ACCURATE MEASUREMENTS OF SURFACE EMISSION RATE FOR LARGE-AREA ALPHA AND BETA REFERENCE SOURCES Doru Stanga*, Pierino De Felice** *National Institute of R&D for Physics and Nuclear Engineering-Horia Hulubei(IFIN-HH) P.O.Box MG-6, 76900, Bucharest-Magurele, Romania **Instituto di Metrologia delle Radiazioni Ionizzanti, ENEA, C.R.Casaccia P.O.Box 400, I-00100, Rome A.D., Italy Abstract The characteristics of large-area reference sources for the calibration of contamination monitors are specified by the international standard ISO 8769, both for alpha and beta emitters. According to this standard, the surface emission rate has to be measured with a relative standard uncertainty which must not exceed 3%. In this paper, the experimental conditions needed for accurate measurements of the surface emission rate are presented. The uncertainty of such measurements is much smaller than 3%. The evaluation of this uncertainty is also described. 1. Introduction Radioactive contamination of surfaces may result from nuclear accidents and spilling, splashing or leakage from unsealed sources. It may give rise to health hazards. Consequently, surface contamination has to be kept under control and for this purpose are used surface contamination monitors. The calibration of these monitors is done by using large-area sources. For alpha emitters and beta emitters with maximum beta energy greater than 0.150 MeV, the characteristics of reference sources for the calibration of surface contamination monitors are specified by the international standard ISO 8769 [1]. According to this standard, the surface emission rate of reference sources has to be measured by absolute methods [,3] or by using an instrument that has been calibrated by means of sources that has been measured absolutely. Moreover, the surface emission rate has to be measured by the national standards laboratory with an uncertainty which must not exceed 3% (one standard deviation). In this paper, the counting system and the experimental conditions needed for accurate measurements of the surface emission rate are presented. The uncertainty of such measurements is much smaller than 3%. The evaluation of this uncertainty is also described in the paper.. Counting system and method of measurement The counting system used for absolute alpha and beta surface emission rate measurements is composed of a large area, gas-flow, windowless proportional detector, an integral and a spectrometric counting channels. The block diagram of this system is shown in Figure 1.
FPD PA A ID GDG DTU QC/T GFC HV PC MCA FPD flow proportional detector GFC gas-flow circuit PA preamplifier Canbera 006 HV high voltage supply A - amplifier Silena 711 ID integral discriminator Ortec 41 PC/MCA- plug-in card Ortec Trump-K-W3 DTU dead time unit QC/T quad counter/timer Ortec 974 GDG gate &delay generator Ortec 416A Figure 1. Counting system used for alpha and beta surface emission rate measurements The proportional detector has an area of 730 cm and the cathod is formed by the source holder and the source itself. The distance between cathod and anod wires is kept constant at 0 mm by means of the source holder. In this way, reproducible conditions are achived with a solid angle of virtually π sr. The detector is operated under continuous flow of a mixture of argon (90%) and methan (10%) supplied by the gas-flow circuit. Before assembling, all components of the gas-flow circuit were cleaned in an ultrasonic bath with detergent and then rinsed in water. The electronic counting system was made by using NIM modules and a MCA plug-in card (see fig.1). The NIM modules were used for obtaining the integral counting channel. In this channel, the signals from the proportional detector are increased by a charge-sensitive preamplifier (PA) and a spectroscopic amplifier (A) which feeds an integral discriminator (ID). Pulses from the discriminator are counted by conventional counting logic. A dead-time unit (8.5 µs) was used to allow precise corrections of dead-time losses [4]. The method of measurement of the alpha and beta surface emission rate is based, as it was shown before, on the use of a large-area, gas-flow, windowless, proportional detector. The source is introduced inside the counter volume and the particles are detected with a counting efficiency of 100% in a solid angle of π sr. According to ISO 8769, the surface emission rate of a beta source is given by the particle counting rate (corrected for dead-time, background and decay) obtained by setting the discrimination level in the counting system to the value corresponding to a photon energy of 590 ev which is 0.1 times the energy of the X k line of Mn following the decay of 55 Fe. For alpha sources, the discrimination level has to be set above the electronic noise of the measurement system. Under the conditions mentioned, the alpha and beta surface emission rate E can be calculated with the following equation R E = B 1 τ. R (1)
where R is the particle counting rate, τ is the dead-time of the counting system and B represents the background. The decay correction was ignored because it is insignificant for the reference sources recommended by ISO 8769. The relative standard uncertainty of the E can be derived [5] from eq. 1. Thus, we have ε ( E) ( E + B) R E = ε ( R) + τ R ε ( τ ) + ε ( B) () R. E 4 ( E + B) where ε(r), ε(τ) and ε(b) are relative standard uncertainties for R, τ and B, respectively. The method of measurement for alpha and beta surface emission rate is very simple but it is necessary to achieve the optimum experimental conditions of counting for obtaining accurate measurement results. Thus, the proportional detector has to work under a continuous and stable gas-flow and its plateau has to be long with a very low slope. To obtain a counting efficiency of 100%, it has to use a tight detector and a very pure counting gas. Additionally, the detector must be flushed and then operated under a steady gas-flow for several hours prior to measurements in order to be cleaned of air impurities. 3. Alpha and beta surface emission rate measurements First, we have obtained the optimum conditions of counting. Thus, the detector has been flushed with P-10 gas and then operated under a steady gas-flow for two hours. For beta sources, the discrimination level has been adjusted by means of a 55 Fe source. The obtained X-ray spectrum is shown in Figure. Using this spectrum, we checked the stability of the counting system because the peak, corresponding to 5.9 kev X-ray, is very sensitive to gas impurities and gas-flow rate. It shifts to high channels when the gas purity inside the detector volume increases and it reachs a stable position when the air imputities from the detector are completely removed by the steady gas-flow [6]. Counts 5000 0000 15000 10000 5000 0 5.9 kev X-ray 1 1 3 34 45 56 67 78 89 100 111 1 133 144 155 166 177 188 199 10 1 3 43 Channel Figure. 55 Fe spectrum
We have measured four 100 x 150 mm sources ( 14 C, 36 Cl, 04 Tl and 41 Am) purchased from Amersham and certified by PTB. The background has been determined before measurements. For alpha sources, the discrimination level has been set just above the electronic noise of the counting system. The measuring conditions used for obtaining 55 Fe spectrum were maintained during all measurements of beta sources. Experimental results obtained and calculated values of the surface emission rate for alpha and beta sources are presented in Table 1. Table 1. Experimental results and calculated values for the surface emission rate obtained in alpha and beta source measurements Source R (s -1 ) B (s -1 ) τ (µs) E (s -1 ) Reference date C-14 469.8 3.7 8.5 4407.0 4.06.001 Cl-36 5096. 3.7 8.5 5303.8 30.05.001 Tl-04 1935. 3.7 8.5 1943.9 9.05.001 Am-41 695.5 0.03 8.5 700.6 7.06.001 The relative standard uncertainty of the surface emission rate has been calculated according to eq.(). In Table are presented both individual components of the uncertainty and overall uuncertainty for each source. The relative standard uncertainty for R has been evaluated by taking into account both Poisson fluctuations and the uncertainty in setting the discrimination level. For the dead-time, we took into consideration both uncertainties due to dead-time unit instability and overload effects. Table. The relative standard uncertainty of the surface emission rate for alpha and beta sources Source ε(r) (%) ε(τ) (%) ε(b) (%) ε(e) (%) C-14 0.3 5 0. 0.40 Cl-36 0.1 10 0. 0.50 Tl-04 0.1 10 0. 0.8 Am-41 0.1 0.1 0.14 In Table 3 are compared our measurement results with certified results given by PTB. As it shown, a very good agreement is obtained. Table 3. Comparison between measured results and certified results given by PTB Source E±u(E) (s -1 ) E(PTB)±u(E(PTB)) (s -1 ) E/ E(PTB) C-14 4407.0±17.6 4397±73 1.00 Cl-36 5303.8±6.5 530±88 0.997 Tl-04 1943.9±5.4 1904±3 1.01 Am-41 700.6±1.0 699±11 1.00 4. Conclusions According to ISO 8769, four large-area sources emitting beta or alpha particles have been measured by using an absolute method. In order to obtain accurate measurements results, we
have achieved the optimum experimental conditions of counting. Under these conditions, we have measured the surface emission rate with a relative standard uncertainty much smaller than the value of 3% required by ISO 8769. These measurement results are compared with the results certified by PTB and a very good agreement is found. References 1.ISO, 1988. Reference Sources for the Calibration of Surface Contamination Monitors- Beta Emitters(Maximum Energy greater than 0.15 MeV) and Alpha Emitters, International Organization for Standardization, ISO 8769, 1 st Edition.. H. Janβen, R. Klein, 1996. Characterisation of large-area sources for the calibration of beta monitors, Nucl. Instrum. and Methods A369(1996)55-556. 3. Jyi-Lan Wuu, Ming-Chen Yuan, Shi-Hwa Su, Wen-Son Hwang, 00. The alpha and beta emitter measurement system system in INER, Appl. Radiat. Isotopes, 56(1-)61-64. 4. ICRU 5, 1994. Particle Counting in Radioactivity Measurements, International Commission on Radiation Units and Measurements, ICRU Report 5, Bethesda. 5. ISO/TAG4/WG3, 1995. Guide to Expression of Uncertainty in Measurement, International Organization for Standardization, Geneva. 6. M. Yoshida, T. Oishi, T. Honda, T. Torii, 1996. A calibration technique for gasflow ionization chamber with short half-lived rare gases, Nucl. Instrum.Methodes, A 383(1996)441-446