For control and monitoring purposes, about 10,000 individually calibrated cryogenic temperature sensors will be installed along the 26.7 km LHC. In order to reduce maintenance constraints these sensor s should be as immune as possible to the high neutron fluence environment. For selecting the sensor to be used, a radiation hardness evaluation program at cryogenic conditions is being performed in an irradiation vault of the ISN SARA Cyclotron (Grenoble, France). The set-up is capable of simulating the whole life of a LHC thermometer: same total neutron dose (1015 n.cm-2), irradiation at low tempe rature (1.8 K) and thermal cycles. Bath temperature and sensor resistance are monitored on-line. This paper presents the latest results of this program

Amand, J F

Casas-Cubillos, J

Junquera, T

Thermeau, J P

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLarge Hadron Collider ProjectLHC Project Report 209Neutron Irradiation Tests in Superfluid Helium of LHC Cryogenic ThermometersJean-François Amand1, Juan Casas-Cubillos1, Tomas Junquera2 and Jean-Pierre Thermeau2AbstractFor control and monitoring purposes, about 10,000 individually calibratedcryogenic temperature sensors will be installed along the 26.7 km LHC. In orderto reduce maintenance constraints these sensors should be as immune as possibleto the high neutron fluence environment. For selecting the sensor to be used, aradiation hardness evaluation program at cryogenic conditions is being performedin an irradiation vault of the ISN SARA Cyclotron (Grenoble, France). The set-upis capable of simulating the whole life of a LHC thermometer: same total neutrondose (1015 n.cm-2), irradiation at low temperature (1.8 K) and thermal cycles. Bathtemperature and sensor resistance are monitored on-line. This paper presents thelatest results of this program.1 CERN, LHC Division, CH-1211 GENEVA 23, Switzerland2 Institut de Physique Nucléaire, CNRS-IN2P3, 91406 ORSAY, FrancePresented at ICEC 17, 14-17 July 1998, Bournemouth, UKAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandGeneva, 29 July 19982Neutron Irradiation Tests in Superfluid Helium of LHC Cryogenic ThermometersJean-François Amand1, Juan Casas-Cubillos1, Tomas Junquera2 and Jean-Pierre Thermeau21 CERN, LHC Division, CH-1211 GENEVA 23, Switzerland2 Institut de Physique Nucléaire, CNRS-IN2P3, 91406 ORSAY, FranceFor control and monitoring purposes, about 10,000 individually calibratedcryogenic temperature sensors will be installed along the 26.7 km LHC. In orderto reduce maintenance constraints these sensors should be as immune as possibleto the high neutron fluence environment. For selecting the sensor to be used, aradiation hardness evaluation program at cryogenic conditions is being performedin an irradiation vault of the ISN SARA Cyclotron (Grenoble, France). The set-upis capable of simulating the whole life of a LHC thermometer: same total neutrondose (1015 n.cm-2), irradiation at low temperature (1.8 K) and thermal cycles. Bathtemperature and sensor resistance are monitored on-line. This paper presents thelatest results of this program.1 INTRODUCTIONThe future Large Hadron Collider (LHC) at CERN needs a huge cryogenic system to maintain themagnets temperature at 1.9 K along the ring. Proton losses in the machine (produced by interaction withthe residual gas in the beam pipe or by collisions with the beam pipe) will result in a high neutron fluence.Simulations [1] for the LHC arc regions give average neutron dose values ranging from 102 to 103 Gy forthe 20 years of machine operation, assuming that the sensors are positioned on the cold mass at 57 cmfrom the beam axis. In other regions of the collider, doses can be significantly higher and furthercalculations are necessary. Sensors will also endure warm-up cycles to 300 K during an LHCmaintenance shutdown. Ideally, the thermometer precision must remain better than 0.25% (5 mK at 2 K)during the whole machine lifetime. The main objective of the irradiation campaign is to expose thethermometers to environmental conditions as close as possible to those expected in the LHC: similarneutron energy spectra (1 MeV-10 MeV) and integrated dose, and irradiation at superfluid heliumtemperature. To study the influence of a thermal cycle between 1.8 K and 300 K a cryostat warm-up hasbeen performed during three experiments and furthermore some sensors were irradiated in successiveexperiments. Ten models of commercial sensors from various manufacturers have been tested. They canbe classified into five families: carbon resistors (Allen-Bradley (AB) and russian TVO), CERNOXTM fromLakeShore Cryotronics (CX), platinum (Pt), rhodium-iron (RhFe) thin-film and wire-wound andGermanium (Ge) thin-film. In addition, some test samples have been irradiated: these are metals or alloysthe behaviour of which under irradiation is well documented. These samples allow us validate our results.Since 1996, eight experiments have been performed (the first in liquid argon, the second [2] in liquidhelium and the last six in superfluid helium) and 281 thermometers have been tested.2 EXPERIMENTAL FACILITY2.1 Irradiation aspectsThe neutron beam is produced by the 9Be(d,n)10B reaction. A deuteron beam (20 MeV, iβ7 µA) hits a thinberyllium target and the gamma dose induced by this reaction is low: about 20% of the total energyemission. The sensors under test are immersed in a cryogenic bath that can be either liquid argon orhelium. The minimum distance between the thermometers and the neutron source (beryllium target) isabout 8 cm (see figure 1). To investigate the effects of a variable total dose, sensors are placed at differentdistances from the source. The total neutron dose is measured for each thermometer by using a smallnickel foil (diameter: 4 mm, thickness: 0.125 mm) attached to it [3]. From the nickel foil activation data, aneutron dose map can be calculated at the end of the experiment (see figure 2). The average dose received3by the sensors is about 20 kGy and the total neutron dose is 1015 n.cm-2. In addition, the deuteron currenthitting the beryllium target is measured during the irradiation and a simple normalisation gives theneutron dose at any time during the experiment.2.2 Cryogenic aspectsThe irradiation cryostat is composed of a cold vessel (diameter 27 cm, height 60 cm), a radiation heatshield and an outer vacuum vessel. The heat load to the cold vessel is about 200 mW, plus 20 mW whenthe beam is on. The bath is re-filled daily with 4.2K liquid helium, then cooled down by pumping and itstemperature is regulated by controlling the saturation pressure with a throttling valve. The referencetemperature is obtained either with a helium vapour pressure bulb (volume 0.3 cm3) or by measuring thebath pressure, both in principle insensitive to ionising radiation. The bulb is immersed in the liquid bath,at the same height as the resistive sensors and a warm sensor connected through a capillary measures thepressure. At 1.8 K, the measured bath temperature stability is σT = 0.023 mK, including the thermal noiseand the pressure measurement incertitude.2.3 Acquisition and measurement systemsThe measurement system is composed of a nanovoltmeter, a programmable current source (current rangesfrom 1 µA to 1 mA depending on the resistor type) and a scanner. The acquisition cycle duration is about7 minutes and the temperature sensors are measured in a 4-wire configuration. In order to eliminatethermocouple effects and current source inaccuracies the voltages across the resistive-type sensors aremeasured with a current reversal technique and the current is measured with a high precision shuntresistor. Other parameters recorded by the acquisition system include the bath and bulb saturationpressures, the deuteron current hitting the target and various monitoring devices to verify the properoperation of the instruments. The measured relative resolution with this acquisition system is σR/R=10-6.Figure 1  Irradiation cryostat Figure 2  Neutron dose repartition at 8.5 cm from target43 PROCEDUREBefore irradiation, the thermometers are calibrated between 1.6 K and 4.2 K (about 15 calibration points),furthermore around the irradiation temperature another five data points with 5 mK interval are recorded.During irradiation the bath temperature is maintained at 1.8 K and all sensors are periodically recorded.When about half the expected total dose is reached, the beam is stopped, the cryostat is warmed-up to 300K and re-cooled down to 1.8 K in order to observe possible annealing effects. At the end of the irradiationcampaign the sensors are recalibrated.4 RESULTSAll the thermometers under investigation are resistive and they can be classified into two families:metallic (Pt and RhFe) and semiconductor (AB, TVO, CX, Ge) types. The results are summarised inTable 1.4.1 Carbon type temperature sensorsTwo carbon type sensors have been tested: TVO sensors [4] and AB resistors. For both types a beamheating effect is observed and the apparent temperature increases proportionally with the neutron doserate. For AB and TVO the heating is respectively ∆T = Φ × 9.0-10 mK and ∆T = Φ × 3.10-10 mK (thesesvalues are given with a 20% incertitude; Φ is the neutron dose rate given in n.cm-2.s-1). Neutron dose rateheating effects in the LHC should be negligible because the dose rate is several orders of magnitude lowerthan that used in these experiments. Figure 3 shows the irradiation shifts versus neutron dose afterdeducing beam heating.Sensors of this type show an exponential increase in the measured temperature (resistancedecrease) until reaching saturation at a total dose of 5.1013 n.cm-2 for AB and 2.1014 n.cm-2 for TVO. ForAB sensors, the temperature measurement shift value at saturation depends on the neutron dose rate; it is1mK for a dose rate Φ = 3.108 n.cm-2.s-1 and 2 mK for Φ = 109 n.cm-2.s-1. For TVO the temperaturemeasurement shift at saturation range from 0.1 mK to 0.5 mK. Low temperature annealing was observedfor AB: when stopping the irradiation while maintaining the bath temperature at 1.8 K, it is possible toobserve that the sensors recover their initial resistance with a time constant of 3 to 10 hours. The 300 Kannealing produced no significant healing.4.2 Metallic type temperature sensorsBoth Pt and RhFe thermometers have been studied. Neutron irradiation induces an increase oftemperature reading that is proportional to the neutron dose (see figures 3 and 4). RhFe show adependency on the fabrication technology and from a radiation hardness point of view wire wound isbetter than thin film, the temperature measurement increase is respectively DOSE × 1.25 10-14 [mK](±20%) and DOSE × 3 10-14 [mK] (±20%). 300K annealing induces a 40 to 50% recovery of the initialsensor characteristics. For Pt a similar behaviour is observed, at 1.8 K we observe ∆R/R = DOSE × 2.50.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 1E+14 2E+14 3E+14Neutron dose (n/cm2)∆∆T(K)CernoxRhFe WireRhFe Thin FilmTVOAB0.00.20.40.60.81.01.21.41.61.82.00 24 48 72 96 120Time (hours)∆∆T(K)Figure 3  Error on temperature measurement on some Figure 4  Irradiation-induced error on temperature read-out  sensors during irradiation (Tbath=1.8 K)  for  Pt and Ge (T=1.8 K, dose=6 1014 n.cm-2)PlatinumGermanium510-17 (DOSE in n/cm-2) and the induced temperature measurement shift is very high because of the poorsensitivity of Pt at these temperatures.4.3 Germanium thermometersGe thermometers were the most sensitive to neutron radiation with a temperature measurement shift ofabout 300 mK. Saturation was observed at a total dose of 1014 n.cm-2 (see figure 3). Curiously during thethermal cycle to 300 K the annealing was very efficient and the sensor recovered its characteristics towithin 98%. Such a behaviour can only be observed if the sensor is at cryogenic conditions and monitoredon-line during the irradiation.4.4 CernoxTMThe CX sensor total temperature measurement shift range from +0.5 mK to +1.5 mK (doses between3.1014 and 1015 n.cm-2). The temperature shift and the neutron dose could not be correlated for this sensortype. The beam-heating effect is almost non-measurable and is of the order of 0.2 mK for 2 109 n.cm-2.s-1.No annealing has been observed for this sensor type.Thermometer(+number tested)R @ 1.8K dR/dT @ 1.8K σT @ 1.8K beam heatingmK/(n.cm-2.s-1)∆T Irradiationfor 4 1014n.cm-2Expected ∆T inLHCAB  (44) 6600 Ω -10600 Ω.K-1 8.10-5 9 10-10 +2 mK < 2 mKTVO  (44) 5700 Ω -3300 Ω.K-1 3.3 10-5 3 10-10 +0.3 mK < 0.5 mKCX  (66) 12600 Ω -12000 Ω.K-1 2.5 10-5 10-10 +1 mK < 2 mKGe  (5) 9000 Ω -8000 Ω.K-1 1.2 10-4 0 +300 mK +300 mKRhFe thin-film (46) 15 Ω +0.7 Ω.K-1 3.10-5 0 +12 mK +3 mK/yearRhFe wire (36) 5.4 Ω +0.6 Ω.K-1 2.6 10-5 0 +5 mK +1.5 mK/yearPt  (22) 1.7 Ω +3.5 10-4 Ω.K-1 - - +1.5 K -Table 1   Results of irradiation at 1.8 K (average values)5 CONCLUSIONSThe results of this irradiation programme give a good overview of radiation resistance of resistive-typetemperature sensors. About 50 sensors of each type have been tested in several experiments, the resultsare reproducible and can be qualitatively explained by experts in radiation damage of materials. Themetallic type and Ge thin-film sensors show the highest errors. It is very important to perform themeasurements on-line and at low temperature in order to understand self-healing processes duringannealing at room temperature.Future plans for irradiation testing under similar conditions are foreseen for other cryogenicsensors such as level, pressure and helium flow gauges. Due to the SARA cyclotron dismantling (fromthe end of 1998), the irradiation cryostat will be moved to the CERI cyclotron in Orléans (France).6 ACKNOWLEDGEMENTSWe would like to thank the following people for their contribution to the experiment: C.Joly, F.Chatelet,P.Bujard (IPN, Orsay); J.Collot, M.Fruneau, P.Martin, G.Mondin and all the cyclotron group (ISN,Grenoble). We also want to thank C.Balle, P.Lebrun, M.Tavlet (CERN), V.Datskov (JINR, Russia) andL.Thomé (CSNSM, Orsay) for their support and advises.7 REFERENCES1 Potter, Schonbacher and Stevenson. CERN-LHC report #18, Nov. 19952 Junquera, Amand, Thermeau and Casas-Cubillos. CERN-LHC Project report #153, Nov. 19973 Collot,J. and al. Nuclear Instrument and Methods A (1994) 350 5254 Datskov,V.I. and Weisend II, J.G. Cryogenics (1994) 34 425

Neutron Irradiation Tests in Superfluid Helium of LHC Cryogenic Thermometers

Abstract

Similar works

Full text

Available Versions

CERN Document Server