24 th ICNTS Design and test of an albedo personal neutron dosemeter based on PADC detectors R. Bedogni a, A. Esposito a, G. Gualdrini b, R. Mishra c, S. P. Tripathy c a INFN–LNF Frascati National Laboratory, via E.Fermi n. 40, Frascati, Italy b ENEA-Radiation Protection Institute, Via dei Colli n. 16, Bologna, Italy c BARC - Bhabha Atomic Research Centre, Trombay, Mumbai, India

24 th ICNTS ● Introduced in late 60s, the albedo dosimetry technique relies on the detection of the neutrons thermalized and back-scattered by the human body. ● Due to the availability of high sensitivity techniques for the measurement of thermal neutrons, the detection of low energy neutrons back-scattered from the body is in principle easier than the direct detection of fast neutrons. ● Together with etched-track detectors (mainly PADC) and superheated emulsions, albedo dosemeters are among the most popular fast neutron personal dosemeters. Introduction

24 th ICNTS ● Albedo dosemeters are typically based on pairs of 6 LiF / 7 LiF based thermoluminescence detectors. Advantages are: high sensitivity to thermal neutrons, reusability, simple read out, low cost. ● Combined TLD-albedo (<0.1 MeV) / track detectors (0.1 – 20 MeV) techniques have been proposed to obtain reasonably flat energy response. Main problems: ● The high photon sensitivity of TLDs causes large uncertainties, especially in workplaces with dominant photon component. ● Complex equipment needed: chemical lab, track reader, TLD reader. ● The response in terms of H p (10) shows an important energy dependence (it decreases with the energy and falls dramatically above 0.1 MeV), ● The variability of the response with the angle of incidence is usually very limited. Introduction

24 th ICNTS This work presents the study of an albedo dosemeter based on a planar PADC covered by a 10 B screen. This will constitute a low-energy section to be combined with a directly exposed PADC (high-energy section) to form a wide energy range personal dosemeter (photon insensitive, only PADC equipment needed). The energy and angle dependence of the response was investigated through extensive simulations (MCNP 4C) and irradiation at different reference fields ( 241 Am-Be, 252 Cf and 252 Cf(D 2 O)) and angles of incidence (0°, ±20°, ±40°, ±60°) in the calibration laboratories of ENEA- Bologna and INFN Frascati. Introduction

24 th ICNTS Detectors: PADC (0.1% DOP) 2.5 cm x 3.5 cm x 0.14 cm from Intercast Europe (Parma, Italy). Converter:BE10 from Kodak (99% 10 B, thickness 50 m, density 1.45 g.cm -3 ) Etching:90 minutes in KOH with Normality 6.25 N at 70°C Read out:semi automatic reader developed at INFN-LNF, total scanned area 1.92 cm 2. Resolution 2.5 m/pixel All irradiations were perfomed in terms of Hp(10,) on ISO water filled phantom. Room- and air-scattered radiation was taken into account with ISO techniques (ISO ). Experimental set up

24 th ICNTS The nine dosemeters were individually modeled. Simulations were performed for all neutron fields and angles of incidence used in the experiment. In addition, a series of mono-energetic neutron beams plus thermal neutrons were simulated to predict the energy dependence of the response. The number of (n,) reactions in the converter was expected to be proportional to the measured track density on PADC. Total and phantom-scattered neutron spectra were calculated at the entrance of the boron converter. Simulations

24 th ICNTS Total and phantom-scattered neutron spectra Pure thermal 252 Cf(D 2 O) 252 Cf 241 Am-Be

24 th ICNTS SF = percentage of (n,)  reactions in the converter due to neutrons back-scattered from the phantom (estimated with a self flagging card) Energy dependence of the fluence response

24 th ICNTS = number of (n,) reactions induced in the converter per unit incident neutron fluence. Energy dependence of the fluence response

24 th ICNTS Energy dependence of response in terms of H p (10)  H = number of (n,) reactions induced in the converter per unit incident neutron fluence.

24 th ICNTS Comparison experiment / simulations Standard deviation 5% Average value = 714±10 Ratio simulation/experiment for all neutron fields and angles of incidence

24 th ICNTS Other dosimetric performances Detection limit DL, Dose level at which a known fraction of false negatives may occur – 5% in this work): 40 Sv for Am-Be (conservative) Angle dependence of the response The average of the responses a different angles differs by less than 30% from normal incidence. Criterion ISO satisfied.

24 th ICNTS ● The response of a PADC based albedo personal dosemeter was investigated with MCNP 4C and experimental tests in reference neutron fields with different angles of incidence. Good agreement (within 5%) was found between calculations and measurements. ● As expected, the energy dependence of the response is very large from thermal to MeV neutrons. ● The angle dependence of the response is very limited. ● The achievable DL in workplace fields may be lower than 40 Sv The studied configuration is a good candidate as “low energy section” of a combined dosemeter (albedo + direct detection) fully based on PADC and photon-insensitive Conclusions