This thin-target project is part of a program focused on the study of radioactive nuclei conducted by IRFU’s nuclear physics research laboratory in Caen, GANIL, on the SPIRAL2 facility (or other ISOL1 machines). The target must be designed to fit the geometry of existing experiments, including the detection of charged particles in a vacuum (for the GASPARD2 project) or photons (for AGATA) around the target. The system will first be validated on the IPN at Orsay or the CEA/DAM accelerator at Bruyères-le-Châtel. Design and build took 30 months, with three institutes working together to develop the entire system, including IRFU’s SACM, SIS and SPhN departments, CEA/DAM and IPNO. Since January 2014, the cryostat has been producing solid hydrogen. Adjustment of the system and studies on target thickness are in progress at LCSE.
Constraints related to the reaction chamber, combined with those of the detection system, determined the geometry of the CHyMENE cryostat. The process chosen to produce the target consists of continuous ribbon extrusion of slush hydrogen, a solution that meets the requirement for non-stop use of the target over an operating period of 15 days. The constant supply of hydrogen to the system is sufficient to sustain the target in the reaction chamber, which requires a vacuum of 5?10-5 mbars for the detection system. The ribbon production that sublimates in the reaction chamber is equivalent to a constant leak of 5 mbars?l/s for a 50-µm target
. A pumping capacity of 100000l/s is necessary to achieve this vacuum specification. Off-the-shelf equipment is out of the question, due to both its cost and excessive size. Although the pumping capacity is undersized, the vacuum currently obtained in our tests is 5?10-4 mbars to produce a target with a 20-µm nozzle (for a 5-mm working width). These initial results currently serve as the reference used by the Institute of Nuclear Physics at Orsay (IPNO), responsible for chamber design. The CHyMENE system consists of a motorized cryostat with vertical movement capability for removing the target from the beam axis and 100° target rotation capability to achieve the angles of analysis required during the experiment. The cryostat is equipped with a cryogenic source (cold head) supplying the 15 W of power at 11 K required to form hydrogen slush in the lower part of the extruder. The nozzle mounted on the end of the cryostat determines the geometry of the target (picture opposite). Cryogenic power is transmitted from the second stage of the cold head to the extruder exclusively by means of conduction across two metals with very high thermal conductivity: aluminum (10 kW/m?K) and copper (2.2 kW/m?K at 11 K). The liquid hydrogen flows by gravity inside the extruder and solidifies in the coldest part of the device at 14 K (5 bars absolute). The hydrogen ice produced is mechanically compressed at 100 bars in the extruder to form the hydrogen film discharged through the nozzle.
Several nozzles are under study, covering a thickness ranging from 180 µm to 20 µm. In addition to meeting thickness criteria, they must also ensure that the ribbon facing the beam remains in a stable position. Obtaining a target of uniform thickness depends, first, on the temperature and flow velocity of the hydrogen slush. Second, thickness uniformity is sensitive to the nozzle design (choice of material, internal geometry and interaction between slush and internal walls), as well as mechanical precision in fabrication and the quality of assembly. Thickness and uniformity measurements are currently underway. The target is characterized using a 241Am source that mainly delivers 5.485-MeV alpha particles. The method chosen consists of using a silicon detector to measure the energy lost by alpha particles crossing the target. Given the limited intensity of the available source (compared to that of a beam), measurement is mainly affected by the thickness of the integrated target across its width over a period of approximately one to two hours, depending on collimation conditions.
The short-term goal is to obtain uniform thickness throughout several days of hydrogen ribbon production, and then switch to producing deuterium ribbon. In the medium term, the CHyMENE cryostat may find applications not only in nuclear physics, but may also contribute to developing secondary proton beams using laser pulses on hydrogen films less than 10 µm thick.