DESIGN OF NEUTRON DOSE RATE METER FOR RADIATION PROTECTION IN THE EQUIVALENT DOSE Hiroo Sato 1 and Yoichi Sakuma 2 1 International University of Health and Welfare, Kitakanemaru 2600-1, Ohtawara 324-8501 Japan 2 National Institute for Fusion Science, Oroshic-cho 322-6, Toki 509-5292 Japan Abstract A neutron dose rate meter designed for radiation protection in the equivalent dose given by ICRP based on a new concept. The instrument measures the neutron dose rate in the unit of µsv/hour with approximately correct equivalent dose response in the energy range from thermal to 20MeV without moderator. The instrument is a complete measuring assembly consisting of a neutron detection unit and of an electric unit and a memory unit combined together. The detection unit consists of a NE-213 or BC-501A liquid scintillator and of high neutron sensitivity surrounded with liquid scintillator by double layer pressurized 3 He ionization chamber without moderator. The pulses from the liquid scintillator are led to a circuit of n-γ discrimination by the pulse shape method or by the digital charge comparison method. The pulses from each layer of 3 He ionization chambers are amplified in the charge sensitive amplifier. The process pulses are indicated on display unit in counting rates from each detector respectively and stored by memory. The memory pulses are analyzed with separate adequate energy ranges by a certain computer. 1.INTRODUCTION This report describes neutron dose rate meter designed for radiation in the equivalent dose given by ICRP. The instrument measures the neutron dose rate in the unit of µsv/hour with approximately correct equivalent dose response in the energy range from thermal to 20MeV without moderator. The instrument is a complete measuring assembly consisting of a neutron detection unit and an electric unit and combined together. 2.EQUIVALENT DOSE The fundamental dosimetric quantity in radiological protection is the absorbed dose D. Furthermore, ICRP recommended to evaluate in the equivalent dose. The average and R radiation dose D T,R in tissue T is given by the expression H T = Σω R D T,R Where H T is symbol of equivalent dose, ω R the radiation weighing factor. For neutron, radiation weighting factors were given in Fig.1. Instrument is designed to detect to the neutron from 0.025eV-20MeV in counting rate and to analyze to absorbed dose based on the counting rate. The ultimate information of the equivalent dose is estimated from the product of the absorbed dose and the radiation weighing factor for neutron of the each groups respectively by microprocessor or computer. RADIATION WEIGHTING FACTORS Wr INCIDENT NEUTRON ENERGY(MeV) Fig.1 Radiation weighting factors for neutron. The smooth curve is to be treated as an approximation. 3.Detection Unit The detection unit consists of three parts of detector as was shown in Fig.2. The first part of the 1
detection unit is selected NE-213 or BC-501A liquid scintillator 2)-5) which is filled in 75mmφ x 75mm cylindrical vessel, to measure the fast neutron. The liquid scintillator vessel is made of 1-1.5mm thick 6 Li-glasse scintillator, which is second part of detection unit, to measure slowdown to thermal neutron from inside of liquid scintillator. Light pulses of both detectors pass through optical guide and are translated to electric pulses by a photomultiplier tube. A bellow and a type off tube are fitted with the glass scintillator side wall to free thermal distortion and to air tight. Furthermore, the third part of detection unit is applied to surrounding of the above detector for measure to the incident thermal neutron. In order construction, the second and third parts of the glass scintillator can be replaced to double layer pressurized 3 He ionization chamber as shown Fig.3 and 4, and gotten same operating function at the glass scintillation. 7 2 1 3 4 8 NEUTRON 5 6 Fig.2 The detection unit of three parts of detector. 1.Liquid scintillator, 2. 6 Li-glass scintillator of second part, 3.Optical guide, 4.Photo-multiplier, 5.Bellow, 6.Tip off tube, 7. 6 Li-glass scintillator of third part, 8.Incident fast, epithermal and thermal neutrons Incidenc To liquid scintillat From liquid scintillator Thermal neutron Thermal neutron 4 1 2 3 First epitherm al neutron Fig.3 Double layer pressurized 3 He ionization chambers Fig.4 The detection unit with double layer ionization chambers. 1.Liquid scintillator, 2.Light guide, 3.Photo-multiplier, 4.Double layer ionization chamber. 2
3 2 12 11 6 8 1 4 5 6 9 4 5 7 10 Fig5 Block diagram of neutron dose rate meter. 1.Liquid scintillator, 2.Glass scintillator of second part, 3. Glass scintillator of third part, 4.Photo-multipliers, 5.Preamplifier, 6.Pulse shape discriminators, 7.Charge sensitive amplifier, 8-10.Display unit for fast, epithermal and thermal neutrons. 11.Memory unit, 12.Computer 4.Electric Unit Figure 5 is a schematic diagram of neutron dose rate meter for counting and analyzer system. The pulses from the liquid and the second part of glass scintillator are led to a circuit of n-γ discrimination by the digital charge compensation method to eliminate the γray pulses. The pulses from the third part glass scintillator are amplified in the charge sensitive amplifier. The process pulses are indicated on display unit in counting rates from each three detectors respectively and stored by memory. The microprocessor or computer system controls the overall system and analyzes to separate to energy group from the memory pulses, and obtain absorbed and equivalent dose. 5.Discussion 5.1.Fast Neutron Detection The liquid scintillation counter is used to measure the fast neutron as the most general instrument. NE- 213 or BC-501A liquid scintillators are suited above measurement from 0.5 to 20MeV neutron without γray. Figure 6 shows the effect of threshold of fast neutron and γ ray spectrums. Peaks of 300 and 470ns are respectively γ ray and fast neutron. The two peaks are completely separated with pulse shape discrimination method above 500keV. Detector arrangements require large volume scintillator to ensure a good detection efficiency of high energy neutrons. Figure 7 show detection efficiency toward energy (MeV) of the incident neutron into 125mm φ x 125mm BC-501A scintillator. The efficiency of 1MeV neutron is 75% at the 0.5MeV discrimination level. 3
Fig.6 Effect of energy threshold. Fig.7 Efficiency of BC-501A. (12.5 cm deep cell) 5.2.Epithermal Neutrons 1eV < Energy < 100keV (Moderate to thermal energies) The glass scintillator measures the epithermal neutron which is moderated to thermal energy in the liquid scintillator. Figure 8 shows a pulse height spectrum of 60 Co gamma ray and thermal neutron with 1mm thick GS20 6 Li glass scintillator. The glass scintillator excellent pulse height discrimination against gamma background is possible. Moreover pulse rise time has large difference between glass scintillator and liquid scintillator. Though, practical electronic circuits difficult to separate each other pulses. We will exchange glass scintillator to double layer pressurized 3 He ionization chambers in the designed instrument. Efficiency of the ionization chamber, if defined as fraction of neutrons which, upon entering the detector, result in a count. Efficiency of the absorption of a neutron passing a distance d through the charged 3 He is ficiency = 1 exp(-a d p) (1) Where a d is the absorption cross section at distance d and p is atmospheric pressure of charged 3 He in the ionization chamber, for the special case of a collimated neutron beam traveling perpendicular to the surface of the ionization chamber. The efficiency of the ionization chamber of 3, 4 and 5cm distance is given in table1 for several charged pressure. The maximum charged gas pressure is limited less than 10 atmospheric pressures by the law in Japan. Above the 99% efficiency, the ionization is constructed with purged 3 He at 8 atmospheric pressure and 4cm distance from Table1. The pulse height of the originated γray and thermal neutron are large different from 1-10, therefore γ ray background can be easily eliminated from by discriminator. Detection efficiency of the epithermal neutron is estimated at 50% as a product of ratios of thermalized and of incidence to the ionization chamber. 4
Table 1 Efficiency of 3,4, 5cm(=d) thick ionization chambers relation to several atmospheric pressures of charged 3 He Efficiency = 1 -exp(-a d p) a 3 =0.45 a 4 =0.45 and a 5 =0.45 Charged pressure P atm Efficiency d=3cm d=4cm d=5cm 1-e -0.45p 1-e -0.60p 1-e -0.75p 0.5 0.225 0.259 0.313 1.0 0.362 0.451 0.528 5.0 0.895 0.950 0.977 7.0 0.957 0.985 0.995 8.0 0.973 0.993 0.998 9.0 0.982 0.995 0.999 10.0 0.989 0.997 1-5 10-4 Fig.8 Pulse height spectra. (1 mm thick GS 6 Li Glass) 5.3.Thermal Neutrons Energy < 1eV (Capture Reactions) Out side of double layer ionization chamber measures the incident thermal neutron. The detection efficiency of the thermal neutron is nearly 100%, same as epithermal neutron. Measured thermal neutron annihilated in the ionization chamber, incident thermal neutron barely passes through mutual double layer ionization chamber. So that the ionization chamber is also absorber of thermal neutron. The effective area for the thermal neutron is very wide (0.3m 2 ) so much so that instrument can be detect from background thermal neutron at real time. 5.4.Electronic Circuits The electronic circuits are being studied. Detail of the electric circuits will be carried out in the near future. Though, in this section, the design of the electronic systems is described. Decay time of the light pulses of the liquid and glass scintillators vary in each other. Same as liquid scintillator large difference from occurred γray and fast neutron. These decay times partially distributes some nano seconds to 600ns each other. Therefore, the fast and epithermal neutron light pulses will be respectively measured at insensitive to γ rays. Now, measuring instrument does not exist to resolve to some nano seconds pulses. Unit mili meter thick glass scintillator is able to detect 99% efficiency for thermal neutron, and is insensitive to γ rays. Above 1.5mm thick glass scintillator, pulse height by the γ ray gradually grows up and irresolves against thermal neutron. Designed instrument will be consisted with liquid scintillation detector and double layer ionization chambers. Measuring circuits for liquid scintillation counter use to measure the n-γ discrimination by the pulse shape method and double layer ionization chambers by ordinary charge sensitive amplifiers. Above pulse signals lead into display in counting rate or absolve dose rate, parallel into memory unit and analyzed by various electro-technique. The design of neutron dose meter is carried out to get easily and steady the equivalent dose. 6.Conclusion 1) Measurement of three energy group neutrons 2) Estimation of equivalent dose 3) Expected to high sensitivity above background neutron 4) Detection of thermal to 20MeV neutron without moderator 5) 4kg light weight instrument References 1. ICRP Pub. 60 (1990). 5
2. M. Moszyński, G. Bizard, G. J. Costa et al., Nucl. Inst. Meth., A317 (1992) 262-272. 3. M. Moszyński, G.J. Costa, G. Guillaume et al., Nucl. Inst. Meth., A350 (1994) 224-234. 4. L. Büemann, S. Ding, S. Guldbakke, H. Klein et al., Nucl. Inst. Meth., A332 (1993) 483-492. 5. K. Shin and Y. Ishii, Nucl. Inst. Meth., A308 (1991) 609-615. 6