Neutron Irradiation Tests of Pressure Transducers in Liquid Helium

The superconducting magnets of the future Large Hadron Collider (LHC) at CERN will operate in pressurised superfluid helium (1 bar, 1.9 K). About 500 pressure transducers will be placed in the liquid helium bath for monitoring the filling and the pressure transients after resistive transitions. Their precision must remain better than 100 mbar at pressures below 2 bar and better than 5% for higher pressures (up to 20 bar), with temperatures ranging from 1.8 K to 300 K. All the tested transducers are based on the same principle: the fluid or gas is separated from a sealed reference vacuum by an elastic membrane; its deformation indicates the pressure. The transducers will be exposed to high neutron fluence (2 kGy, 1014 n/cm2 per year) during the 20 years of machine operation. This irradiation may induce changes both on the membranes characteristics (leakage, modification of elasticity) and on gauges which measure their deformations. To investigate these effects and select the transducer to be used in the LHC, a neutron irradiation program is being performed at the CERI cyclotron (CNRS Orléans, France): a cryostat is installed on a beam line, transducers are immersed in liquid helium and irradiated by neutrons (1-20 MeV, 1015 n/cm2). The tested transducers measure the helium bath pressure, the true value of which is given by a warm, unirradiated sensor. Every readout is acquired on-line. This paper presents the results of the first experiments performed during spring, 1999.