# The DESIR facility at SPIRAL2

J.C. Thomas, Bertram Blank

► To cite this version:

J.C. Thomas, Bertram Blank. The DESIR facility at SPIRAL2. French-Japanese Symposium on Nuclear Structure Problems, 2011, Wako, Japan. pp.224-229, 10.1142/9789814417952_0045 . in2p3-01003705

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Distributed under a Creative Commons Attribution 4.0 International License
THE DESIR FACILITY AT SPIRAL2

J.-C. THOMAS

Grand Accélérateur National d'Ions Lourds, CEA/DSM - CNRS/IN2P3, Bd. Becquerel,
BP 55027, 14076 CAEN Cedex 05, France

B. BLANK

Centre d'Etudes Nucléaires de Bordeaux Gradignan - Université Bordeaux 1 – UMR
5797 CNRS/IN2P3, Chemin du Solarium, BP 120, 33175 Gradignan, France

The DESIR collaboration proposes the construction of a low-energy beam facility at GANIL-SPIRAL2 to study the properties of exotic nuclei in unexplored regions of the nuclide chart. Beam preparation devices including gas catchers, radiofrequency quadrupoles, high resolution separators and ion traps are under construction in order to provide high quality beams to the users. The DESIR Physics program addresses by means of complementary experimental techniques most of the current interrogations regarding the structure of exotic nuclei, the fundamental interactions driving their properties, as well as their formation in the universe.

Keywords: low-energy facility, nuclear physics, weak interaction, astrophysics

# 1. Introduction

The DESIR facility is proposed in the frame work of the SPIRAL2 facility presently under construction at GANIL, Caen [1]. SPIRAL2 proposes to its future users a large variety of exotic nuclei produced by fission of $^{238}$U, by fusion-evaporation reactions, and by transfer of a few nucleons. The most exotic species – many of which will be produced for the very first time – will be transmitted to the DESIR facility for experiments at low energies that will determine their fundamental, ground-state properties such as decay mode, half-life, mass, charge radius and shape. In addition, DESIR can also receive beams from SPIRAL1, the ISOL radioactive beam facility already existing at GANIL which produces exotic species by projectile and, in the near future, by target fragmentation. Finally, DESIR is also meant to receive beams from the low energy branch of S3 [2], the superconducting separator spectrometer proposed for construction in the context of SPIRAL2. A layout of the SPIRAL2 facility is proposed in figure 1.
The DESIR collaboration proposes the construction of a large experimental hall into which the different beams from the three production sites can be injected and used in world-class experiments. The DESIR buildings and infrastructure are presented in paragraph 2. Their construction will last from 2012 to mid 2014, when the installation of the beam preparation and experimental equipment can start. The former are described in paragraph 3.

<figure><img src="image_2.png" /><figcaption>Fig. 1. General view of SPIRAL2 with the DESIR building in the upper right corner and the existing GANIL below. The beam preparation equipment located in the S3 experimental area, in the SPIRAL2 production building and at the entrance of the DESIR experimental room are identified by means of red contours.</figcaption></figure>

They consist (see figure 1) in a radioprotection monitoring station and stable ion sources coupled to a general purpose ion buncher (IS+GPIB). In addition, dedicated beam preparation devices located in the Low Energy Branch part of the S3 facility (S3-LEB) and after the SPIRAL2 production module (RFQ & HRS) will help making the best use of the stable and radioactive ion beams delivered to the DESIR experimental hall.

The experimental equipment are presently under construction in different laboratories across Europe or already used in other laboratories. This equipment comprises three different facilities devoted to the study of the atomic nuclei: The DETRAP facility will consist in ion and atomic traps dedicated to high-precision mass measurements, to studies dealing with nucleosynthesis scenarios and the standard model of weak interaction, and to the high-purity preparation of radioactive species for decay studies. The LUMIERE facility, the laser-spectroscopy facility of DESIR, will allow measurements of static properties of the exotic nuclei's ground and long-lived isomeric states like their magnetic moment, their quadrupole moment and thus their
deformation as well as their spin and parity. The BESTIOL facility will enable
scientists to perform precision measurements of beta decay to study fundamental
properties like decay modes, half-lives, branching ratios, spins and parities of these
exotic nuclei. Finally, the basement of the DESIR facility will contain a highly
protected room for high-intensity experiments devoted to material science and
industrial applications. The DESIR Physics program is detailed in paragraph 4. The
facility is expected to go online by the end of 2015.

## 2. Buildings and infrastructure

A layout of the DESIR building and the beam transport tunnels is shown in figure 2. The tunnels will allow to transport the beams produced by SPIRAL1, SPIRAL2 and S3 to the DESIR building where the experimental equipment will be located. The tunnels will have a height and a width of 3m and a total length of 100 m for the three branches.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Layout of the DESIR building and beam transport tunnels linking the different GANIL-SPIRAL2 (radioactive-) ion beams production sites to the DESIR experimental hall.</figcaption></figure>

The DESIR experimental area (see figures 2 and 3) is a hall of 1500 m² with a
basement of roughly half this size. The basement will contain the radioprotection
and control equipment needed to determine the quality and intensity of the
radioactive beams sent into the DESIR hall, a mechanical workshop, clean rooms, assembly rooms, a radioprotection laboratory, storage rooms and different supply rooms. The main experimental area will be located at ground floor and will receive most of the experimental equipment installed at DESIR (see figure 3). The detailed description of the equipment can be found in the DESIR Letter of Intent [3] and in the Technical Design report [4].

The construction of the DESIR buildings should start in 2012 and be delivered by
mid 2014.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Schematic view of the DESIR experimental hall (ground level).</figcaption></figure>

**3. Beam preparation**

In the SPIRAL2 production building, radioactive ion beams will be produced combining different techniques (neutron-induced fission of <sup>238</sup>U, transfer reactions as well as fusion-evaporation reactions) and different ion-source ensembles (ECR, FEBIAD, surface-ionization and laser ion sources). A large variety of radioactive ion beams will therefore be available, from light neutron-deficient nuclei to heavy neutron-rich isotopes, but might be delivered in rather poor purity and optical conditions. In order to make the best use of the produced beams, a high-intensity radiofrequency cooler, the SHIRaC device [5], will be coupled to a high resolution separator (HRS) [6], in order to provide DESIR with high quality mass-separated beams with energies ranging from 10 to 60 kV and emittances of a few π.mm.mrad.
The aim of the SHIRaC equipment is to cool single charged ion beams with an intensity and an emittance as large as 1 eµA and 80 π.mm.mrad, respectively, in order to optimize their further cleaning in the HRS. A first prototype shown in figure 4 has been designed and successfully tested using a 25 enA cesium beam for which a transmission of 25 % has been achieved by means of a high frequency (5 kHz) and high voltage (2 kV<sub>ptp</sub>) quadrupole. A second prototype with a larger aperture (5 mm instead of 2), stronger electrical fields (10 kV<sub>ptp</sub>) and working at a higher frequency (10 MHz) is currently being tested in order to reach the µA intensity regime.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Test bench of the SHIRaC high-intensity radiofrequency cooler at the LPC Caen Laboratory, Caen, France.</figcaption></figure>

The design of the high resolution separator has been derived from the CARIBU device installed at the Argonne National Laboratory. It consists in two mirror symmetric arms associating two matching quadrupoles, a focusing sextupole, a matching quadrupole and a 90° dipole separated by a multipole magnet. Its nominal dispersion of $(x/δ) = -31.5$ cm/% should allow the transmission from point to point in X and Y of up to 3 π.mm.mrad ion beams with a mass resolving power $m/δm$ as large as 20000. The equipment and its mechanical integration in the SPIRAL2 production building has been fully simulated (see figure 5) and it will be tested in 2013 at the CEN Bordeaux Gradignan institute before being installed at SPIRAL2 in 2014.
<figure><img src="image_6.png" /><figcaption>Fig. 5. Drawing of the high resolution separator to be installed together with SHIRaC in the SPIRAL2 production building.</figcaption></figure>

In the S3 facility, the high-intensity heavy ion beams delivered by the SPIRAL2 LINAG will undergo fusion-evaporation reactions to produce intense radioactive ion beams of N~Z nuclei as well as very heavy isotopes in the transactinide region. Despite the large acceptance and primary-beam suppression provided by the S3 equipment, an additional purification of the cocktail beam collected at its focal plane is needed in order to select the radioactive nuclei to be studied in DESIR. That is the aim of the low-energy branch of S3 (see figure 6), which will either consist in a radiofrequency gas catcher such as the one designed by the Argonne National Laboratory in the framework of the RIA project, or in an equipment similar to the LISOL device run at Louvain-la-Neuve, Belgium, by the IKS group of Leuven. In both case, single charged ions, including refractory elements, will be extracted and separated in a HRS similar to the one located in the SPIRAL2 production building before being sent to DESIR.
<figure><img src="image_7.png" /><figcaption>Fig. 6. Schematic drawing of the S3 low-energy branch consisting in a gas catcher device coupled to a high resolution separator.</figcaption></figure>

The radioactive ion beams produced and selected in the SPIRAL2 production building and in the S3 facility, as well as those produced by SPIRAL1 need to be identified and characterized before they are used in the DESIR experimental hall. It allows checking the composition and the intensity of the ion beams, as well as the transmission through the beam transport lines in between the different production sites and the DESIR building. It is also mandatory regarding safety issues such as the dose rate induced by the radioactive beams collected in the experimental equipment and the risks associated with the accidental release of radioactive species inside the DESIR building. The characterization of the radioactive beams will be performed at the entrance of the DESIR building by means of an identification station consisting in a fast tape transport station and dedicated charged particle and gamma-ray detectors. The design of the identification unit was performed by the IPHC institute of Strasbourg and a first equipment is currently being used at the ISOLDE facility (see figure 7).
<figure><img src="image_8.png" /><figcaption>Fig. 7. Views of the identification station to be installed in the DESIR basement in order to characterize the radioactive beams produced in the SPIRAL2 production building, in S3 and in SPIRAL1 and sent to DESIR.</figcaption></figure>

Many experiments need bunched beams in order to increase their signal to noise
ratio (e.g. experiments at the LUMIERE facility) or for an efficient injection in
their setup (trap-assisted experiments). A general purpose ion buncher (GPIB)
will therefore be installed at the entrance of the DESIR experimental hall, just
before the main line feeding the experimental equipement. Such a buncher is
continuously filled with the incoming beam and release the ions in packets to the
users with an expected efficiency of more than 50%. This buncher will also work
in transmission mode with an efficiency close to 100%. The design of the
apparatus will be derived from the ISCOOL equipment operated at ISOLDE (see
figure 8). In order to tune the device and to tune the experimental equipment prior
to the experiment, different standard stable ion sources will be used, directly
connected to the GPIB.
<figure><img src="image_9.png" /><figcaption>Fig. 8. Photograph of the ISCOOL ion buncher operated at ISOLDE from which the DESIR GPIB will be derived.</figcaption></figure>

# 4. Physics program

## 4.1. *Beta-decay spectroscopy studies at the BESTIOL facility*

Beta decay offers great insights into the structure of exotic nuclides. In addition, the decay energy, the half-life and the generation of delayed neutrons by fission fragments is of great practical importance for running nuclear power plants. The BESTIOL facility will allow to perform detailed spectroscopy studies of both neutron-deficient and neutron-rich nuclides, with RIB intensities ranging from more than $10^5$ pps down to a few ions per hour. It consists (see figure 9) in a double Penning-trap isobar separator for trap-assisted spectroscopy in combination with a total absorption spectrometer (TAS), neutron arrays, charged-particle detectors such as the Si Cube and standard beta-gamma detection devices.
<figure><img src="image_10.png" /><figcaption>Fig. 9. Views of some of the detection systems composing the BESTIOL equipment.</figcaption></figure>

The physics cases that will be addressed using the BESTIOL facility are
manyfold:

* High-precision measurements of super-allowed and mirror beta decays.

* Lifetime and decay spectroscopy properties of neutron-rich nuclei relevant to nuclear structure and astrophysics studies, as well as to nuclear energy.

* Beta delayed charged-particle correlation studies

* Cluster emission mechanisms

* Shape coexistence, deformation and Gamow-Teller strength around A~100 and at the N=50, 82 closed shells.

## 4.2. Collinear Laser spectroscopy at the LUMIERE facility

Different techniques based on collinear laser spectroscopy as well as experiments based on optically polarized ion beams are proposed using the LUMIERE facility at DESIR (see figure 10). It aims at determining the nuclear moments and spins of nuclear ground-state and long-lived isomeric states, as well as the mean-square charge radius variations over isotopic chains. The sensitivity of the collinear laser spectroscopy technique using optical detection of the fluorescence light is expected to be enhanced by several orders of magnitude using the collinear resonance ionisation spectroscopy method that is currently being developed at ISOLDE.

This technique requiring bunched ion beams, the LUMIERE facility will be
coupled to the general purpose ion cooler and buncher device (GPIB) presented
above and will allow studying extremely exotic species of refractory and heavy
elements produced at rates as low as a few ions per second.

By using optical pumping inside the cooler, it is possible to access now almost all
elements for collinear laser spectroscopy. Pumping in the cooler has been used
recently at Jyväskylä to access the Mn isotopes, and can be extended towards and
beyond N=40, N=50 and N=82 at DESIR.

<figure><img src="image_11.png" /><figcaption>Fig. 10. Possible layout of the LUMIERE facility at DESIR.</figcaption></figure>

By optical pumping using circularly-polarized laser light in the collinear laser
beam line, it is possible to reach very large nuclear polarization. Such polarized
laser beams can be used for different applications, such as β-NMR studies or for
β-γ coincidence spectroscopy to determine asymmetry parameters that are a direct
indication for the spin of the populated daughter states.

4.3. *Mass measurement and fundamental interaction studies at the*
*DETRAP facility*

The confining capability of ion traps offers different possibilities for the
preparation of exotic beams for observation. Penning traps, employing highly-
homogeneous magnetic and electric fields, now play the leading role for
measurements of the nuclear binding energy. Such measurements are crucial to
understand the specificities and the evolution of the nuclear structure in the N~Z,
A~100 and transactinide regions of the nuclide chart. They are also particularly
relevant for the study of the astrophysical rapid-neutron capture process the
features of which are intimately related to the mass and decay properties of the
isotopes involved. In addition, mass differences allow the precision
determination of beta-decay Q values, of capital importance for weak-interaction
studies based on super-allowed beta decaying nuclei. Mass measurement will be
performed at DESIR with the MLL Penning trap (see figure 11), which is now
being commissioned in Munich. It combines a multi-reflection time-of-flight
spectrometer with a Penning trap, leading to a mass resolving power m/δm of
about 10⁹.

<figure><img src="image_12.png" /><figcaption>Fig. 11. Photograph of the MLL Trap mass-measurement equipment commissioned at Munich before its installation at DESIR.</figcaption></figure>

Traps can also be used to hold radioactive species and wait for their decay. The additional property of nicely defining the initial-ion conditions makes traps very effective for the study of decay kinematics and hence correlation coefficients of weak-interaction decays to probe for physics beyond the standard model. The LPCtrap system (see figure 12) that has been used for the study of the ⁶He decay at SPIRAL1 is planned to be installed at DESIR to pursue such studies on selected beta decaying nuclei in order to further probe the contribution of scalar and tensor like currents to the weak interaction. In addition, the measurement of the beta-neutrino angular correlation coefficient in the beta decay of mirror nuclei will allow building up a new systematic of Ft values from which an independent value of the V ud term of the Cabbibo-Kobayashi-Maskawa quark-mixing matrix will be extracted.
<figure><img src="image_13.png" /><figcaption>Fig. 12. Left: Photograph of the LPCtrap setup operated at GANIL/SPIRAL. Right: Schematic drawing of the decay trap in which the β-v angular correlation coefficient is measured.</figcaption></figure>

**5. Conclusion**

The DESIR collaboration gathers today about 100 scientists belonging to more
than 30 institutes worldwide. It proposes to build up a low-energy beam facility
at SPIRAL2 where a large variety of high quality radioactive beams, produced by
neutron-induced fission, fragmentation, transfer and fusion-evaporation reactions
will be available. The complementarity of up-to-date experimental techniques,
like laser and trap-assisted spectroscopy, will open unique opportunities for
experiments with low-energy exotic nuclei. The construction of the facility should
start in 2012 and be operational by the end of 2015.

**Acknowledgments**

The construction of parts of the experimental equipment was supported by the
French ANR via the Contract ANR-06-BLAN-0320.

**References**

1. http://www.ganil-spiral2.eu/

2. http://pro.ganil-spiral2.eu/spiral2/instrumentation/s3

3. http://www.cenbg.in2p3.fr/desir/IMG/pdf/DESIR LOI LONG.pdf

4. http://www.cenbg.in2p3.fr/desir/IMG/pdf/DESIR-Technical-Proposal-V090105.pdf