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The period of radioactive phosphorus 15P32
Physica Ill, no. 10
December 1936
THE PERIOD OF RADIOACTIVE PHOSPHORUS ISP32 by G.]. SIZOO and C. P. KOENE Communication from the Natuurkundig Laboratoriurn der Vrije Universiteit, Amsterdam
Summary An ionisation method for the measurement of the relative intensities of very weak activities is described, which has the advantage that the correction for the background ionisation is eliminated. The period of radioactive phosphorus was determined with this method and found to be 15.0 ± 0.1 days.
1. For the period of radioactive phosphorus 15P32 A m a I d i and his collaborators 1) found a value of 14 days, whilst Am b r 0 sen 2) gives 17.5 ± I.! days as the result of a measurement expanded over 3,5 days. The discrepancy between these results lead us to determine this constant with higher accuracy, using a method of measurement which we had already developed for an investigation on the radioactive content of different soils. As this method has proved to be very useful in practice, a short description of it may be given here. 2. The principle of the method is that the ionisation current produced by the radioactive material is compared each time with the ionisation current of a constant source, in such a way that the correction for the background ionisation is eliminated. The electrodes of two ionisation chambers are connected to each other and to an electrometer of high sensivity, placed between them. The chambers are of equal dimensions and have semi-circular crosssection. The larger part of the flat bottom is formed by a removable slide, in which the active material can be spread out. Because of the symmetry of the arrangement, the slides are interchangeable and because of the very accurate working of the apparatus the replacement is an absolutely reproducable operation. To the sidewall of each of the chambers a small cavity is connected, in which the "standard source" can be placed. This source consists of a piece of pitchblende, Physica III
1053 66**
1054
G.
J.
S1Z00 AND C. P. 1WENE
enclosed in a wooden box, fitting very closely in each of those cavities. Therefore also the position of this source is very accurately fixed. As the walls of the two chambers are brought to opposite tensions, the difference of the two ionisation currents is measured. Therefore the remaining background current is very small, but still it might influence the results if the activities to be measured are very weak. \Ve may show however that the very tedious measurement of the remaining background can be avoided, if measurements are performed with four positions of the "standard source" (5) and the material of which the activity has to be measured (M). These positions are: a) M and 5 at chamber 1. b) M at chamber 1,5 at chamber 2. c) M and 5 at chamber 2. d) M at chamber 2, 5 at chamber 1. In each measurement the time required for the movement of the electrometer wire along the same part of the scale (between about -0.03 Volt and +0.03 Volt), is measured. The currents produced by M, resp. 5, placed at chamber 1 may be M 1 resp. 51> those produced if .M, resp. 5, are placed at chamber 2 may be given by M 2 resp. 52. Because of the symmetry of the arrangement MI> resp. 51' is practically equal to M 2 , resp. 52' but at any rate their ratios are the same in all measurements. Let fa, fb,t eand fdbe the times found in the four measurements. Let T be the time corresponding to the background current and the same part of the scale, and suppose the.background currrent to be in the same direction as .i\1 1 and 51. Besides, in order to obtain opposite movements in the cases a and b, and also in the cases c and d, the strength of the standard may be chosen in such a way that 1111 and 1112 are smaller than 51 and 52Then we have:
THE PERIOD OF RADIOACTI\'E PHOSPHORUS 151'32
lOSS
From these equations we find: 2(M 1 +M2) 2(5 1
=
+ 52) =
I
I
i,
t,
a: ( - - -
I o: ( -t ~
I I) +--t, t d
I + -tIb +e T + tI ) . d
Therefore: 111 1 51
+ M2 +52
Putting I +M2/M 1 1 5
+
2/5 1
we conclude: M. tb Id(la C . 51 = Ib Id(la
1
=
C (constant),
+ Ie) - la lAl + Id) + Ie) + fa te(t + Id) . b
b
111 1 and 51 are proportional to the activities of the active material and the "standard source", and C remains constant during all measurements. The right side of the equation therefore may be taken as a mesure for the relative intensity of the material. If, as in our case, the activities are very small, the statistical fluctuations become appreciable. This makes it necessary to repeat the measurements a number of times. This repetition is made very easy by our arrangement, because the change of position a into b, or of position c into d, only requires the deplacement of the standard from one cavity to the other. As this manipulation takes only a few seconds, it is not necessary to earth the electrometer in the mean time. Acomplete measurement, in which fa, tb , Ie and td were measured ten times each, took about one or two hours. Therefore it is allowed to suppose the background current constant during each measurement. 3. For the determination of the decay of radioactive phosphorus a quantity of sulphur, purified by extraction with carbondisulphide, was radiated with a neutron source of 100 mg (Ra + Be) during about 14 days. Ten grams of the irradiated sulphur was spread out in one of the slides, and the decay of the intensity was followed during 50 days with the method described above. In fig. I the intensities and the logarithms of the intensities, expressed in percents of the "standard source", are plotted against time.
1056
THE PERIOD OF RADIOACTIVE PHOSPHORUS ISP 32
The satisfactory fitting of the points to the curves proves the reliability of our method of measurement for these very weak activities. For the logarithms of the intensity the root mean square of the relative deviations of the measured points from the straight line is somewhat less than one percent. r---~-----.-----r----,Lt.S
~·_--+----+----+---_tLt.o
~----'\+----+----+----t3.S
3,0
>-
I-
iii :z
as
>-
t:
:z
u..
til
0
:z
,.j
UJ
~
w
I-
151-----+----,"~--_f\7""--_t 2,0 0
..J
o
1,5
IS
30
liS
60
DAYS
Fig. 1. Dec a y of radioactive phosphorus "P" with time. The radiation intensity is expressed in percents of the radiation intensity ot the standard source.
From our measurements we conclude that the period of radioactive phosphorus is ISP32 15.0 .l..- 0.1 days. We wish to thank Mr. H. K u i per for his valuable help with the measurements. Received October IS, 1936. REFERENCES I) E. A m a I d i, O. d'A g 0 s tin 0, E. Fer m i, B. Po n t e cor v set t i and E. S e g r e, Proc. roy. Soc., London, 113, 522, 1935. 2) J. A 111 bra sen, Z. Phys. UI, 43,1934.
0,
F.
R a-
December 1936
THE PERIOD OF RADIOACTIVE PHOSPHORUS ISP32 by G.]. SIZOO and C. P. KOENE Communication from the Natuurkundig Laboratoriurn der Vrije Universiteit, Amsterdam
Summary An ionisation method for the measurement of the relative intensities of very weak activities is described, which has the advantage that the correction for the background ionisation is eliminated. The period of radioactive phosphorus was determined with this method and found to be 15.0 ± 0.1 days.
1. For the period of radioactive phosphorus 15P32 A m a I d i and his collaborators 1) found a value of 14 days, whilst Am b r 0 sen 2) gives 17.5 ± I.! days as the result of a measurement expanded over 3,5 days. The discrepancy between these results lead us to determine this constant with higher accuracy, using a method of measurement which we had already developed for an investigation on the radioactive content of different soils. As this method has proved to be very useful in practice, a short description of it may be given here. 2. The principle of the method is that the ionisation current produced by the radioactive material is compared each time with the ionisation current of a constant source, in such a way that the correction for the background ionisation is eliminated. The electrodes of two ionisation chambers are connected to each other and to an electrometer of high sensivity, placed between them. The chambers are of equal dimensions and have semi-circular crosssection. The larger part of the flat bottom is formed by a removable slide, in which the active material can be spread out. Because of the symmetry of the arrangement, the slides are interchangeable and because of the very accurate working of the apparatus the replacement is an absolutely reproducable operation. To the sidewall of each of the chambers a small cavity is connected, in which the "standard source" can be placed. This source consists of a piece of pitchblende, Physica III
1053 66**
1054
G.
J.
S1Z00 AND C. P. 1WENE
enclosed in a wooden box, fitting very closely in each of those cavities. Therefore also the position of this source is very accurately fixed. As the walls of the two chambers are brought to opposite tensions, the difference of the two ionisation currents is measured. Therefore the remaining background current is very small, but still it might influence the results if the activities to be measured are very weak. \Ve may show however that the very tedious measurement of the remaining background can be avoided, if measurements are performed with four positions of the "standard source" (5) and the material of which the activity has to be measured (M). These positions are: a) M and 5 at chamber 1. b) M at chamber 1,5 at chamber 2. c) M and 5 at chamber 2. d) M at chamber 2, 5 at chamber 1. In each measurement the time required for the movement of the electrometer wire along the same part of the scale (between about -0.03 Volt and +0.03 Volt), is measured. The currents produced by M, resp. 5, placed at chamber 1 may be M 1 resp. 51> those produced if .M, resp. 5, are placed at chamber 2 may be given by M 2 resp. 52. Because of the symmetry of the arrangement MI> resp. 51' is practically equal to M 2 , resp. 52' but at any rate their ratios are the same in all measurements. Let fa, fb,t eand fdbe the times found in the four measurements. Let T be the time corresponding to the background current and the same part of the scale, and suppose the.background currrent to be in the same direction as .i\1 1 and 51. Besides, in order to obtain opposite movements in the cases a and b, and also in the cases c and d, the strength of the standard may be chosen in such a way that 1111 and 1112 are smaller than 51 and 52Then we have:
THE PERIOD OF RADIOACTI\'E PHOSPHORUS 151'32
lOSS
From these equations we find: 2(M 1 +M2) 2(5 1
=
+ 52) =
I
I
i,
t,
a: ( - - -
I o: ( -t ~
I I) +--t, t d
I + -tIb +e T + tI ) . d
Therefore: 111 1 51
+ M2 +52
Putting I +M2/M 1 1 5
+
2/5 1
we conclude: M. tb Id(la C . 51 = Ib Id(la
1
=
C (constant),
+ Ie) - la lAl + Id) + Ie) + fa te(t + Id) . b
b
111 1 and 51 are proportional to the activities of the active material and the "standard source", and C remains constant during all measurements. The right side of the equation therefore may be taken as a mesure for the relative intensity of the material. If, as in our case, the activities are very small, the statistical fluctuations become appreciable. This makes it necessary to repeat the measurements a number of times. This repetition is made very easy by our arrangement, because the change of position a into b, or of position c into d, only requires the deplacement of the standard from one cavity to the other. As this manipulation takes only a few seconds, it is not necessary to earth the electrometer in the mean time. Acomplete measurement, in which fa, tb , Ie and td were measured ten times each, took about one or two hours. Therefore it is allowed to suppose the background current constant during each measurement. 3. For the determination of the decay of radioactive phosphorus a quantity of sulphur, purified by extraction with carbondisulphide, was radiated with a neutron source of 100 mg (Ra + Be) during about 14 days. Ten grams of the irradiated sulphur was spread out in one of the slides, and the decay of the intensity was followed during 50 days with the method described above. In fig. I the intensities and the logarithms of the intensities, expressed in percents of the "standard source", are plotted against time.
1056
THE PERIOD OF RADIOACTIVE PHOSPHORUS ISP 32
The satisfactory fitting of the points to the curves proves the reliability of our method of measurement for these very weak activities. For the logarithms of the intensity the root mean square of the relative deviations of the measured points from the straight line is somewhat less than one percent. r---~-----.-----r----,Lt.S
~·_--+----+----+---_tLt.o
~----'\+----+----+----t3.S
3,0
>-
I-
iii :z
as
>-
t:
:z
u..
til
0
:z
,.j
UJ
~
w
I-
151-----+----,"~--_f\7""--_t 2,0 0
..J
o
1,5
IS
30
liS
60
DAYS
Fig. 1. Dec a y of radioactive phosphorus "P" with time. The radiation intensity is expressed in percents of the radiation intensity ot the standard source.
From our measurements we conclude that the period of radioactive phosphorus is ISP32 15.0 .l..- 0.1 days. We wish to thank Mr. H. K u i per for his valuable help with the measurements. Received October IS, 1936. REFERENCES I) E. A m a I d i, O. d'A g 0 s tin 0, E. Fer m i, B. Po n t e cor v set t i and E. S e g r e, Proc. roy. Soc., London, 113, 522, 1935. 2) J. A 111 bra sen, Z. Phys. UI, 43,1934.
0,
F.
R a-