Fehrenbacher, G.

Kozlova, E.

Radon, T.

Sokolov, A.

GSI Repository

ANDREA, a compact neutron dosimeterA. Sokolov1, G. Fehrenbacher1, E.Kozlova1, and T.Radon11GSI, Darmstadt, GermanyAccelerator facilities are widely used for research andmedical treatments, still they are sources of radiation po-tentially harmful for the surrounding environment and peo-ple. For the radiation survey the dose rates inside the accel-erator tunnels as well as outside the shielding must be mon-itored. The highest contribution to the dose rate especiallyin case of high energy and heavy ion acceleration comesfrom neutron radiation, meaning that the neutron dose de-tection should be of highest priority. At GSI the approvedtechnique for this purpose implies the GSI ball [1], appro-priate for the ambient dose equivalent measurements forneutrons from 10 meV up to 100 GeV energies. Neverthe-less this passive dosimeter is quite heavy and large with adiameter of D=31cm. In order to decrease the dimensions,weight and production costs, as well as to improve the char-acteristics of the dosimeter this work was done.The main principle of the dosimeter to provide the read-out values close to the ambient dose equivalent [2] is to usethe moderating outer layer, typically – Polyethylene (PE)or Polypropylene (PP) for lower neutron energies and touse a lead layer in order to improve the response of the de-tector for higher energies, at which the spallation in form ofneutrons ejection takes place. Next to the lead layer there isan additional small moderating layer (PP/PE) with a TLD-card, TLD=ThermoLuminescence Dosimeter.Figure 1: Dosimeter of type 3 (left) and type 5 (right).Numerous simulations were performed using theFLUKA code [3] in order to obtain a simple and com-pact design of the dosimeter. First of all the dosimeterwas chosen in a form of a cylinder (Fig.1, left), which stillhad the height of 31cm, but the diameter is only 20 cm.The smaller diameter for this type 1 dosimeter, is possibledue to the introduction of an additional rubber layer witha small amount of boron, 5 % of the total mass. One canfind the read-out signal in Fig.2, where it is seen that theresponse function of the dosimeter is close to the desiredfluence-to-dose values H*(10) presented in [2]. To improvethe response in the higher energy region, especially around10 MeV, the outer dimensions were kept, while the innercylinder was increased in diameter providing better mod-eration in the region of interest. The result is the type 2detector, see Fig.2. Decreasing the outer diameter signif-icantly influences the response in the low energy region,leading to an overestimation of the dose. This can still bemanaged changing the concentration of boron in the rub-ber layer. For the type 3 detector, with a diameter of 16 cmthe boron portion was increased to 10 %. The improvedresponse function can be seen in Fig.2.Figure 2: Ambient dose and comparison of response func-tions for different types of dosimeter.Still the presence of the rubber layer makes the designmore complicated and construction costs higher. To getrid of the rubber another material was chosen for the outerlayer - Polyvinylchloride (PVC), which is a cheap and al-most non-flammable plastic. In Fig.2 one can see the re-sponse functions for the dosimeter of type 4 with differentouter PVC diameters from 16 cm down to 10 cm. Usageof the PVC as a moderating material together with a big-ger diameter of the inner cylinder provides a possibility toreduce the height of the lead cylinder and get rid of its bot-tom and top caps, making the design simpler (Fig.1, right),the dosimeter is distinctly lighter and the over-response inthe high energy region smaller (Fig.2, type 5). So finally incomparison to the GSI ball the weight is reduced from 19kg to 4.5 kg and the size from 31 cm to 14 cm, both dosime-ters can be seen in Fig.3. The compact design reduces theMU-SAFETY-02 GSI SCIENTIFIC REPORT 2014386 DOI:10.15120/GR-2015-1-MU-SAFETY-02Figure 3: Neutron Dosimeters: ANDREA, type 5 (left) andGSI ball (right).overall absorption and increases the total signal. Still aslight under-response in the vicinity of 10 MeV and over-response in the low energy region remain and are the issuesfor further improvements. The name of this type 5 dosime-ter is ANDREA=A Neutron DosemeteR for Efficient Areamonitoring.References[1] G. Fehrenbacher et al, Radiation Protection Dosimetry(2007), Vol. 125, No. 1–4, pp. 209–212 Advance Access pub-lication 16 September 2007.[2] M. Pelliccioni, Radiation Protection Dosimetry Vol. 88, No.4, pp. 279–297 (2000) Nuclear Technology Publishing.[3] A. Fasso et al, ”FLUKA: a multi-particle transport code”,CERN-2005-10 (2005), INFN/TC 05/11, SLAC-R-773.GSI SCIENTIFIC REPORT 2014 MU-SAFETY-02DOI:10.15120/GR-2015-1-MU-SAFETY-02 387

ANDREA, a compact neutron dosimeter

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ANDREA, a compact neutron dosimeter

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