Performance Requirements for Monitoring Pulsed, Mixed Radiation Fields around High-energy Accelerators

RADIATION FIELDS AND MONITORS In preparation of the installation of a radiation monitoring system for the future LHC and its injectors comprehensive studies were performed to evaluate the suitability of different existing monitors for radiation protection measurements in pulsed and complex, mixed radiation fields. Different ionisation chambers were exposed to a mixed, high-energy reference field and it was shown that the results agreed well with Monte-Carlo calculations. In addition the chambers were exposed to short, high-intensity radiation pulses, where recombination effects and the capability of the electronics to process a high number of charges within very short time were studied in detail. These results are being used to optimize the design of the read-out electronics. Moreover, the results demonstrate that the investigated chambers are well suited to measure ambient dose equivalent and dose rates in these fields up to certain limits from where on recombination corrections should be taken into consideration. The so-called prompt, mixed radiation fields around high-energy accelerators are composed of charge d hadrons (protons, pions, kaons, etc.), neutrons, leptons (electrons, positrons, muons) and photons. The energies of these particles range from fractions of eV to several GeV. The particle and energy composition of the fields at a given point in or outside the accelerator tunnel depends strongly on its position with respect to the location of th e beam loss and the shielding used. As a general ru le radiation fields around high-energy accelerators are similar to cosmic radiation fields in the atmosphere. The time behaviour of the radiation fields follows the tim e structure of the pulsed particle beam. The pulse length s depend on the accelerators and their various beam modes and range from some few nanoseconds to seconds wit h repetition times of some tens of nanoseconds up to several tens of seconds. A thorough market survey revealed that no detector system from the shelf will be able to fulfil the special CERN needs. The radiation monitors that are in use at CERN’s existing accelerators seem to be the mo st promising candidates for a reliable radiation monitoring of pulsed, mixed radiation fields. The detectors are ionisation chambers equipped with special, read-out electronics developed in-house. In a first step three types of monitors were studied in detail: a 3 litre, non-confine d air ionisation plastic chamber (PMI, manufactured by PTW, Type 34031), operated under atmospheric pressure and 5.2 litre steel chambers filled with 20 bar of hydrogen or argon (Centronic IG5-H20 or IG50-A20). INTRODUCTION The monitoring of ionising radiation around highenergy accelerators like the Large Hadron Collider (LHC) and its injectors represents a major technical and metrological challenge due to the complex composition of the radiation field and the time structure of the pulsed particle beams. The radiation monitoring system RAMSES # [1] foreseen for LHC and its injectors has to meet these very specific physics requirements as well as the regulatory requirements of current radiation protection legislations. A state-of-the-art system has to be installed with the performance suitable for future certification by an authorised metrological institution. PHYSICS PERFORMANCE OF IONISATION CHAMBERS IN PULSED, MIXED RADIATION FIELDS Therefore it is essential to understand the response of the monitors to the various radiation fields and to demonstrate that the relevant quantities like ambient dose equivalent and ambient dose equivalent rate are reliably measured. At present, international standards exist only for monitors in conventional radiation fields, the outcome of the study will be provide an input into setting standards for ambient dose equivalent measurements in pulsed, mixed radiation fields around high-energy accelerators. The ionisation chambers were exposed to various stray radiation fields at CERN. The comparison between experiments and Monte Carlo simulations for the detector response was performed at the CERN-EU High Energy Reference Field Facility CERF [2], the influence of th e ion recombination mechanism on the detector response was studied in the target area of the Antiproto n Decelerator (AD) and close to the beam dump of the Proton Synchrotron Booster (PSB). This paper gives an overview on the studies of the radiation monitors envisaged to be used by RAMSES for the radiation protection part; a description of the overall RAMSES can be found in reference [1]. * [email protected] # RAMSES: Radiation Monitoring System for Environment and Safety Proceedings of EPAC 2004, Lucerne, Switzerland