As part of the program organized by IRFU’s nuclear physics department (SPhN) to study exotic nuclei, from 2013 to 2015 SACM/LCSE built two cryogenic systems to meet the needs for thick hydrogen targets (100 and 150 mm) for the MINOS project (funded by the ERC), already in operation, and thin targets (20 and 50 µm) for the CHyMENE project (funded by the French National Research Agency or ANR), currently under development.
The liquid targets of the MINOS project have been used as part of an international collaboration (12 countries) in Riken, Japan since May 2014. Four experiments of three weeks each were carried out using the MINOS device associated with detection systems belonging to the host laboratory. The cryostat has been designed to provide the space required for detection, particularly for the MINOS time projection chamber (TPC).
The cryostat (pictured above), used to obtain liquid hydrogen (picture on the right), is equipped with a commercial cryocooler with cold head power of 15 W (at 20 K). The choice of this cryocooler (compressor and cold head) was motivated by the simplicity of its use (requiring only water and electricity). The absence of a cryogenic infrastructure in the host laboratory (no liquid helium available) and the lack of specialist personnel were two factors that were also decisive. Finally, the compact size of the cold head is a perfect response to detection constraints.
The cryogenic cycle begins by cooling the system without hydrogen for 12 hours. The liquefaction phase begins by transferring gaseous hydrogen from its storage tank, at an initial absolute pressure of 1.5 bars, to the cryostat via a cryogenic bay. The hydrogen liquefies at a temperature of 20.4 K at atmospheric pressure in the one-liter condenser mounted on the second stage of the cold head. The condenser gradually feeds the target with liquid hydrogen by gravity flow. The resulting cold vapor returns to the condenser via a closed loop and is then liquefied. To complete the operation, during temperature-controlled heating, the liquid hydrogen vaporizes and returns to its dedicated storage tank through the check valves. This cryogenic system design minimizes the quantity of hydrogen used (120 g at normal pressure and temperature) and ensures the safety of the system, which uses an explosive fluid under cryogenic conditions.
Several types of target are available depending on the characteristics of the beam used at Riken. The cylindrical target, made of Mylar®, consists of an inlet window (Ø 39 mm) and a cylinder (Ø 52 mm) with 3 different lengths (60, 100 or 150 mm). Target rupture tests at a temperature of 77 K conducted in our laboratory give a rupture pressure of 5.9 bars compared to the maximum pressure of 1 bar. To prevent the ultimate fault scenario in the event of a liquid target rupture, the hydrogen must remain confined inside the cryostat. The 280-liter buffer tank connected to the cryostat keeps the absolute pressure of the H2 below 700 mbars if the gas vaporizes on the cryostat walls at room temperature. Management of this type of accident is provided by a specific target monitoring program.
The command & control system supervises temperature control and equipment operation. It also provides an answer to physicists’ request to be able to take measurements on an “empty” target, which is actually filled with H2 gas of much lower density (with a density ratio between liquid and gaseous H2 of 106 at 18 K). This makes it possible during the experiment to quickly measure the signal-to-noise ratio created by the Mylar® cell and to check that settings on the various detection devices are optimal.
About 15 data analyses are underway. Three new additional experiments are scheduled in 2016 and 2017.