Personal dosimetry in a mixed field of high-energy muons and neutrons.

High energy accelerators quite often emit muons. These particles behave in matter as would heavy electrons and are thus difficult to attenuate with shielding in many situations. Hence, these muons can be a source of radiation exposure to personnel and suitable methods of measuring the absorbed dose received by these people is obviously required. In practical situations: such muon radiation fields are often mixed with neutrons, well-known to be an even more troublesome particle species with respect to dosimetry. In this paper, we report on fluence measurements made in such a mixed radiation field and a comparison. of dosimeter responses. We conclude that commercial self-reading dosimerers and film badges provide an adequate measure of the dbsorbed dose due to muons. INTRODUCTION At high energy proton acceierator laboratories. radiation exmosure due to ..~ muons is often quite significant. The properties of such muon -fields have been studied at Fermilab using scintillation telescopes (1, 2). The muon intensity has been observed to be peaked in the direction of the incident proton beam (350800 GeV) in a cone typically iess thsn six degrees FWHM. The nature of the high energy physics exueriments that use particie beams incident on targets at rest in the iabo:atory frame of reference requires that the majority of the experimentai apparatus be placed in, or very near, this cone. The signal cables leading from these particle detectors to areas of significant occupancy by experimenters are often restricted in length by puise quality and delay considerations. Since muons, which have a rest energy of 105.7 MeV, behave in matter as would eiectrons of such iarge mass: they have very long ranges (e.g., about 700m of soil at 400 GeV). It is, therefore, generaliy impractical to use shielding to reduce dose equivaient rates in nearby areas. In specialized cases. magnetic fields are effective in reducing the muon fmence by deflection, but this technique is limited by the expense of magnets sufficientiy iarge to deflect muons with momenta typically of the order of tens of GeV/c. Tnus, high occupancy aress are sometimes locations of significant muon fluence. It is, therefore, necessary to be abie to accurately assess the dose equivaients received by individuals in muon fields having pooriy known energy spectra. This note presents a comparison of the response of dosimeters in a radiation field that is a mixture o:* muons and neutrons. PROPERTIES OF THE RADIATION FIELD Figure 1 shows the geometry at the location where measurements were carried out. The muons arise primarily in the decay of pions formed by the interaction of 800 GeV protons from the Tevatron in two tungsten targets. The radiation field at the location of the dosimetry measurements also includes neutrons emerging from the targets, beam dump, and associated iron and concrete shielding. The dosimetry measurements were made in a plane 4.3m above that of the beam. This location was not a high occupancy area but was chosen because the radiation field in this area, although of higher intensity, is similar in composition to that at locations typically occupied by personnel. ~“*ilo,, YlEW IN IHE PimE w THE SEtM Figure 1: Geometry of the source of muons and the dosimezry measurements. The longitudinal scale differs from the transverse scale for both the plan and and ’ elevatior views. The characteristics of the radiation field at this location were determined from measurements performed with various detectors which were mounted in a vehicle equipped with counting electronics. The neutron fiuence as a function of energy was measured by a Banner multisphere spectrometer (,3? 4). Th% aetecto:, calied a phoswich, consists of an &mm diameter by Fxnm longscintillation crystal embedded in a LiI(Eu) scintillator (5). It is placed at 12.7mm diameter by 12.7mm iong p&tic the center of one of a set of moderating polyethylene spheres of various diameters? and inserted into the radiation field; the procedure has been described a number of times (6, 7). ‘The peak associated with neutrons in the pulse-height spectrum is clearly discernible above the charged particie background events even though large muon fiuences are also observed (see beiow). In these measurements, the counting electronics were Fated-on during the 23 second beam spill, which occurred 0% iapproximateiy) 60 second accelerator cycie, and a orecisior. during each BF, long counter (8) provided the reiative normal&&ion for the indi;idual Banner gphere measurements. A threshold set on the long counter output rendered it insensitive to muons and 7 rays. If the energy-dependent multisphere response functions are known (9), the neutron fiuenct a~ a function of energy (i.e.; the neuxon spectrum) can be obtained from measured Banner sphere counting rates by unfoiding methods (see, e.g., 10). To gain some confidence ir the reasonableness of the unfolded specxum, we have used two programs, SWIFT (11: 12, 13), based on Monte Cario techniques, and BUNK1 (14: 15) an iterative recursion method. The unfoided specxa associated with a good fit to the data are shown in Fig. 2, piotted as fluence per unit logarithmic energy interval. The results from