Multiwire proportional counters for low-level 14C and 3H measurements

NUCLEAR

INSTRUMENTS

AND METHODS

156 ( 1 9 7 8 )

441-446

; ©

NORTH-HOLLAND

P U B L I S H I N G CO.

MULTIWIRE PROPORTIONAL COUNTERS FOR LOW-LEVEL 14C AND 3H MEASUREMENTS PAVEL POVINEC

Department o1 Nuclear Physics, Comenius University, Mlynsk6 dofina, 816 31 Bratislava, Czechoslovakia Received 29 May 1978 Two types of proportional counters for low-level counting of 14C and 3H are described. The 14C counter is of the Oeschger type with copper foil used for separation of the inner and the ring counter. The 3He counter is a new type of wall-less counter which does not use the internal cathode between the inner and the ring counter. Both counters have very low background and enable to reach a high counting sensitivity.

1. Introduction Low-level radioactivity measurements of soft beta-ray emitters 3H and 14C require to use internal methods of counting. Because beta-particles emitted in 3H and ~4C decays have low penetrating power, the sample must be part of the counter filling. Proportional and liquid scintillation counters are at present mostly used for this purpose. The latter have been the object of very thorough research and several improvements have been reached (see e.g. ref. 1). However, if low-level counting of very small samples with very high accuracy is required, e.g. in the tree ring dating2), proportional counters are still superior, as they enable to reach a higher sensitivity per g of sample. A high sensitivity of proportional counters is reached by applying low-level techniques. The inner counter filled with a radioactive gas is protected against an external radiation by a ring counter connected with the inner counter in anticoinci-

dence. There are several possibilities how to arrange the inner and ring counters to form a lowlevel system. The most simple is to use the inner counter in the form of a metal tube surrounded by a ring of GM counters. The lowest background is reached by inserting a quartz tube, coated with a metallic film, inside a copper or steel tube3). By a careful selection of counter parts from the point of radioactive contamination it is possible to reach a background of about 1 cpm per liter of sensitive volume4). Another possibility is to use both the inner and the ring counters in the form of coaxial cylinders7). The ring counter, located between the cylinders that serve as cathodes, for reducing the dead time, works in proportional region. The shielding properties of this type of ring counter are better because there is no dead space between individual counters and by filling the counter to

Fig. 1. Cross section of the multiwire proportional counter. 1 - stainless steel, 2 - brass, 3 - " O " ring, 4 - teflon, 5 - output, 6 - high voltage connector, 7 - ring anodes, 8 - Cu foil, 9 - valve, 10 - cathode contact.

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higher pressures it is possible to reach a higher efficiency for an external radiation. However, the most important advantage of this design is a possibility to choose a thickness of the inner cathode according to requirements of an experiment. In the case of low-level 3H and 14C counting it is advantageous to use such a thickness of the inner cathode that there will be only a small probability for beta particles from the inner counter to reach the ring counter. If the counter is designed for measuring soft beta-ray emitters, e.g. 3H (maxim u m beta-energy of 18.6 keV) or 14C (maximum beta-energy of 156 keV), the optimum thickness for stopping beta-electrons in the inner cathode is about 1 m g / c m 2 for 3H and about 10 m g / c m 2 for 14C, respectively. This possibility has been utilized for the first time by H o u t e r m a n s and Oeschger 8) who used gold-coated Mylar foil for the inner cathode. Moljk et al. 9) constructed a so-called wallless counter in which the inner cathode is replaced by a grid of wires. These counters became famous because of extremely low background. As there is only a limited a m o u n t of material between the inner and the ring counters, the partial background produced by Compton electrons liberated from the inner cathode is very low in comparison with a thick tube counter. Both the Oeschger 8) and Curran 9) counters are two outstanding examples of low-level counting systems. In this paper a multiwire proportional counter for low-level counting of ~4C of similar design as the Oeschger counter is described. The counter developed for 3H counting is a new type of wall-less counter because it does not use the cathode between the inner and the ring counter.

2. Counter for 14C counting The diagram of the proportional counter is shown in fig. 1. The construction features of the ""

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counter were already described1°), therefore will be only shortly mentioned here. The body of the counter is constructed of stainless steel tube with a diameter of 124 m m and a lenght of 400 ram. The cathode between the inner and the ring counter is made of copper foil with a thickness of 35 zzm. The volume of the inner counter is 3.30 1. T h e total v o l u m e of the inner and the ring counter is 4.60 1. The inner counter is not physically separated from the ring counter, therefore the same gas filling is used in both counters. A highvoltage and v a c u u m isolation is made of teflon. During the testing of the counter the anode wire diameter was chosen between 10 and 200/~m. The ring counter has 40 anodes with a diameter of 100/~m. All wires are made of tungsten with gold coating. A negative high-voltage is connected to the counter body, therefore a high-voltage capacitor which is usually a source of spurious pulses need not be used. This enables also to use a higher input sensitivity even at working voltages about 10 kV. The counter is able to evacuate down to 10 -4 torr and to fill with a counting gas up to 10 arm. Fig. 2 shows a dependence of the counter background on the thickness of the ring counter. It is seen that a thickness of 10 m m is enough for obtaining a full anticoincidence effect of the ring counter. The thin ring counter enables to reach a large sensitive volume of the inner counter in comparison with counters described earlierl~). For obtaining a large sensitive volume it is necessary to construct large diameter and short [

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length Oeschger-type counters. For a given length of the counter the sensitive volume depends on the diameter of the inner and the ring counter. The sensitive volume is given by the formula 12)

ing are registered at lower discrimination level and therefore the beginning of the plateau is shifted to lower voltages and simultaneously the quality of the plateau is better. At higher input sensitivity the end of the plateau is shifted to lower voltages. 100 This is because of overloading effects in the linear Vs = 1 + 2 (d/r) + (d2/r2) ' amplifier. For 5 mV input sensitivity the plateau where d is the thickness of the ring counter and lenght is 500V with a rise of 2%/100V. To improve the plateau and the long-term star is the radius of the inner counter. Fig. 3 shows bility of the counter, field tubes have been built in that it is desirable to use at least 10 cm diameter the counter. Plateau lengths of more than 1000 V inner counters. have been obtained with a slope of about In order not to meet difficulties with stability of l % / 1 0 0 V . CO2-filled proportional counters during long-term Fig. 5 shows the dependence of the background radioactivity measurementsl3), methane has been and the NBS 14C standard counting rate as a funcused as a gas filling of the counter. tion of the gas pressure. It can be seen that the The working characteristics of the inner counter background is changing with the pressure slowly, filled with methane to a pressure of 800 torr are while the 14C counting rate rises sharply. shown in fig. 4 for different input sensitivities. The dependence of the counting rate as a funcThe anode wire diameter was in this case 30 ~m. tion of the position of an external gamma-ray Particles causing a lower ionisation of the gas fillsource (150 kBq of 137Cs) is shown in fig. 6 for the source moving in the cross direction and in fig. 7 I f I i I I [ [ [ I I J I I for the source moving along the axis. From these ,-.,gl0 curves the anticoincidence effect of the ring counter is noticeable. / The background of the inner and the ring counter without any shielding is 750 and 1400 cpm, respectively. In the shielding box (20 cm of Fe) the I 5 10 20 50 I00 500 mV background of the inner and the ring counter is I I I I I I t I I 1 I I I I 3.2 3,4 3,6 3.8 4.0 4.2 4,4 250 and 480 cpm, respectively. The background of VlkV) the inner counter with an anticoincidence shieldFig. 4. W o r k i n g characteristics of the c o u n t e r filled with m e ing but without the iron shielding is 500 cpm. t h a n e to 800 torr and for different input sensitivities. This drop in the background does not depend on 140 the passive shielding and it is therefore due to the contribution of the hard component of cosmic 120 I rays. A two times higher drop in the background

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Fig. 7. l'he same as in fig. 4 but for the source moving along the axis. is obtained by the passive shielding. This decrease does not depend on the anticoincidence shielding, therefore it is due to the filtration of the soft component of cosmic-rays. T h e background of the counter in the shielding box with active anticoincidence shielding is 5.2 cpm. This is for the counter filled with m e t h a n e up to 800 torr and for the input sensitivity of the electronics of 5 m V for the inner counter and 3 m V for the ring counter. The total detection efficiency determined using the NBS ~4C standard is 95%. Using a differential discriminator in the ~4C channel with levels at 5 and 500 mV it is possible to lower the background to 5.0 cpm. However, the dependence of the factor of merit on the position of the upper discrimination level does not have an expressive maximum. The optimum counter parameters were chosen using a new formula for the gas amplification factor 14) and after optimalisation of conditions for saturated ion collection'S): anode diameter 1 0 0 # m , gas filling 1500 torr of methane, working voltage 7 kV. At these parameters the gas amplification factor is 6 x 103. It is c o m m o n to compare proportional counters according to a m a x i m u m measurable age which can be determined by a given counter. Using the formula Tma

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measurable age can be calculated. If n - - 4 (criterion 4a) the result will be in interval _+4t; with a probability of 99.99%. The m a x i m u m measurable age for the described counter using 4 a criterion and 48 h counting time is 52 000years. Fig. 8 shows the dependence of the m a x i m u m measurable age on the m e t h a n e pressure in the counter. It is advantageous to operate the described counter for pressures up to 5 atm. An important characteristic of a proportional counter used for radiocarbon dating is the dependence of the standard deviation on the measurable age. This dependence, as well as the dependence of the relative standard deviation on the age is shown in figs. 9 and 10. The m i n i m u m relative standard deviation is 1% for ages between 2000 and 10000 years and 5% for the m a x i m u m age. The described counter is suitable for radiocarbon dating and it has been used for several years with very good performances for ~4C measurements in dendrochronological samples~6).

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3. C o u n t e r for 3H c o u n t i n g For low-level counting of 3H we have developed a new multiwire proportional counter of wall-less type. This is a wall-less type counter indeed because it does not have any cathode between the inner and the ring counter. In comparison with the previously described counter it has two advantages; it enables to reach a minimum background and highest volume efficiency. Using a suitable working voltage of the inner and the ring counter it is possible to operate both counters without the inner cathode. If the voltage on the central anode i,~ higher than the voltage on the ring anodes, and this is higher than the voltage on the outer cathode, then ring anodes work simultaneously as the inner cathode. A modified version of the laC counter was used for testing the idea of the wall-less type 3H counter. A negative high voltage was connected to the counter body. An auxiliary negative voltage was simultaneously applied to the ring anodes. The working characteristics for the counter filled with methane to 900 torr and with the input sensitivitY of 5 and 2 mV for the inner and the ring counter respectively, are shown in fig. l l. This figure I

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shows that the anticoincidence effect of the ring counter is fully utilized. The background is 890 and 1350cpm for the unshielded inner and the ring counter, respectively. The background in the shielding box with the active anticoincidence is 2.0cpm. Further lowering of the counter background was possible using a differential discriminator in the tritium channel. Using two-channel electronics for the inner counter with discrimination levels of 5 and 65 mV in the first channel and 65 mV in the second channel we measured a dependence of the background in these channels on the working voltage. This dependence is shown in fig. 12. It can be seen that the background in the first channel decreases with increasing voltage, and opposite in the second channel. The total background at 3.5 kV is (2.0_+ 0.1) cpm ; the background in the tritium window is (1.1_+ 0.1) cpm. Using a new type of wall-less counter the total counter background was decreased from 1.5 cpm// of the sensitive volume 04C counter) to 0.6 cpm/1, or 0.3 cpm/1 in the tritium window (3H counter). 4. C o n c l u s i o n s

Two types of proportional counters for low level counting of 3H and ~4C have been described. The counters enable to reach a very high counting sensitivity. Counters of similar design can be used for counting other soft beta emitters. Further improvements in low background proportional counting are possible by utilization of an internal anticoincidence in multielement counters 6']7) and by simultaneously applying the pulse height and pulse shape discriminationS'6']8). Results obtained with these counters are in preparation and will be published in a separate paper.

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The author is deeply grateful to Dr. G. Rajagopalan, for valuable discussions about the wall-less type 3H counter.

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References I) j. E. Noakes, M. P. Neary and J. D. Spaulcling, Nucl. Instr. and Meth. 109 (1973) 177. 2) H.E. Suess, Proc. 12thNobelSymp. onRadiocarbonvariations and absolute chronology, ed. I. U. Olsson (Almqvist and Wiksell, Stockholm, 1970) p. 303, 3) H. de Vries, M. Stuiver and I. Olsson, Nucl. Instr. and Meth. 5 (1959) 111. 4) M. F. A. F. EI-Daoushy and I. U. Olsson, Proc. Low-radioactivity measurements and applications, eds. P. Povinec and S. Usa~ev (SPN, Bratislava, 1977) p. 85. 5) H. Oeschger and H. H. Loosli, ibid., p.13. 6) p. Povinec, J. Szarka, S. Usadev and M. Chud~, ibid., p. 71. 7) R. Nydal, Rev. Sci. lnstr. 33 (1962) 1313. 8) F. G. Houtermans and H. Oeschger, Heir. Phys. Acta 28 (1955) 464; 31 (1958) 117.

9) A. Moljk, R. W. P. Drewer and S. C. Curran, Proc. Roy. Soc. A239 (1957) 117. 10) p. Povinec, Acta Phys. Comen. 16 (1975) 125. 11) H. yon Buttlar, K. Farzine and H. D. Wohlfahrt, Nucl. Instr. and Meth. 37 (1965) 288. 12) M. A. Geyh, Proc. Radioactive dating and methods ollowlevel counting (IAEA, Vienna, 1967) p. 575. 13) M. Alessio, F. Bella and S. Improta, Nucl. Instr. and Meth. 124 (1975) 597. 14) p. Povinec and L. 13urana, Acta Phys. Comen. 15 (1975) 169. 15) p. Povinec, Acta Phys. Comen. 16 (1975) 115. 16) p. Povinec, Acta Phys. Comen. 18 (1977) 137. 17) p. Povinec, Nucl. Instr. and Meth. 101 (1972) 613. 18) S. Sudfir, L. Vas and T. Bit6, Nucl. Instr. and Meth. 112 (1973) 399.