The CHyMENE project (Cible d'Hydrogène Mince pour l'Etude des Noyaux Exotiques - a thin cryogenic target for the studies of exotic nuclei) is part of the instrumentation necessary for the exploitation of the low energy beams (~ 5 to 25 MeV/n), such as SPIRAL2 beams. The aim is to develop a thin cryogenic target of pure hydrogen (H2 or D2), the characteristics of which will be well adapted to the conditions of future direct reaction experiments.
The DPhN and DACM departments of IRFU are responsible for the project management. The project involves three laboratories: IRFU, IPN Orsay and the CEA-DAM/DIF SPN. It is funded by the ANR (Agence Nationale de la Recherche), the French National Research Agency.
Nuclear physics experiments with hydrogen targets most often use polypropylene (CH2)n targets for their simplicity of implementation, whereas pure hydrogen targets would be better adapted to the desired measurements. In the absence of a carbon ion, the H2 targets increase the luminosity while reducing the straggling of the reaction products within the target. Moreover, the carbon ions present in the (CH2)n targets are also the origin of parasitic nuclear reactions which create contamination of the measured quantities. It is then necessary to subtract these contaminations by performing complementary measurements on target of 12C, at the cost of a loss of beam time. The CHyMENE project proposes the realization of a cryogenic target, the characteristics of which will be adapted to the conditions of future experiments of direct reactions. It will be a pure target of solid hydrogen H2 without a window that will flow in the form of a ribbon in front of the beam in the vacuum of a reaction chamber. To use this target with low energy ion beams, its thickness is a very important constraint. It is fixed by the shape of the extrusion nozzle and can vary in a range currently comprised between 20 and 100 μm.
We are currently considering studying with the CHyMENE device the interaction of a high-power laser beam with a hydrogen target, in order to produce a proton beam. While several installations will soon be operational in Europe with increasing repetition rates, the target destroyed at each interaction and the pollution of the installation by the debris becomes a major problem to which a device of the CHyMENE type can provide solutions.
The method used for the CHyMENE device is an extrusion technique, developed by the PELIN laboratory in St. Petersburg. The pure hydrogen gas is introduced at room temperature and cooled in the vicinity of the triple point.
This volume of hydrogen in an amorphous state is compressed by a worm and pushed to an extrusion nozzle. At the exit of the nozzle in the vacuum, the hydrogen is in a transparent solid state. The shape of the nozzle defines the geometric characteristics of the target. This technology must be adapted to provide a homogeneous thin target (thickness of 200 to 50 μm) compatible with vacuum and beam conditions.
Since the first test carried out in June 2007 in St. Petersburg (stable production of a target with a thickness of 200 μm), the method has been validated with the production of increasingly thin targets.
In an initial test campaign carried out from November 2009 to April 2010, an operating point for the production of the H2 film was obtained, allowing a 100 μm thick ribbon to flow.
The study of the target (thickness and homogeneity) was carried out in the DAM / DIF department of the CEA of Bruyères-le-Châtel under a proton beam.
The results of the tests were published in the European Physical Journal A in 2013.
A new device of the collaboration was built in 2013 and tested with a radioactive source. Beam tests are planned for 2018.
The operating principle has been fully validated. Important R&D work is being carried out on the definition and realization of extrusion nozzles which must simultaneously guarantee good thermal conductivity and mechanical resistance to the flow of hydrogen ribbon in a micrometric range.
Up to now, the CHyMENE device has been developed as an H2 hydrogen target. We will soon test the operation with the deuterium gas D2 which has thermodynamic properties close to H2.
SACM Jean-Marc GHELLER