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Energy conversion of short-lived radioactive isotopes
Letters to the editors
125
1234567 Absorber
o
I
2
Absorber
thickness,
3
4
thickness,
Absorber
cm
5
0
6
cm
Absorber 1.
5. When shielding three-dimensional y-sources materials of high atomic numbers should be used and when composite shielding is used the part made of low atomic number material should be placed nearest to the source. G. V. GORSHKOV V. M. KODIUKOV
1. GORSHKOVG. V. and KODIUKOVV. M., Atomnaya Energiya
5, 71 (1958). 2. GOR~HKOVG. V., Gamma Radiation From Radioactive Bodies L. Izd. LGU (1956).
Energy conversion of short-lived radioactive isotopes* (Received 22 April 1960) THE cc-and p-active isotopes, formed as a result of the interaction of neutrons with the atoms of a substance, can serve as an emitter of charged particles, which, on being collected on a collecting electrode, set up a difference of potential. Based on this principle, it is possible to build a transducer consisting of an * Translated by W. E.
JONES
from Atomnaya
2
3
4
thickness,
cm
5
6
7
cm
(a-d)
emitter and a collector separated by a solid dielectric or a vacuum. The number of charged particles given off by the emitter in unit time, proportional to the current, which can be produced in the transducer : NptG A=---
A4
REFERENCES
(1961).
I
thickness,
Energiya
10, 12
where N, is Avogadro’s number; CJis the capture cross section for the neutrons; n is the neutron flux; G is the mass of the emitter; M is the atomic weight of the material of the emitter; Tis the half-life of the isotope formed; t is the irradiation time of the emitter’“. For small half-lives (t > T) the number of charged particles is independent of the time. For large half-lives (t < T), the number of charged particles is proportional to the irradiation time. In order to obtain a stable and time-independent operation of the transducer, the aim should be to choose a substance for the emitter with the least possible half-life. In order to best utilize the neutrons, this substance should have a large neutron capture cross section. Experiments were carried out in which isotopes of rhodium (lo3Rh), with a capture cross section of 150 barns, were used as the emitter. As a result of interaction of the neutrons with rhodium, the isotope lo4Rh is formed, emitting /?-particles with energies of 2.5 MeV and a half-life of 41.8 sec. A diagram of the energy transducer is shown in Fig. 1. A rhodium wire, with a diameter of 0.8 mm and a weight of
126
Letters to the editors
FIG. I.-Diagram of the energy converter. 1. Emitter; 2. Dielectric; 3. Collector. 0.42 g, covered with insulating lacquer and a layer of polyethylene of thickness 1.5 mm, is inserted in an aluminium sleeve which serves as the electron collector. During assembly of the transducer, particular attention was paid to the accuracy of finish of the insulation. The prepared transducer was installed in a channel of the research reactor in the Kurchatov Institute of Atomic Energy, Academy of Sciences USSR. A television cable PK-1 was used for the tapping leads. Currents and potentials were measured with an electrostatic voltmeter and a string electrometer. By placing the transducer in a neutron flux of 10ra n/cm2 set, currents of 4.2 x 10e8A were obtained with an external resistance of lOlo ohm and a potential of 420 V. The number of Bparticles given off by the emitter for the given neutron flux can ensure a current of 6 x 1OWamp. The reduction in the observed current, relative to the maximum possible, is explained by the absorption of p-particles in the material of the emitter and in the dielectric. After the transducer had been moved into a zone of the reactor with a neutron flux of 10’0-1011n/cm” set, its current over 2 minutes was reduced to 1.6 x 10-8amp. This result does not contradict theoretical assumptions. A transducer of this type can be used for obtaining a high voltage source of constant potential. In addition, the transducer can be used for determining the neutron flux and, consequently, also the power of the reactor according to the current given by the transducer. The production of a steady value of the current for a given
neutron flux depends on the half-life of the isotope, formed as a result of irradiation of the emitter. As a result of transition from the lower value of the neutron flux to the greater value, build-up of the radioactive isotope occurs. Equilibrium is attained in practice for r/T = 6.65 (deviation 1 per cent), which, by using rhodium, corresponds to 280 sec. For transition from the higher value of the neutron flux to the lower, a certain time is necessary for the decay of the radioactive isotope which has been built up. In the case of a change of neutron flux by a factor of 10, equilibrium is approached in practice for t/i” = 10 (deviation 1 per cent), which, by using rhodium, corresponds to 418 sec. An instrument with this inertia can be used only in the case when the neutron flux is changing slowly. For rapidly changing processes, the derivative of the current with respect to time should be determined, which is also proportional to the neutron flux: 2
_ N,,onG 0.693 ,_F
dtMT
*
For small values of t/T (< 0.0145) it can be assumed that dA NanGO. =dt MT
n=constn
,
i.e. the derivative is independent of time. Then, in the case of using rhodium, t d 0.6 sec. Thus, with a transducer using a rhodium emitter measurements can be carried out over a time either less than 0.6 set or greater than 418 sec. By choosing the emitter material we can measure the instrument inertia and use it for processes which differ in rate and duration. For example, the /?-active isotope 6nFe with a half-life of 46 days is obtained by the 68Co(n,p)68Fe reaction; measurements can be carried out by means of it over a period of t < 11 hours. In all the cases considered, the inertia of the measurement circuit is not taken into account. M. G. MITEL’MAN R. S. EROFEEV N. D. ROZENBLYUM REFERENCE 1. MERREIR., Introduction
to Nuclear
Technology,
Foreign Literature Publishing House (1955).
Moscow,
125
1234567 Absorber
o
I
2
Absorber
thickness,
3
4
thickness,
Absorber
cm
5
0
6
cm
Absorber 1.
5. When shielding three-dimensional y-sources materials of high atomic numbers should be used and when composite shielding is used the part made of low atomic number material should be placed nearest to the source. G. V. GORSHKOV V. M. KODIUKOV
1. GORSHKOVG. V. and KODIUKOVV. M., Atomnaya Energiya
5, 71 (1958). 2. GOR~HKOVG. V., Gamma Radiation From Radioactive Bodies L. Izd. LGU (1956).
Energy conversion of short-lived radioactive isotopes* (Received 22 April 1960) THE cc-and p-active isotopes, formed as a result of the interaction of neutrons with the atoms of a substance, can serve as an emitter of charged particles, which, on being collected on a collecting electrode, set up a difference of potential. Based on this principle, it is possible to build a transducer consisting of an * Translated by W. E.
JONES
from Atomnaya
2
3
4
thickness,
cm
5
6
7
cm
(a-d)
emitter and a collector separated by a solid dielectric or a vacuum. The number of charged particles given off by the emitter in unit time, proportional to the current, which can be produced in the transducer : NptG A=---
A4
REFERENCES
(1961).
I
thickness,
Energiya
10, 12
where N, is Avogadro’s number; CJis the capture cross section for the neutrons; n is the neutron flux; G is the mass of the emitter; M is the atomic weight of the material of the emitter; Tis the half-life of the isotope formed; t is the irradiation time of the emitter’“. For small half-lives (t > T) the number of charged particles is independent of the time. For large half-lives (t < T), the number of charged particles is proportional to the irradiation time. In order to obtain a stable and time-independent operation of the transducer, the aim should be to choose a substance for the emitter with the least possible half-life. In order to best utilize the neutrons, this substance should have a large neutron capture cross section. Experiments were carried out in which isotopes of rhodium (lo3Rh), with a capture cross section of 150 barns, were used as the emitter. As a result of interaction of the neutrons with rhodium, the isotope lo4Rh is formed, emitting /?-particles with energies of 2.5 MeV and a half-life of 41.8 sec. A diagram of the energy transducer is shown in Fig. 1. A rhodium wire, with a diameter of 0.8 mm and a weight of
126
Letters to the editors
FIG. I.-Diagram of the energy converter. 1. Emitter; 2. Dielectric; 3. Collector. 0.42 g, covered with insulating lacquer and a layer of polyethylene of thickness 1.5 mm, is inserted in an aluminium sleeve which serves as the electron collector. During assembly of the transducer, particular attention was paid to the accuracy of finish of the insulation. The prepared transducer was installed in a channel of the research reactor in the Kurchatov Institute of Atomic Energy, Academy of Sciences USSR. A television cable PK-1 was used for the tapping leads. Currents and potentials were measured with an electrostatic voltmeter and a string electrometer. By placing the transducer in a neutron flux of 10ra n/cm2 set, currents of 4.2 x 10e8A were obtained with an external resistance of lOlo ohm and a potential of 420 V. The number of Bparticles given off by the emitter for the given neutron flux can ensure a current of 6 x 1OWamp. The reduction in the observed current, relative to the maximum possible, is explained by the absorption of p-particles in the material of the emitter and in the dielectric. After the transducer had been moved into a zone of the reactor with a neutron flux of 10’0-1011n/cm” set, its current over 2 minutes was reduced to 1.6 x 10-8amp. This result does not contradict theoretical assumptions. A transducer of this type can be used for obtaining a high voltage source of constant potential. In addition, the transducer can be used for determining the neutron flux and, consequently, also the power of the reactor according to the current given by the transducer. The production of a steady value of the current for a given
neutron flux depends on the half-life of the isotope, formed as a result of irradiation of the emitter. As a result of transition from the lower value of the neutron flux to the greater value, build-up of the radioactive isotope occurs. Equilibrium is attained in practice for r/T = 6.65 (deviation 1 per cent), which, by using rhodium, corresponds to 280 sec. For transition from the higher value of the neutron flux to the lower, a certain time is necessary for the decay of the radioactive isotope which has been built up. In the case of a change of neutron flux by a factor of 10, equilibrium is approached in practice for t/i” = 10 (deviation 1 per cent), which, by using rhodium, corresponds to 418 sec. An instrument with this inertia can be used only in the case when the neutron flux is changing slowly. For rapidly changing processes, the derivative of the current with respect to time should be determined, which is also proportional to the neutron flux: 2
_ N,,onG 0.693 ,_F
dtMT
*
For small values of t/T (< 0.0145) it can be assumed that dA NanGO. =dt MT
n=constn
,
i.e. the derivative is independent of time. Then, in the case of using rhodium, t d 0.6 sec. Thus, with a transducer using a rhodium emitter measurements can be carried out over a time either less than 0.6 set or greater than 418 sec. By choosing the emitter material we can measure the instrument inertia and use it for processes which differ in rate and duration. For example, the /?-active isotope 6nFe with a half-life of 46 days is obtained by the 68Co(n,p)68Fe reaction; measurements can be carried out by means of it over a period of t < 11 hours. In all the cases considered, the inertia of the measurement circuit is not taken into account. M. G. MITEL’MAN R. S. EROFEEV N. D. ROZENBLYUM REFERENCE 1. MERREIR., Introduction
to Nuclear
Technology,
Foreign Literature Publishing House (1955).
Moscow,