Marc Tavlet

Comparison of high-dose dosimetry systems for radiation damage studies in collider detectors and accelerators

Abstract Measurements of absorbed dose in accelerator tunnels around primary beam areas are carried out on a routine basis at CERN. Dosimetric surveillance of high-energy particle accelerators has a great importance for the assessment of the radiation induced damage to materials and components used in high-level radiation areas. Standard dosimeters used at CERN for this purpose are polymer-alanine dosimeters (PAD) and radiophotoluminescent glass dosimeters (RPL). Ethanol-chlorobenzene dosimeters (ECB) for high-dose dosimetry, developed at the Ruder Boskovic Institute (RBI), have several interesting properties making their use in future multi-TeV colliders and detectors promising. These and RPL dosimeters were compared using the CERN alanine dosimetry as the reference system and 60 Co gamma rays as the reference radiation. A very good agreement between the ECB and PAD was obtained for 60 Co gamma irradiation whereas RPL overestimated the dose by about 15%. In mixed accelerator radiation fields the combination of the three dosimeters opens the possibility to estimate the total dose and the quality of the principal radiations contributing to the total radiation field.

PSAIF: the PS-ACOL irradiation facility at CERN

Since the end of 1990, CERN has been operating an irradiation facility behind the antiproton production target. The target is hit by 26 GeV protons from the PS (proton synchrotron); the antiprotons are collected by the ACOL machine (antiproton collector) The strong emission of secondary high-energy particles allows the irradiation of small items, in an environment typical of the particle accelerators and of future multi-TeV detectors. On-line measurements can be carried out during the irradiation. Dosimetry, as well as neutron-flux measurements by activation detectors, were carried out. The irradiation facility, as well as the dosimetric methods, are described and the results are given.<<ETX>>

AGING OF ORGANIC MATERIALS AROUND HIGH-ENERGY PARTICLE ACCELERATORS

Abstract Around particle accelerators used for fundamental research on the basic structure of matter, materials and components are exposed to ionizing radiation caused by beam losses in the proton machines and by synchrotron radiation in the lepton machines. Furthermore, with the high-energy and high-intensity collisions produced from future colliders, radiation damage is also to be expected in particle-physics detectors. Therefore, for a safe and reliable operation, the radiation aging of most of the components has to be assessed prior to their selection. An extensive radiation-damage test program has been carried out at CERN for decades on a routine basis and many results have been published. The tests have mainly concentrated on magnet-coil insulations and cable-insulating materials; they are carried out in accordance with the IEC 544 standard which defines the mechanical tests to be performed and the methods of degradation evaluation. The mechanical tests are also used to assess the degradation of composite structural materials. Moreover, electrical properties of high-voltage insulations and optical properties of organic scintillators and wave guides have also been studied. Our long-term experience has pointed out many parameters to be taken into account for the estimate of the lifetime of components in the radiation environment of our accelerators. One of the main parameters is the dose-rate effect, but the influence of other parameters has sometimes to be taken into account.

Dose and neutron-fluence measurements in mixed gamma-neutron fields by means of semi-conductor dosimeters

Semiconductor detectors (PIN diodes and MOS transistors) were irradiated around high-energy particle-accelerators. The radiation fields contain mainly gammas and neutrons; they are characterized by means of alanine and RPL dosimeters and activation foils. PIN diodes are sensitive to neutrons, MOS transistors are sensitive to gammas; their responses are compared to the ones given after reactor- and cobalt-irradiations respectively. These comparisons show that it is possible to discriminate the gamma and the neutron components of the irradiation. The fading of the various sensors is also recorded.<<ETX>>

Radiation-damage measurements on PVT-based plastic scintillators

Abstract Samples of PVT-based plastic scintillators produced by Nuclear Enterprise Technology Ltd. (NET) were irradiated up to 9 kGy, both with a gamma source and within a typical accelerator radiation field (CERN PS ACOL Irradiation Facility). The consequent reduction of scintillating efficiency and light transmission were measured, as well as subsequent recovery, over a period of several months. The main results show that irradiation affects more the light transmission than the light emission. The radiation type does not affect either the amount of transmission reduction or the recovery. Observations were also made by means of polarized light. Non-uniformities and internal stresses were observed in scintillator bulks which were polymerized too quickly. These defects influence the light transmission.

Effects of ageing of electrical insulators after exposure for 11 years in the large electron–positron collider at CERN

Abstract All polymer-based materials used as electrical insulators in the large electron–positron collider (LEP) at CERN, were qualified for their radiation behaviour. The selection was based on mechanical tests, according to the recommendations of the IEC 60544 Standard. The lifetime of these insulators was assessed also taking into account the expected “dose-rate effect”, following the recommendations of the IEC 61244 Technical Report. During the decommissioning of LEP, samples have been taken out from equipment that had a full life exposure. They were tested according to the same mechanical tests and the results are compared with the expectations. The results show that lifetime estimate is sometimes hazardous over such long periods of time.

Qualification of coolants and cooling pipes for future high-energy-particle detectors

Abstract In the next generation of high-energy-particle detectors to be installed at the Large Hadron Collider (LHC) at CERN, materials and components will be exposed to a significant level of ionising radiation. Silicon detectors and related electronics will have to be cooled down to −20 °C and therefore appropriate cooling fluids and cooling pipes have to be selected. Analytical methods such as UV–visible and FT–IR spectrometries, electronic microscopy and gas chromatography were used to characterise the radiation-induced effects on some organic coolants irradiated with both gamma and neutron fields. Some impurities were identified as a major source for radio-induced polymerisation and also for hydrofluoric acid (HF) evolution. Mechanical tests were performed to assess the operability of the rubber hoses and plastic pipes. Possible synergistic effects between the pipe material and the environment had to be considered.

Behaviour of organic materials in radiation environment

Radiation effects in polymers are reminded together with the ageing factors. Radiation-ageing results are mainly discussed about thermosetting insulators, structural composites and cable-insulating materials. Some hints are given about high-voltage insulations, cooling fluids, organic scintillators and light-guides. Some parameters to be taken into account for the estimate of the lifetime of components in radiation environment are also shown.

Shear tests on adhesives for magnet collars for the LHC

Thermal tests and radiation tests have been carried out on four epoxy adhesive systems which cure at room temperature. Results show that one system is suitable for application at 1.8 K in the radiation environment of the future Large Hadron Collider to built at CERN.

Evaluation of the radiation resistance of electrical insulation materials

Abstract The qualification of insulating materials for electrical cables is often accomplished according to the IEC 60544 standard of the International Electrotechnical Commission. The mechanical properties of the polymeric insulators are tested prior and after irradiation at relatively high dose rates. To assess the ageing of selected materials under realistic service conditions, usually at lower dose rate, an IEC Working Group has proposed extrapolation methods (IEC 61244-2), one of which is applied here for a cable sheathing material from Huber+Suhner. The method is found to be suitable to compare radiation resistance data of different materials irradiated under different conditions.