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Internal-gas radioactivity-counting with pure H2S as counting gas
lnternauonal
Journal or A&cd
Radiation and Isotopes,
1977. Vol. 28. pp. 7 13-718. Pergamon
Press. Printed in Great Britam
Internal-gas Radioactivity-counting Pure H,S as Counting Gas WALTER
ROEDEL,
Institut
fir Umweltphysik,
LOTHAR Universitlt
LEIDNER Heidelberg,
and WOLFGANG D-69 Heidelberg.
Federal
(Received 27 Drcemher 1976; in final form 10 February
with
JUNKERMANN Republic
of Germany
1977)
The present paper shows that it is possible to use pure H2S as filling gas for an internal-gas radioactivitycounter. H,S used as counting-gas is self-quenching. but spurious after-pulses occur. Therefore a selfclosing gate triggered by the primary pulse and closed for several msec must be introduced in order to produce satisfying plateaux in the counting characteristics. The results obtained with two different types of counting tubes and with H2S pressures between 100 and 450 mbar are reported. Some possible processes taking place in the counting tubes are discussed.
1. INTRODUCTION
now reports our studies on internal-gas-counting pure H,S as counting gas.
THE WORK reported here has been started to develop a highly sensitive technique for measuring very weak samples of radioactive sulfur. Such samples are collected in tracer studies using natural cosmic-ray-produced radioactive sulfur.“’ Various methods have been studied in our laboratory: Stoepp1er’2.3) measured elementary sulfur as an external sample in a low-level proportional counter with thin window; Hofmann and Roedel’4v5’ developed a sensitive technique using gel scintillation counting of appropriate sulfidesThe most promising method, however, seemed to be the internal-gas counting of a suitable sulfur compound. By this technique, a gaseous compound of the nuclide to be measured is added to the filling gas or is used as the filling gas of a counting tube. In 1960, Merritt and Hawking&‘) reported for the first time (as far as is known to the authors) intemalgas-counting of a mixture of methane and sulfur dioxide; the highest possible SOz partial pressure without serious destruction of useful counting characteristics. however, was as low as 15 mbar. Some years later, in our laboratory, Marwinskq7) studied the counting properties of several gaseous sulfur compounds (CS2, H,S, SO,) added to a 90: lo-mixture of argonmethane (partially at Ar-CH, pressures beyond atmospheric pressure). He found H,S to be the most appropriate sulfur compound for intemal-gas-counting. If an “electronic dead time” (see Section 2) is applied in order to suppress spurious after-pulses, H,S partial pressures as high as 400mbar can be allowed. Marwinsky also mentions that it is possible to use pure H,S as counting gas. The present paper
713
2. DETAILS
with
OF OPERATION
The counting-rate vs high-voltage characteristic of a HIS-filled counting tube measured in the usual arrangement (this means tube-preamplifier-amplifierdiscriminator-scaler) does not show any plateau (Fig. 1, curve a). This fact is due to spurious after-pulses following the primary pulse. The rate of these afterpulses depends on high voltage and amounts to five (or more) after-pulses per primary pulse. After-pulses occur statistically during times in the order of a few msec. A typical wave-form is shown in Fig. 2. By a simple and cheap self-closing gate triggered by the primary pulse (the “electronic dead time” mentioned above) these spurious pulses can be suppressed. Then a counting characteristic according to curve b of Fig. I is obtained. This means that H,S is self-quenching
h,ghvollog.lk”,
L 2.1 FIG.
1.
Counting the “electronic
22
2.3
2.4
2.5
characteristics without (a) and with (b) dead time” (H,S pressure 200 mbar).
714
Waiter Roedrl, Lothar
FLH
381
I..ond-gale)
Leidrwr and Woljgang Junkermann
SN 74121 N IFLKIOII lunwbratorl
FG. 3. Block diagram closing
of the electronics (a) with the selfgate (b); R and C control the gating time.
and that no external quenching by lowering the high voltage at the counting tube is necessary. The gating time must be in the order of IO msec. For this reason, the counting rate of an H,S-internal-gas-counter is limited to about 1000 events/min. But this does not seem to be a serious restriction in low-level studies. During the work mentioned above. Marwinsky also performed experiments with external quenching by lowering the high voltage at the counting tube. This, however, does not suppress the after-pulses. The afterpulses seem to be delayed by times in the order of the external quenching time, and appear when the full high voltage has been re-established. The deadtime is thus increased, whereas no improvement of counting characteristics could be found. Figure 3 shows a block diagram of the electronics, with the self-closing gate in detail. The material for building up the counting tubes must be resistant against H,S. For the present work, two counting tubes have been used. The first is made of quartz glass, the cathode surface being coated with graphite spray. In Fig. 4 a schematic drawing of the design of this tube is given. The second tube is very similar to the first; the wall, however, is made of copper coated with chromium (chromium is resistant against H,S). The central wires of both tubes consist of Chroman 25 (Vacuum-Schmelze, Hanau) and have diameters of 50 pm. In spite of the fact that both tubes show nearly the same geometric shape and size. the counting
FK. 4. Schematic drawing (cross-sectlon) of the quartzgraphite-tube; on the right hand. the connections to the preamplifier and the high voltage and to the ground; on the left. the gas inlet. Numbers indicate the length and the diameter (mm).
hqh
2.5
10
is
voiklgr
ikV/ .
4.0
FIG. 5. Counting characteristxs of the copperxhromlumtube at 400 mbars gas pressure. with CH, or H,S as filling gas.
characteristics of the tubes are quite different. cussed in Section 3.
3. COUNTING
as dis-
CHARACTERISTICS
The determinations of pulse-rate vs high-voltage characteristics discussed in this paper have been carried out with commercially available H,S-gas (99X50<, H,S. according to the manufacturer) and with an external gamma-radiation source. For the sake of standardization, the counting characteristics have also been determined with methane as filling gas. Additionally, a few measurements have been made with internal 35S sources’ t H 235S has been prepared by reduction of radioactive sulfate in a hydrogen flow at elevated temperatures, according to Stoeppler.‘3’ Good counting characteristics with sufficient lengths of plateaux have been obtained with H,S pressures within the tubes between 100 and 450 mbar. Beyond 500 mbar, no satisfying plateaux in the counting characteristics have been measured. Figure 5 shows a typical characteristic of the copper-chromium-tube at 400 mbar H,S-pressure, in comparison with the counting characteristic of the same tube filled with methane. The length of the H,S-plateaux is about 25t&3OOV; the slope is about 3% per IOOV (with the external radiation source; with an internal 3’S-source slopes of 1% or less per 100 V have been measured). The counting tubes must not be operated at high voltages beyond the upper ends of the plateaux. If high voltages a little beyond the upper ends of the plateaux are used, the counting characteristics show a hysteresis (Fig. 6 illustrates this effect); if still higher voltages are applied, the counting gas is irreversibly destroyed. The two types of counting tubes (cathode coated with chromium and cathode coated with graphite. re-
715
FIG. 2. Typical waveform of a pulse with after-pulses (horiz. 0.5 mscc/div, vert. 0.5 V/div).
717
Internal-gas radioactivity-counting
, CO”“,
role
forb,lr
cwn,
umh,
role
Wpml
loo-
I
+
0
d
6
50-
hwh
,, l,k
5’7
1,‘s
I,$
YOIIOQC
IkVl
H25 prcssurr
210
loo
FIG. 6. Hysteresis of the counting characteristic after an application of a high voltage beyond the upper end of the plateau.
spectively) show striking differences of electron multiplication in H,S gas and thus of the position of plateaux. The curves of Fig. 7 give the high voltages at the beginning of the plateaux as functions of pressure of H,S and CH,, respectively. The electron multiplication with CH* as the filling gas is nearly the same in both tubes. Whereas, in the quarti-graphite-tube, the electron multiplication is in H2S a little higher than in methane, in the copper-chromium-tube, the electron multiplication in H$ is appreciably lower than in CH,. It should be mentioned, however, that the chromium-copper-tube produces better plateaux (concerning plateau lengths as well as plateau slopes); also, the operation of the copper-chromium-tube is more stable. A further striking difference between the two types of tubes lies in the dependence of counting rates on H,S pressure: in the quartz-graphite-tube the counting rate (normalized at the counting rate with methane as the filling gas) decreases with increasing HIS
300
200
volloge (k VI
8. Count rate vs H,S-pressure in the quartz-graphitetube, with constant radiation source strength; the count rate “loo” corresponds to the count rate with CH, as the filling gas.
pressure, as shown in Fig. 8, whereas such an effect could not be found with the copperchromium-tube. 4. DISCUSSION It is not the aim of the present paper to “explain” the mechanisms occurring in a H,S counting tube. Nevertheless, some possible processes shall be discussed here. Neuert and Clasen’s’ and Neuert”’ studied by mass spectroscopy the ions produced by electron impact on H,S molecules. Besides several species of positive ions they found S- (with a yield of about 1% of the total of ions). They did not find H,S-. That means that H$ is electronegative, the electron attachment being dissociative. According to ref. (9) the appearance potential of the reaction H,S+
e-+H,S--+S-
+ 2H
is 2.9 eV. On the other hand, the reaction + 2H
has an appearance potential of 6.5 eV; thus the electron attachment energy (electron affinity) of H,S should bc 3.6 eV. For the reaction
4.0
H,S-
+ S- + H,,
the dissociation energy amounts dissociative attachment H,S + e--+S
lb0
IdO
360
4dO
7. High voltages at the beginning of the plateaux functions of the gas pressure (---, copperchromium-tube; -, qur.rtz-graphite-tube). FIG.
A.R.I
28/a--c
(mbors, 500
FIG.
H,S-+Shjgh
4w
0
as
to 2.0 eV; thus the + H,
is exothetmic. Neuert and Clasen did not find HSions produced by electron impact, but according to Lewis and White”” it is possible that neutral HS radicals are produced (which cannot be detected by ion mass spectroscopy, as was used by Neuert and Clasen). HS radicals are also electronegative with an electron affinity of 2.19 eV.(’ l)
718
Walter Roedel, Lothar Leidner and Wolfgang Junkermann
Due to these processes, the filling gas is highly electronegative, ard it is to be expected that the electron multiplication factor is low in a counter tube filled with H,S (compared, for instance, with a tube filled with methane). The higher multiplication factor in the quartz-graphite-tube is then most probably due to photon interaction with the cathode material. On the other hand, the electron work functions for chromium (4.374.7 eV) and for carbon (4.WI.84 eV)(‘” are not very different, and it is not clear where the observed difference comes from (perhaps the imbedding material of the graphite spray is responsible for the effect). The after-pulses are most probably due to electrons produced by interaction of positive ions with the cathode wall of the tube. H,S+ has an ionization potential of 10.4eV. “‘) Thus the energy released by neutralization of an ion at the cathode wall is more than twice the electron work energy and thus sufficient to release a free electron which may induce a pulse. The time intervals between primary and secondary pulses are then controlled by the propagation time of positive ions; this is most probably the reason for the fact that the lowering of the high voltage increases the time, which elapses between primary pulse and after-pulses. It should not be ruled out, however, that after-pulses may also be produced by free radicals having captured electrons. On the other hand, only a very few positive ions
produce after-pulses. It is probable that in most cases the neutralization energy (10.4eV for H,S’, 10.5eV for HS + )Cl‘r is used for dissociation of H,S molecules. If the dissociation energy (see above) is subtracted from the neuralization energy released, the remainder is less than twice the electron work function, and no free electron is produced. This is probably a reason for the fact that the discharge in H,S is self-quenching and therefore a reason for the fact that H,S can be used as a counting gas in quite a simple manner.
REFERENCES W. and LEIDNERL. paper I. ROEDELW.. JUNKERMANN submitted to Nature. M. thesis, University of Mainz (1963). 2. STOEPPLER M. Nukleonik 6, 335 (1964). 3. STOEPPLER 4. HOFMANNH. and ROEDELW. Int. J. Appl. Radiat. Isotopes 24, 37 (1973). 5. HOFMANNH. and ROEDELW. Int. J. Appl. Radiat. Isotopes 24, 47 (1973). 6. MERRIIT W. F. and HAWKINGSR. C. Analyt. Chem. 32, 308 (1960). 7. MARWINSKYD. thesis, University of Heidelberg (1969). 8. NEUERT H. and CLASEN H. Z. hatar/: 7a, 410 (1952). 9. NEUERTH. Z. Naturf 8a. 459 (1953). 10. LEWIS M. N. and &ITT J. c. Phys. Rev. 255, 894 (1939). I1 WEASTR. C. (ed.) Handbook of Chemistry and Physics. 53rd edition, CRC, Cleveland (1973).
Journal or A&cd
Radiation and Isotopes,
1977. Vol. 28. pp. 7 13-718. Pergamon
Press. Printed in Great Britam
Internal-gas Radioactivity-counting Pure H,S as Counting Gas WALTER
ROEDEL,
Institut
fir Umweltphysik,
LOTHAR Universitlt
LEIDNER Heidelberg,
and WOLFGANG D-69 Heidelberg.
Federal
(Received 27 Drcemher 1976; in final form 10 February
with
JUNKERMANN Republic
of Germany
1977)
The present paper shows that it is possible to use pure H2S as filling gas for an internal-gas radioactivitycounter. H,S used as counting-gas is self-quenching. but spurious after-pulses occur. Therefore a selfclosing gate triggered by the primary pulse and closed for several msec must be introduced in order to produce satisfying plateaux in the counting characteristics. The results obtained with two different types of counting tubes and with H2S pressures between 100 and 450 mbar are reported. Some possible processes taking place in the counting tubes are discussed.
1. INTRODUCTION
now reports our studies on internal-gas-counting pure H,S as counting gas.
THE WORK reported here has been started to develop a highly sensitive technique for measuring very weak samples of radioactive sulfur. Such samples are collected in tracer studies using natural cosmic-ray-produced radioactive sulfur.“’ Various methods have been studied in our laboratory: Stoepp1er’2.3) measured elementary sulfur as an external sample in a low-level proportional counter with thin window; Hofmann and Roedel’4v5’ developed a sensitive technique using gel scintillation counting of appropriate sulfidesThe most promising method, however, seemed to be the internal-gas counting of a suitable sulfur compound. By this technique, a gaseous compound of the nuclide to be measured is added to the filling gas or is used as the filling gas of a counting tube. In 1960, Merritt and Hawking&‘) reported for the first time (as far as is known to the authors) intemalgas-counting of a mixture of methane and sulfur dioxide; the highest possible SOz partial pressure without serious destruction of useful counting characteristics. however, was as low as 15 mbar. Some years later, in our laboratory, Marwinskq7) studied the counting properties of several gaseous sulfur compounds (CS2, H,S, SO,) added to a 90: lo-mixture of argonmethane (partially at Ar-CH, pressures beyond atmospheric pressure). He found H,S to be the most appropriate sulfur compound for intemal-gas-counting. If an “electronic dead time” (see Section 2) is applied in order to suppress spurious after-pulses, H,S partial pressures as high as 400mbar can be allowed. Marwinsky also mentions that it is possible to use pure H,S as counting gas. The present paper
713
2. DETAILS
with
OF OPERATION
The counting-rate vs high-voltage characteristic of a HIS-filled counting tube measured in the usual arrangement (this means tube-preamplifier-amplifierdiscriminator-scaler) does not show any plateau (Fig. 1, curve a). This fact is due to spurious after-pulses following the primary pulse. The rate of these afterpulses depends on high voltage and amounts to five (or more) after-pulses per primary pulse. After-pulses occur statistically during times in the order of a few msec. A typical wave-form is shown in Fig. 2. By a simple and cheap self-closing gate triggered by the primary pulse (the “electronic dead time” mentioned above) these spurious pulses can be suppressed. Then a counting characteristic according to curve b of Fig. I is obtained. This means that H,S is self-quenching
h,ghvollog.lk”,
L 2.1 FIG.
1.
Counting the “electronic
22
2.3
2.4
2.5
characteristics without (a) and with (b) dead time” (H,S pressure 200 mbar).
714
Waiter Roedrl, Lothar
FLH
381
I..ond-gale)
Leidrwr and Woljgang Junkermann
SN 74121 N IFLKIOII lunwbratorl
FG. 3. Block diagram closing
of the electronics (a) with the selfgate (b); R and C control the gating time.
and that no external quenching by lowering the high voltage at the counting tube is necessary. The gating time must be in the order of IO msec. For this reason, the counting rate of an H,S-internal-gas-counter is limited to about 1000 events/min. But this does not seem to be a serious restriction in low-level studies. During the work mentioned above. Marwinsky also performed experiments with external quenching by lowering the high voltage at the counting tube. This, however, does not suppress the after-pulses. The afterpulses seem to be delayed by times in the order of the external quenching time, and appear when the full high voltage has been re-established. The deadtime is thus increased, whereas no improvement of counting characteristics could be found. Figure 3 shows a block diagram of the electronics, with the self-closing gate in detail. The material for building up the counting tubes must be resistant against H,S. For the present work, two counting tubes have been used. The first is made of quartz glass, the cathode surface being coated with graphite spray. In Fig. 4 a schematic drawing of the design of this tube is given. The second tube is very similar to the first; the wall, however, is made of copper coated with chromium (chromium is resistant against H,S). The central wires of both tubes consist of Chroman 25 (Vacuum-Schmelze, Hanau) and have diameters of 50 pm. In spite of the fact that both tubes show nearly the same geometric shape and size. the counting
FK. 4. Schematic drawing (cross-sectlon) of the quartzgraphite-tube; on the right hand. the connections to the preamplifier and the high voltage and to the ground; on the left. the gas inlet. Numbers indicate the length and the diameter (mm).
hqh
2.5
10
is
voiklgr
ikV/ .
4.0
FIG. 5. Counting characteristxs of the copperxhromlumtube at 400 mbars gas pressure. with CH, or H,S as filling gas.
characteristics of the tubes are quite different. cussed in Section 3.
3. COUNTING
as dis-
CHARACTERISTICS
The determinations of pulse-rate vs high-voltage characteristics discussed in this paper have been carried out with commercially available H,S-gas (99X50<, H,S. according to the manufacturer) and with an external gamma-radiation source. For the sake of standardization, the counting characteristics have also been determined with methane as filling gas. Additionally, a few measurements have been made with internal 35S sources’ t H 235S has been prepared by reduction of radioactive sulfate in a hydrogen flow at elevated temperatures, according to Stoeppler.‘3’ Good counting characteristics with sufficient lengths of plateaux have been obtained with H,S pressures within the tubes between 100 and 450 mbar. Beyond 500 mbar, no satisfying plateaux in the counting characteristics have been measured. Figure 5 shows a typical characteristic of the copper-chromium-tube at 400 mbar H,S-pressure, in comparison with the counting characteristic of the same tube filled with methane. The length of the H,S-plateaux is about 25t&3OOV; the slope is about 3% per IOOV (with the external radiation source; with an internal 3’S-source slopes of 1% or less per 100 V have been measured). The counting tubes must not be operated at high voltages beyond the upper ends of the plateaux. If high voltages a little beyond the upper ends of the plateaux are used, the counting characteristics show a hysteresis (Fig. 6 illustrates this effect); if still higher voltages are applied, the counting gas is irreversibly destroyed. The two types of counting tubes (cathode coated with chromium and cathode coated with graphite. re-
715
FIG. 2. Typical waveform of a pulse with after-pulses (horiz. 0.5 mscc/div, vert. 0.5 V/div).
717
Internal-gas radioactivity-counting
, CO”“,
role
forb,lr
cwn,
umh,
role
Wpml
loo-
I
+
0
d
6
50-
hwh
,, l,k
5’7
1,‘s
I,$
YOIIOQC
IkVl
H25 prcssurr
210
loo
FIG. 6. Hysteresis of the counting characteristic after an application of a high voltage beyond the upper end of the plateau.
spectively) show striking differences of electron multiplication in H,S gas and thus of the position of plateaux. The curves of Fig. 7 give the high voltages at the beginning of the plateaux as functions of pressure of H,S and CH,, respectively. The electron multiplication with CH* as the filling gas is nearly the same in both tubes. Whereas, in the quarti-graphite-tube, the electron multiplication is in H2S a little higher than in methane, in the copper-chromium-tube, the electron multiplication in H$ is appreciably lower than in CH,. It should be mentioned, however, that the chromium-copper-tube produces better plateaux (concerning plateau lengths as well as plateau slopes); also, the operation of the copper-chromium-tube is more stable. A further striking difference between the two types of tubes lies in the dependence of counting rates on H,S pressure: in the quartz-graphite-tube the counting rate (normalized at the counting rate with methane as the filling gas) decreases with increasing HIS
300
200
volloge (k VI
8. Count rate vs H,S-pressure in the quartz-graphitetube, with constant radiation source strength; the count rate “loo” corresponds to the count rate with CH, as the filling gas.
pressure, as shown in Fig. 8, whereas such an effect could not be found with the copperchromium-tube. 4. DISCUSSION It is not the aim of the present paper to “explain” the mechanisms occurring in a H,S counting tube. Nevertheless, some possible processes shall be discussed here. Neuert and Clasen’s’ and Neuert”’ studied by mass spectroscopy the ions produced by electron impact on H,S molecules. Besides several species of positive ions they found S- (with a yield of about 1% of the total of ions). They did not find H,S-. That means that H$ is electronegative, the electron attachment being dissociative. According to ref. (9) the appearance potential of the reaction H,S+
e-+H,S--+S-
+ 2H
is 2.9 eV. On the other hand, the reaction + 2H
has an appearance potential of 6.5 eV; thus the electron attachment energy (electron affinity) of H,S should bc 3.6 eV. For the reaction
4.0
H,S-
+ S- + H,,
the dissociation energy amounts dissociative attachment H,S + e--+S
lb0
IdO
360
4dO
7. High voltages at the beginning of the plateaux functions of the gas pressure (---, copperchromium-tube; -, qur.rtz-graphite-tube). FIG.
A.R.I
28/a--c
(mbors, 500
FIG.
H,S-+Shjgh
4w
0
as
to 2.0 eV; thus the + H,
is exothetmic. Neuert and Clasen did not find HSions produced by electron impact, but according to Lewis and White”” it is possible that neutral HS radicals are produced (which cannot be detected by ion mass spectroscopy, as was used by Neuert and Clasen). HS radicals are also electronegative with an electron affinity of 2.19 eV.(’ l)
718
Walter Roedel, Lothar Leidner and Wolfgang Junkermann
Due to these processes, the filling gas is highly electronegative, ard it is to be expected that the electron multiplication factor is low in a counter tube filled with H,S (compared, for instance, with a tube filled with methane). The higher multiplication factor in the quartz-graphite-tube is then most probably due to photon interaction with the cathode material. On the other hand, the electron work functions for chromium (4.374.7 eV) and for carbon (4.WI.84 eV)(‘” are not very different, and it is not clear where the observed difference comes from (perhaps the imbedding material of the graphite spray is responsible for the effect). The after-pulses are most probably due to electrons produced by interaction of positive ions with the cathode wall of the tube. H,S+ has an ionization potential of 10.4eV. “‘) Thus the energy released by neutralization of an ion at the cathode wall is more than twice the electron work energy and thus sufficient to release a free electron which may induce a pulse. The time intervals between primary and secondary pulses are then controlled by the propagation time of positive ions; this is most probably the reason for the fact that the lowering of the high voltage increases the time, which elapses between primary pulse and after-pulses. It should not be ruled out, however, that after-pulses may also be produced by free radicals having captured electrons. On the other hand, only a very few positive ions
produce after-pulses. It is probable that in most cases the neutralization energy (10.4eV for H,S’, 10.5eV for HS + )Cl‘r is used for dissociation of H,S molecules. If the dissociation energy (see above) is subtracted from the neuralization energy released, the remainder is less than twice the electron work function, and no free electron is produced. This is probably a reason for the fact that the discharge in H,S is self-quenching and therefore a reason for the fact that H,S can be used as a counting gas in quite a simple manner.
REFERENCES W. and LEIDNERL. paper I. ROEDELW.. JUNKERMANN submitted to Nature. M. thesis, University of Mainz (1963). 2. STOEPPLER M. Nukleonik 6, 335 (1964). 3. STOEPPLER 4. HOFMANNH. and ROEDELW. Int. J. Appl. Radiat. Isotopes 24, 37 (1973). 5. HOFMANNH. and ROEDELW. Int. J. Appl. Radiat. Isotopes 24, 47 (1973). 6. MERRIIT W. F. and HAWKINGSR. C. Analyt. Chem. 32, 308 (1960). 7. MARWINSKYD. thesis, University of Heidelberg (1969). 8. NEUERT H. and CLASEN H. Z. hatar/: 7a, 410 (1952). 9. NEUERTH. Z. Naturf 8a. 459 (1953). 10. LEWIS M. N. and &ITT J. c. Phys. Rev. 255, 894 (1939). I1 WEASTR. C. (ed.) Handbook of Chemistry and Physics. 53rd edition, CRC, Cleveland (1973).