Application of air proportional counters to a tritium-in-air monitor

Application of air proportional counters to a tritium-in-air monitor

620 Nuclear Instruments and Methods in Physics Research A254 (1987) 620-626 North-Holland, Amsterdam A P P L I C A T I O N OF AIR P R O P O R T I O ...

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620

Nuclear Instruments and Methods in Physics Research A254 (1987) 620-626 North-Holland, Amsterdam

A P P L I C A T I O N OF AIR P R O P O R T I O N A L C O U N T E R S TO A T R I T I U M - I N - A I R Takahiko AOYAMA,

Hiroki SUGIURA

MONITOR

and Tamaki WATANABE

Department of Nuclear Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan Received 24 June 1986

A tritium-in-air monitor using air proportional counters as a detector has been devised. The detector consists of two layers of identical multiwire counters, and it operates without using any counting gas other than sampled air. The background due to internal a-rays was eliminated bypulse-height discrimination, and that due to external, penetrating y-rays and cosmic rays was evaluated from the coincidence counting rate of the two counters. The counting rate by tritium r-rays was obtained from the anticoincidence counting rate Na by subtracting the product of the coincidence counting rate Nc and a constant ratio k = Na/Nc for the background. The ratio k was verified to be independent of the intensity, energy and incident direction of the ),-rays, though it varied with pressure and humidity of the sampled air. A detection efficiency of about 14% was obtained for tritium r-rays, and a detection limit of less than 1 pCi/cm 3 was obtained with a normal background level and a counting time of 30 s.

1. Introduction In tritium handling laboratories it is legally required to monitor tritium in air to a low concentration of the order of 1 p C i / c m 3 or less. This is, however, difficult to perform, since commercially available tritium-in-air monitors have not a low enough detection limit or they have limitations in their practical use. It would therefore be significant to develop highly sensitive and convenient tritium monitors which can be used without limitations. Both ionization chambers and proportional counters are used as detectors of real time tritium monitors since they can detect tritium in air directly with a large detection efficiency. For the application of these detectors various background eliminating methods have been developed to detect a low level of tritium activity, especially with room monitors which cannot use a heavy shielding. The background considered here is that due to environmental ),-rays and cosmic rays, and that due to a-rays from radon contained in natural air. If radioactive nuclides other than tritium are contained in the sampled air, radiations from these nuclides are added to the background. In fission and fusion facilities they are high energy r - and -y-rays from radioactive rare gases and from 13N and 16N. In ionization chambers total charges created by incident radiations are measured. In this case the a-background becomes signifiCant, since a-rays have a large ionizing power though they usually have a low incident rate. For eliminating external )'-background, either compensating chambers [1] or the source modulation technique [2,3] has been used. These methods, however, 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

have such disadvantages as that statistical accuracy is never improved with compensating chambers, and therefore a low detection limit cannot be obtained, and that response time is extremely long for the source modulation technique [3], and therefore the change of tritium concentration cannot be followed. F o r eliminating a-background, radon separation with semipermeable membranes [4] and cancellation of asignals by an electric circuit [5] have been developed. The former is effective only for the separation of tritium oxide from radon, and is difficult to separate gaseous tritium from radon [4]. Semipermeable membranes are preferably used to separate tritium according to its chemical forms. Tritium-in-air monitors using this principle have been developed [4,6,7] to determine the concentration of H T O and H T separately, since the relative hazard is largely different for these two species. F o r the elimination of high energy fl- and y-background due to nuclides other than tritium, compensating chambers have also been used [8-10], which again cannot improve statistical accuracy. In proportional counters it is not the total charge created in the counters but the number of radiations that is measured. This results in the insignificance of the a-background in this case, and, if necessary, it can easily be eliminated by pulse height discrimination. For external background due to environmental )'-rays and cosmic rays, and for internal r - and ),-background from sampled air, we can discriminate or separate the tritium signal by using pulse-shape discrimination [11-13] or anticoincidence counting with guard counters [14-16], or track-length discrimination [17]. These, belonging to low background counting, improve statistical accuracy,

621

T. A ovama et a L / Application of air proportional counters

and therefore increase the sensitivity of tritium monitors. This is the advantage of proportional counters over ionization chambers. The only problem is that the sampied air must be mixed with a large amount of counting gas such as combustible methane or expensive P-10 gas to operate the counters properly, which limits the practical use of proportional counters. To improve the drawback of proportional counters we utilized air proportional counters which had been developed as detectors of tritium surface contamination monitors [18,19] and applied to the detector of the tritium-in-air monitor. The final goal of our study is to develop a highly sensitive and conventional tritium monitor. In the present monitor, however, the detector was designed to be as simple as possible to examine the applicability of air proportional counters to the monitor, and the improvement of the monitor sensitivity was postponed to future developments.

2. Principle and construction 2.1. Principle o f the monitor

Fig. 1 shows the scheme of the present monitor. The detector consists of two layers of identical multiwire counters, which have a common mesh cathode in a gas volume. Signal pulses derived from each counter were not only counted independently but also fed to a coincidence circuit. We call the independent counting rates N 1 and N2, and the coincidence counting rate Nc. Scalers were computer-controlled and each counting rate was processed to obtain an anticoincidence counting rate N a = ( N 1 - No) + ( N 2 - No), a ratio k = N~,/Nc and a total counting rate Nt = N, + Nc. Sampled air introduced at atmospheric pressure into the counters was used as a counting gas. In this case it is known from our previous study [20] that the effective regions of the counters were limited in the vicinity of anode wires to less than 2 mm from each anode wire. The range of tritium r-rays in air at atmospheric pressure is about 7 mm at maximum, whereas the effective

Air n

regions of the counters are separated by about 12 mm. This made it impossible from tritium r-rays to be counted in coincidence. On the other hand, cosmic rays and electrons converted in the counter wall by the incidence of environmental y-rays have generally long ranges, and they can pierce both effective regions and be counted in coincidence. By this principle it would be possible to separate the signal of tritium r-rays from that of background radiations. In addition, the present monitor is expected to have no memory effect for tritium, since r-rays emitted from tritium deposition on the counter wall can ~carcely reach the effective regions. 2.2. Construction o f the detector

Fig. 2 shows the cross-sectional views of the detector. The detector consists of two layers of multiwire counters with an anode-cathode distance of 7.5 mm. Each counter has 45 anode wires with a spacing of 4 mm. To avoid electric field distortion at the sides of the anode wire row, potential wires and strips possessing the same potential as the anode wires were added to both sides. The anode was made of tungsten wires, 50 /xm in diameter, which were soldered on glass-epoxy printed boards. As a cathode, 5 mm thick aluminum plates were used for the counter wall, and a fine stainless-steel wire mesh, 50 p m in diameter and 50 per inch, was used between the two layers of the counter in order not to generate photon conversion in this part. To avoid the influence of leakage current through the surface of glass-epoxy frames, guard strips were installed at a small distance of 1.5 mm from the anode wire planes and silicon resin was applied inside the frames. A high voltage was applied to the anode wires, the cathode being at ground potential. Signal pulses derived from the anode wires of each counter were fed to a charge sensitive preamplifier, and then amplified wi[h a linear amplifier with a gain of 125. Bipolar pulseshaping with a 2 ps time constant was adopted for the amplifier, since a good S I N ratio was obtained for microphonic noise with this condition. The pulses were

+HV

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Computer ]

Scale r ( Nz )

+HV

Fig. 1. Scheme of an air proportional counter-type tritium-in-air monitor. Dotted regions show the effective volume of the counters.

T. Aovarna et al. / Application of air proportional counters

622 Air

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Fig. 2. Cross-sectional views of the detector.

then height-selected with a usual single channel analyzer (SCA) and counted. The lower level of the SCA was set at 0.4 V, which was about a factor 2 higher than the maximum noise level. The input sensitivity in this case was 2.2 × 10 4 ions. The upper level of the SCA was set at 10 V, since there appeared huge saturated pulses exceeding 10 V, which were considered to be due to a-background in the sampled air and counter materials.

3. Operationalcharacteristicsand performance 3.1. Determination o f the operational voltage

Fig. 3 shows the total counting rate N t and the ratio k of the anticoincidence rate N a to the coincidence rate N¢ as a function of applied voltage, which were obtained from a constant irradiation of 137Cs 7-rays and with dry air flow. The dry air was produced by passing r o o m air through a cylinder filled with silica gel. The integral counting curve in fig. 3 was obtained when pulses with heights from 0.4 to 10 V were counted, and this corresponds to usual counting characteristic curve. It is seen from this curve that the counting rate increases with applied voltage until the corona begins at

i 4.2

4.4

4.6

4.8

5.0

APplied v01tage(kV) Fig. 3. Total counting rate Nt and the ratio k = N a / N c as a function of applied voltage, which were obtained on a constant irradiation of 137Cs y-rays. Integral and differential curves are interpreted in the text. The operational voltage is indicated by Vow

5 kV and no plateau region exists. The differential counting curve in fig. 3 was obtained when pulses with heights around threshold voltage from 0.3 to 0.5 V - were counted. The differential counting rate was also increased with applied voltage and a local minimum which indicates a good separation of signal pulses from noise pulses [18,19,21] could not be observed. It is seen from fig. 3 that the value of k increases at first and then decreases exponentially with applied voltage. When neither a plateau nor a local minimum can be obtained on integral and differential counting curves, respectively, it is difficult to determine the optimum operational voltage. In the present case, an operational voltage of 4.8 kV was determined as the voltage at which the detection efficiency for minimum ionizing electrons was as high as possible and yet corona discharge would not be caused by a normal change of environmental conditions such as pressure, temperature and humidity of the sampled air.

T. Aovama et al. / Application of air proportional counters 3.2. Dependence of the total counting rate Nt and the ratio k on the pressure and humidity of the sampled air Fig. 4 shows the dependence of the total counting rate N t and the ratio k on the pressure of the sampled air, which was obtained with an operational voltage of 4.8 kV, a constant temperature of 24°C, dry air flow, and a constant irradiation of 88y ),-rays. The abscissa gives the ratio of atmospheric pressure P0 to the pressure P of the sampled air. It is seen from fig. 4 that both N t and k change exponentially with the inverse of P. Closed circles in the figure show the change of detection efficiency 7/ for tritium fl-rays with pressure, where the efficiency ,/ was fitted to the N t line at atmospheric pressure. The value of ,/ also changes exponentially with the inverse of P. This is understood as follows. Since the values of Nt (or ,/) and k are controlled by the magnitudes of gas amplification and electron attachment, these would be a function of electric field strength E over P or applied voltage V over P at constant temperature [20]. As is shown in fig. 3, the values of N t and k change exponentially with the applied voltage V around the operational voltage at constant pressure and temperature. If the voltage V and the temperature are constant they would change exponentially with the inverse of P. With increasing pressure the total counting rate N t and the detection efficiency *1 decreases, and the ratio k increases. These changes of Nt, 7/ and k can be explained by the decrease of the effective volume or the effective regions around each anode wire of the counters. Strictly, these changes of N t, ,/ and k are due to

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(1)

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0

030 2 I 0.98

the increase of the density of the sampled air with increasing pressure. Since the density of the air varies inversely with the absolute temperature T, the same changes would be expected with decreasing T. In the present study, however, all the experiments were carried out in a controlled temperature of (25 + 1)°C. The changes of N t and k with the humidity of the sampled air were examined by flowing air with controlled humidity through the detector. The humid air was produced by mixing a constant ratio of dry air to air bubbled through water. Since the detector became instable with a relative humidity of about 50% at 25 o C, measurements were carried out with a humidity of less than 50%. When the humidity increased from 0 to 47% at a constant pressure of 1005 mb and a temperature of 2 5 ° C the value of N t decreased to 77% of the initial value and the value of k increased from 5.1 to 6.7. These changes of N t and k correspond to a change of Po/P from 1.01 to 0.995 in fig. 4, and therefore these would be explained by the same reason as that by the increase of pressure, i.e. the decrease of the effective volume of the counters. The decrease of the effective volume of the counters with increasing humidity ~s considered to be due to the promotion of three-body attachment of electrons to oxygen molecules by the presence of H 2 0 in the sampled air. The three-body attachment is believed to be carried out in two steps [22]. In the first step, an excited oxygen ion is created through collision of an electron with an oxygen molecule,

In the second step, the excited ion is stabilized through collision with a constituent molecule M of the sampled air to a stable oxygen ion,

6

5 X

623

2 1.02

(2)

though the excited ion is mostly deexcited by the reverse process of reaction (1). The values of the stabilizing cross section q~ for molecules in air were given by Ogawa [22] and are shown in table 1. Since the value of q~ is larger for H 2 0 than for N 2 and O~, three-body attachment would be promoted by the presence of H 20. Since the detection efficiency 7/ for tritium fl-rays and the ratio k for the background varies with pressure, temperature and humidity for the present monitor, we must use calibrated values of 71 and k to calculate the tritium concentration in the sampled air (see sect. 3.6).

Po/P

Fig. 4. Dependences of the total counting rate Nt (open circles) and the ratio k = Na/N c on the pressure of the sampled air, which were obtained with an operational voltage of 4.8 kV and a constant irradiation of SSy ),-rays. Closed circles show the change of detection efficiency for tritium fl-rays with pressure.

Table 1 Stabilizing cross-section qs for the constituent molecules of air Molecule

N2

02

H 2°

qs [ × 10- t4 cm2]

0.03

1.3

4.55

624

T. Aqvama et a L / Application of air proportional counters

3.3. Dependence of the total counting rate N t and the ratio k on -/-ray exposure rate

Fig. 5 shows the dependence of the total counting rate N t and the ratio k on the exposure rate of 137Cs -/-rays, which was obtained with a constant pressure and temperature and with dry air flow. It is seen from this figure that N t increases linearly with exposure rate while k is constant independent of the rate to at least 10 m R / h . This is entirely consistent with our expectation. 3.4. Dependence of the ratio k on the energy and incident direction of -/-rays The values of k were measured for various energies and incident directions of "/-rays using several -/-ray sources such as 137Cs, 88y, 22Na and 241Am. The -/-rays from these sources entered either perpendicularly to the counter plane or parallel to the plane from the side of the detector. Normalizing the value of k for the case where 137Cs -/-rays were incident perpendicularly to the counter plane, the relative values of k are shown in tables 2 and 3. The measurements were carried out with

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dry air flow, and the observed values of k were corrected for pressure and temperature. It is found from tables 2 and 3 that, except for 241Am, the relative values of k were unity including that for background within the limits of the standard deviation regardless of the energy and incident direction of -/-rays. For the case of 241Am a small coincidence counting rate, and therefore a large value of k was obtained. This is because the range of photoelectrons for 60 keV -/-rays from 2 4 1 A l l l is about 4 cm in air and is not long enough compared with the thickness of the detector. We should, however, note that these low energy y-rays can easily be shielded. In this case the value of k would be constant independent of -/-ray energy. 3.5. Tritium monitoring in a -/-ray field

When the value of k is known at a given pressure, temperature and humidity, background counting rates can be evaluated in real time from observed values of the coincidence counting rate Nc. Fig. 6 shows the change of the anticoincidence counting rate N a and the coincidence counting rate N~ with lapse of time, which was measured with a constant pressure and temperature and with a counting time of 1 min. After 25 min from the start of the measurement, tritium gas with an activity of 44 nCi was injected into the detector from a slit opened at the center of the counter wall. At 40 min 137Cs -/-rays were irradiated for 10 min with an exposure rate of 1.7 m R / h . At 65 min the tritium gas was purged by a constant flow of dry air with a rate of 0.8 1/min. It is seen from this figure that the value of N~ was independent of the presence of tritium and it increased solely when -/-rays were irradiated. Net counting rates from tritium t - r a y s were calculated by subtraction of the product of Nc and k from N o and are shown in fig. 6 as closed squares. It can be found that the calculated values of the net counting rate were on a single line independent of -/-ray irradiation. For the purging of tritium gas, air to an amount of three times the detector volume was enough, and the values of N, returned to the same background level as at the beginning. 3.6. Detection efficiency and the minimum detectable tritium concentration in air

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Fig. 5. Dependences of the total counting rate Nt and the ratio k = N a / N c on the exposure rate of t37Cs "y-rays, which was measured by changing the distance between the source and the detector.

In the case of fig. 6 tritium gas with an activity of (1620 + 20) Bq was injected into the detector and a net counting rate of (230 _+ 2) cps was obtained. The detection efficiency 7/ for tritium t-rays, therefore, becomes 0.142 _ 0.002 at a pressure of 1000 mb and a temperature of 24°C. Aproximating the effective volume of the counters as cylinders surrounding each anode wire, we obtained 1.8 mm as the effective radius. Since the anode

T. Aovama et al. / Application of air proportional counters

625

Table 2 Relative values of k for various y-ray sources having different T-ray energies. 3,-rays were incident perpendicularly to the counter plane excepting background. T-ray source

137Cs

SSy

22Na

241Am

Background

y-ray energy [MeV]

0.662

-

1

0.511 1.275 0.99 + 0.03

0.060

Relative value of k

0.898 1.836 1.01 + 0.02

5.51 + 0.04

0.99 + 0.04

Table 3 Relative values of k for different incident directions of y-rays Incident direction

Perpendicular to the counter plane

Relative value of k

from direct side

from reverse side

1

0.98 _+0.02

anticoincidence c o u n t i n g rate Nab for the b a c k g r o u n d is given by the p r o d u c t of N~ and a c o n s t a n t value of k, so that the net c o u n t i n g rate N, becomes

137Cs

Parallel

N n = N : - Nab = N..,s - kN¢.

(3)

The s t a n d a r d deviation a. for the net c o u n t i n g rate N. is therefore

i

On=

1.01 _+0.03

-N: +

t

k2Nc t

1 (Nn+k(k+l)N~ ~t

(4)

where t is the c o u n t i n g time. The m i n i m u m net counting rate N with which we can detect the presence of tritium activity in the sampled air with a 99.7% confidence level is o b t a i n e d as the net c o u n t i n g rate N. equal to three times the s t a n d a r d deviation o,

wire spacing was 4 m m for the present counters, the effective regions would not be connected between the wires a n d there seem to be insensitive regions. The detection limit of the m o n i t o r for tritium conc e n t r a t i o n in air will b e evaluated as follows. Since the presence of tritium in the sampled air affects the anticoincidence c o u n t i n g rate alone, the net c o u n t i n g rate N, from tritium fl-rays is o b t a i n e d b y the difference between the anticoincidence rates N2, for the sampled air, a n d N b, for the background. O n the o t h e r h a n d , the

3 N = ~tt ~/N + k ( k + l ) N c.

(5)

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9[j4,

N=

10n

]

l+ffk(k+l)N~

1+

.

(6)

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Fig. 6. Tritium monitoring in a y-ray field. 1000 mb, 24°C. Closed squares show the net counting rate Nn for tritium B-rays calculated by Nn = Na - kNc, where k = 5.0 is a constant.

626

T. Aoyama et a L / Application of air proportional counters

Taking 0.5 rain for the counting time t, and substituting k = 5 . 0 and N c = 9 0 cpm - the mean value of Nc without y-ray irradiation as shown in fig. 6 - into eq. (6), we obtain N = 230 cpm. The minimum detectable tritium concentration S in air is calculated from the formula N/60

S = 0.037,/~

( p C i / c m 3 or ~ C i / m 3 ) ,

(7)

where ,/ is the detection efficiency and A (cm 3) is the volume of the detector. Substituting ~1= 0.142 and A = 1140 cm 3 into eq. (7), we obtain S = 0.64 p C i / c m 3, which is small enough compared with the maximum permissible concentration (MPC) in air for tritium of 5 p C i / c m 3 in Japan. If the value of S can be equalized to the M P C a y-background level of about 2.5 m R / h will be allowed.

4. Conclusion A tritium-in-air monitor consisting of two layers of multiwire-type air proportional counter was devised and tested for operational characteristics and performance on T-background elimination. These are summarized as follows. (1) Background due to environmental y-rays and cosmic rays was estimated by the coincidence counting rate No, since the value of Nc was invariable in the presence of tritium in the sampled air. (2) The counting rate from tritium r-rays was obtained from the anticoincidence counting rate N a subtracted by the product of Nc and a constant ratio k = N J N c for the background. (3) The detection efficiency for tritium r-rays and the ratio k varied with pressure and humidity of the sampled air. These variations were explained by a change of effective regions around each anode wire of the counters. (4) The value of k was verified to be independent of the intensity, energy and incident d i r e c t i o n of y-rays except for low energy y-rays. (5) A detection efficiency of about 14% and a minim u m detectable tritium concentration of less than 1 p C i / c m 3 were obtained with a normal background level and a counting time of 30 s. (6) A memory effect due to tritium deposition on the counter wall was not observed for repeating tritium gas injection and purge. In the present monitor having a simple construction of the detector, the detection limit could not be low enough because of a relatively large value of k for the background. By using conventional anticoincidence guard counters the value of k would be close to zero, i.e. an ideal value of k, and hence the detection limit would be improved. Even in this case the data of

pressure, temperature and humidity of the sampled air would be required to calibrate the detection efficiency for tritium r-rays. Since air proportional counters require no counting gas other than air, they can be used as flowthrough detector of tritium-in-air monitors, though the instability problem of the detector for humid air must be solved before their practical use.

Acknowledgement The authors are grateful to Dr. C. Mori and Mr. Y. Fujii for their assistance to measure activities of tritium gas.

References [1] R.A. Jalbert and R.D. Hiebert, Nucl. Instr. and Meth. 96 (1971) 61. [2] M. Ogawa, M. Okamoto, E. Arai, N. Horiuchi and Y. Murata, Nucl. Instr. and Meth. 176 (1980) 591. [3] N. Horiuchi, Nucl. Instr. and Meth. 217 (1983) 479. [4] R.H. Howell, J.C. Cate and C. Wong, Nucl. Instr. and Meth. 124 (1975) 579. [5] Y. Matsumoto, Y. Mito, N. Nakamura and I. Kumabe, J. At. Energy Soc. Japan 26 (1984) 157. [6] L. Beach and S.S. Hoots, Nucl. Instr. and Meth. 175 (1980) 369. [7] R.G.C. McElroy, R.V. Osborne and R.A. Surette, IEEE Trans. Nucl. Sci. NS-29 (1982) 816. [8] R.D. Hiebert and M.A. Wolf, IEEE Trans. Nucl. Sci. NS-28 (1981) 264. [9] R.A. Jalbert, Los Alamos National Laboratory report LA-9382-MS (1982). [10] R.A. Jalbert, Los Alamos National Laboratory report LA-9514-MS (1982). [11] S. Sudhr, L. Vas and T. Birr, Nucl. Instr. and Meth. 112 (1973) 399. [12] R.C. Hochel and D.W. Hayes, Nucl. Instr. and Meth. 130 (1975) 183. [13] S.A. Cox, T.J. Yule and E.F. Bennett, IEEE Trans. Nucl. Sci. NS-29 (1982) 822. [14] P. Povinec, Nucl. Instr. and Meth. 156 (1978) 441. [15] P. Povinec, J. Szarka and S. Usa~ev, Nucl. Instr. and Meth. 163 (1979) 369. [16] P. Povinec, Nucl. Instr. and Meth. 177 (1980) 465. [17] I. Carmi, A. Bresldn, R. Chechik, M. Cohen and N. Zwang, Proc. Int. Syrup. on Methods of Low-Level Counting and Spectrometry (IAEA, Vienna, 1981) p. 477. [18] T. Aoyama, H. Miyai and T. Watanabe, Nucl. Instr. and Meth. 221 (1984) 644. [19] T. Aoyama and T. Watanabe, Health Phys. 48 (1985) 773. [20] T. Aoyama and T. Watanabe, Nucl. Instr. and Meth. 205 (1983) 311. [21] H. Houtermans, M. Miguel and E. Werner, Proc. Symp. on Standardization of Radionuclides (IAEA, Vienna, 1967) p. 115. [22] I. Ogawa, Oyo Buturi 43 (1974) 1088.