL". Journal of Physics G: Nuclear and Particle Physics. 44 (4): 044012. Bibcode:2017JPhG...44d4012B. doi: 10.1088/1361-6471/aa5c4e . ISSN   0954-3899.
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