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Production of 235Um by nuclear excitation by electron transition in a laser produced uranium plasma
Volume 88B, number 1,2
PHYSICS LETTERS
3 December 1979
PRODUCTION OF 235um BY NUCLEAR EXCITATION BY ELECTRON TRANSITION IN A LASER PRODUCED URANIUM PLASMA Yasukazu IZAWA and Chiyoe YAMANAKA Instttute of Laser Engmeering, Osaka University, Suita, Osaka 565, Japan Recewed 17 September 1979
Production of the 26 min isomer of 23s U has been observed in a laser produced uranium plasma by detecting internal conversion electrons from 23Sum. It can be attributed to _nuclear excitation by electron transition (NEET).
In 1973, Morlta [1] pointed out that in the deexcitation of the orbital electron system, a third mechanism named "nuclear excitation by electron transition (NEET)" is theoretically possible besides X-ray and Auger electron emission. He also discussed the applicability of NEET to isotope separation, especially to that of 235U. The existence of NEET has been verified experimentally in 189Os by Otozai et al. [2]. Recently Okamoto [3] proposed a new method to improve the efficiency of NEET for uranium isotope separation, in which a scheme similar to pellet implosion in inertial confinement fusion is utilized. A pellet or a hollow shell of natural uranium is bombarded by lasers or charged particle beams. The electrons produced by the incident beam collide with the uranium atoms and ionize them to give rise to NEET. In this letter we report the first production of a 235U isomer by the NEET process in a laser produced plasma. The nuclear and atomic energy levels in 235U are shown in fig. 1. The nucleus 235U has a first excited isomeric level, which decays to the ground state with a half-life of 26 min. This transition is not accompanied by "r-rays but only by very soft internal conversion electrons. The transition energy was determined by Neve de Mevergnies to be 73 -+ 5 eV [4,5]. If an electron hole is produced in the 5d3/2 level in 235U and an electron transition A occurs from 6P3/2 to 5d3/2, an isomer 235um may be produced by a nuclear excitation B. The combination of deexcitation A and excitation B satisfies the required conditions for
1/2"
73-*5eV (26mln)
6P3/2
B 7/2-
~' 235U(Nucleus)
1
-325eV
A 70.6eV 5d3/2
U(Atom)
-1031eV
Fig. 1. Nuclear levels in 23Su and atomic levels in U.
NEET, because these transitions have common E3 components and approximately equal transtion energies. In order to produce the electron hole in the 5d3/2 level of U, we used a laser produced plasma. The reason for this is that by laser irradiation a plasma with a high electron density (n e ~ 1019 cm - 3 ) and moderate temperature (several tens to a few hundreds of eV) IS easily obtained. A TEA CO 2 laser beam, whose energy is 1 J in 100 ns at 10.6/~m, was focused by a Ge lens on a metallic natural uranium target in a vacuum chamber. Uranium ions in the laser produced blow-off plasma were deflected by an external electric field and collected on a stainless steel plate. After 100 laser pulses on the target with a repetition rate of 0.5 s -1 , the collector plate was transferred to the detector which is similar to the one described by Neve de Mevergnies [4]. We used a channel electron multiplier (Murata type EMW6081B), and a control grid (made in stainless steel, 150 mesh, and 70% open area) located between the collector plate and the multiplier. A variable retarding voltage for the 59
Volume 88B, number 1,2
PHYSICS LETTERS
electrons was applied between the plate and the grid. The grid and the multiplier input were kept at the same potential. The output from the multiplier was counted through an amplifier. Counting was started within 2 min after laser irradiation. The decay curve of the counting rate without retarding voltage ~s shown m fig. 2 by the open circles. The decay consists of three components, which are fast decay, a slow decay and a time-independent component. The decaying components are also shown in fig. 2 by the solid circles, which were obtained by subtracting the time-independent component from the total counting rate. Half-lives of the decays were estimated from least-squares fits to be 1.0 -+0.1 min for the fast and 25.7 -+ 0.4 min for the slow component. When a retarding voltage of 90 V was applied between the collector plate and the grid, only the fast decay and the time-independent component were observed but the slow decay was not observed. This shows that the slow decay is caused by electrons with energies below 90 eV. Therefore it is concluded that the slow decay can be attributed to the internal conversion electrons from the isomers 235um, which were produced by NEET in the laser produced plasma.
1000Ii* • °...
100
........
t--.
E Z O (D
10
T,n= 2 5 . 7 mln~'~t~ 1
10
i
i
i
I
,
,
,
,
I
50 100 TIME( rain )
=
i
i
i
150
Fig. 2. Decay curve of the counting rate. The open circles are the total counting rate in one minute. The sohd circles show the decay components, which were obtained by subtracting the time-independent component from the total counting rate.
60
3 December 1979
The fast decay was observed, whether the collector plate was transferred to the detector or not. But the time-independent component disappeared when the plate was not transferred. When a high power laser beam is focused on a target, vaporization of the target material occurs at the surface, accompanied by ejection of material. This material consists of neutral atoms, ions and electrons. In our experiments, almost all of the ions and electrons ejec-' ted are deflected by the external electric field and collected on the plates. But the neutral atoms disperse mto the chamber without being deflected. Finally they are pumped out by the vacuum system. The origin of the fast decay is supposed to be txemission from the neutral 234U and 238U atoms evaporated from the metallic uranium target. The time-independent component seems to be due to a-emission from the 234U and 238U ions collected on the plate. It was tried to roughly estimate the NEET cross section. The number of isomers produced by NEET in our experimental scheme is described [3] by
N =nen V(ONV)At, where n e is the electron density in the laser produced plasma, n is the density of 235U m the plasma, V is the volume of the plasma, o N is the NEET cross section, o is the electron velocity and At is the laser pulse width. The bracket indicates the average over the maxwellian distribution of the electron velocities. From the counting rate for the slow decay and the efficiency of the counting system, the rate was estimated to be N ~ 103 nuclei/laser pulse. In the laser produced plasma, the electron density varies rapidly in space from the solid density downwards. Here we assume n e = 1019 cm -3, which is the cut-off density for the CO 2 laser. This assumption is not so improbable, because in the higher density region in the plasma, the electron temperature is too low to produce electron holes in the 5d3/2 level, and m the under-dense region the electron density is too low to produce enough NEET. A value of (ONO) = 1.4 X 10 -20 cm 3 s -1 was obtained by using n = 1019/140 cm -3, V = 10 -6 cm 3, and At = 100 ns. This value is one order of magnitude smaller than that calculated by Okamoto [3]. Possible competitive processes with NEET are Coulomb excitation due to inelastic electron scattering by the nucleus and photoexcitation of the nucleus by
Volume 88B, number 1,2
PHYSICS LETTERS
Table 1 Measured values of R = Cisomer/Cion for CO2 and Nd: YAG laser irradiation; Cisomer and Cion are the counting rates for conversion electrons from the isomers at the time t = 0 and a-emission from the uranium ions, respectively. Wavelength Cut-off density R = Cisomer/Cion CO2 laser Nd:YAG laser
(~m)
(era-3)
10.6 1.06
1019 1021
2.9 +- 0.2 (2.0 -+ 0.3) X 102
X-ray emission from the laser produced plasma. The cross sections for these processes are far smaller than that o f the NEET process. The ratio R o f the counts for the fast decay, due to the internal conversion electron emission from the isomers, to that for the time-independent component, due to a-emission from the uranium ions, gives a measure of the NEET efficiency. F r o m the above considera-
3 December 1979
tions, in order to get a higher efficiency, a laser with a shorter wavelength is desirable, because the cut-off density is inversely proportional to the square o f the laser wavelength. Experimental results for CO 2 (~, = 10.6/am) and N d : Y A G (~, = 1.06 lam) laser irradiation are compared in table 1. It is clear t h a t R (= 2.0 × 102) at 1.06/~m is two orders of magnitude greater than R (= 2.9) at 10.6 gm. It is concluded that 235U isomers were produced b y the NEET process in a laser produced uranium plasma, which was confirmed b y detecting the internal conversion electrons from the isomeric level.
References [1] M. Monta, Prog. Theor. Phys. 49 (1973) 1574. [2] K. Otozai, R. Arakawa and M. Morita, Prog. Theor. Phys. 50 (1973) 1771. [3] K. Okamoto, J. Nucl. Sci. Teeh. 14 (1977) 762. [4] M. Neve de Mevergnies, Phys. Lett. 32B (1970) 482. [5] M.R. Schmorak, Nuclear Data Sheets 21 (1977) 117.
61
PHYSICS LETTERS
3 December 1979
PRODUCTION OF 235um BY NUCLEAR EXCITATION BY ELECTRON TRANSITION IN A LASER PRODUCED URANIUM PLASMA Yasukazu IZAWA and Chiyoe YAMANAKA Instttute of Laser Engmeering, Osaka University, Suita, Osaka 565, Japan Recewed 17 September 1979
Production of the 26 min isomer of 23s U has been observed in a laser produced uranium plasma by detecting internal conversion electrons from 23Sum. It can be attributed to _nuclear excitation by electron transition (NEET).
In 1973, Morlta [1] pointed out that in the deexcitation of the orbital electron system, a third mechanism named "nuclear excitation by electron transition (NEET)" is theoretically possible besides X-ray and Auger electron emission. He also discussed the applicability of NEET to isotope separation, especially to that of 235U. The existence of NEET has been verified experimentally in 189Os by Otozai et al. [2]. Recently Okamoto [3] proposed a new method to improve the efficiency of NEET for uranium isotope separation, in which a scheme similar to pellet implosion in inertial confinement fusion is utilized. A pellet or a hollow shell of natural uranium is bombarded by lasers or charged particle beams. The electrons produced by the incident beam collide with the uranium atoms and ionize them to give rise to NEET. In this letter we report the first production of a 235U isomer by the NEET process in a laser produced plasma. The nuclear and atomic energy levels in 235U are shown in fig. 1. The nucleus 235U has a first excited isomeric level, which decays to the ground state with a half-life of 26 min. This transition is not accompanied by "r-rays but only by very soft internal conversion electrons. The transition energy was determined by Neve de Mevergnies to be 73 -+ 5 eV [4,5]. If an electron hole is produced in the 5d3/2 level in 235U and an electron transition A occurs from 6P3/2 to 5d3/2, an isomer 235um may be produced by a nuclear excitation B. The combination of deexcitation A and excitation B satisfies the required conditions for
1/2"
73-*5eV (26mln)
6P3/2
B 7/2-
~' 235U(Nucleus)
1
-325eV
A 70.6eV 5d3/2
U(Atom)
-1031eV
Fig. 1. Nuclear levels in 23Su and atomic levels in U.
NEET, because these transitions have common E3 components and approximately equal transtion energies. In order to produce the electron hole in the 5d3/2 level of U, we used a laser produced plasma. The reason for this is that by laser irradiation a plasma with a high electron density (n e ~ 1019 cm - 3 ) and moderate temperature (several tens to a few hundreds of eV) IS easily obtained. A TEA CO 2 laser beam, whose energy is 1 J in 100 ns at 10.6/~m, was focused by a Ge lens on a metallic natural uranium target in a vacuum chamber. Uranium ions in the laser produced blow-off plasma were deflected by an external electric field and collected on a stainless steel plate. After 100 laser pulses on the target with a repetition rate of 0.5 s -1 , the collector plate was transferred to the detector which is similar to the one described by Neve de Mevergnies [4]. We used a channel electron multiplier (Murata type EMW6081B), and a control grid (made in stainless steel, 150 mesh, and 70% open area) located between the collector plate and the multiplier. A variable retarding voltage for the 59
Volume 88B, number 1,2
PHYSICS LETTERS
electrons was applied between the plate and the grid. The grid and the multiplier input were kept at the same potential. The output from the multiplier was counted through an amplifier. Counting was started within 2 min after laser irradiation. The decay curve of the counting rate without retarding voltage ~s shown m fig. 2 by the open circles. The decay consists of three components, which are fast decay, a slow decay and a time-independent component. The decaying components are also shown in fig. 2 by the solid circles, which were obtained by subtracting the time-independent component from the total counting rate. Half-lives of the decays were estimated from least-squares fits to be 1.0 -+0.1 min for the fast and 25.7 -+ 0.4 min for the slow component. When a retarding voltage of 90 V was applied between the collector plate and the grid, only the fast decay and the time-independent component were observed but the slow decay was not observed. This shows that the slow decay is caused by electrons with energies below 90 eV. Therefore it is concluded that the slow decay can be attributed to the internal conversion electrons from the isomers 235um, which were produced by NEET in the laser produced plasma.
1000Ii* • °...
100
........
t--.
E Z O (D
10
T,n= 2 5 . 7 mln~'~t~ 1
10
i
i
i
I
,
,
,
,
I
50 100 TIME( rain )
=
i
i
i
150
Fig. 2. Decay curve of the counting rate. The open circles are the total counting rate in one minute. The sohd circles show the decay components, which were obtained by subtracting the time-independent component from the total counting rate.
60
3 December 1979
The fast decay was observed, whether the collector plate was transferred to the detector or not. But the time-independent component disappeared when the plate was not transferred. When a high power laser beam is focused on a target, vaporization of the target material occurs at the surface, accompanied by ejection of material. This material consists of neutral atoms, ions and electrons. In our experiments, almost all of the ions and electrons ejec-' ted are deflected by the external electric field and collected on the plates. But the neutral atoms disperse mto the chamber without being deflected. Finally they are pumped out by the vacuum system. The origin of the fast decay is supposed to be txemission from the neutral 234U and 238U atoms evaporated from the metallic uranium target. The time-independent component seems to be due to a-emission from the 234U and 238U ions collected on the plate. It was tried to roughly estimate the NEET cross section. The number of isomers produced by NEET in our experimental scheme is described [3] by
N =nen V(ONV)At, where n e is the electron density in the laser produced plasma, n is the density of 235U m the plasma, V is the volume of the plasma, o N is the NEET cross section, o is the electron velocity and At is the laser pulse width. The bracket indicates the average over the maxwellian distribution of the electron velocities. From the counting rate for the slow decay and the efficiency of the counting system, the rate was estimated to be N ~ 103 nuclei/laser pulse. In the laser produced plasma, the electron density varies rapidly in space from the solid density downwards. Here we assume n e = 1019 cm -3, which is the cut-off density for the CO 2 laser. This assumption is not so improbable, because in the higher density region in the plasma, the electron temperature is too low to produce electron holes in the 5d3/2 level, and m the under-dense region the electron density is too low to produce enough NEET. A value of (ONO) = 1.4 X 10 -20 cm 3 s -1 was obtained by using n = 1019/140 cm -3, V = 10 -6 cm 3, and At = 100 ns. This value is one order of magnitude smaller than that calculated by Okamoto [3]. Possible competitive processes with NEET are Coulomb excitation due to inelastic electron scattering by the nucleus and photoexcitation of the nucleus by
Volume 88B, number 1,2
PHYSICS LETTERS
Table 1 Measured values of R = Cisomer/Cion for CO2 and Nd: YAG laser irradiation; Cisomer and Cion are the counting rates for conversion electrons from the isomers at the time t = 0 and a-emission from the uranium ions, respectively. Wavelength Cut-off density R = Cisomer/Cion CO2 laser Nd:YAG laser
(~m)
(era-3)
10.6 1.06
1019 1021
2.9 +- 0.2 (2.0 -+ 0.3) X 102
X-ray emission from the laser produced plasma. The cross sections for these processes are far smaller than that o f the NEET process. The ratio R o f the counts for the fast decay, due to the internal conversion electron emission from the isomers, to that for the time-independent component, due to a-emission from the uranium ions, gives a measure of the NEET efficiency. F r o m the above considera-
3 December 1979
tions, in order to get a higher efficiency, a laser with a shorter wavelength is desirable, because the cut-off density is inversely proportional to the square o f the laser wavelength. Experimental results for CO 2 (~, = 10.6/am) and N d : Y A G (~, = 1.06 lam) laser irradiation are compared in table 1. It is clear t h a t R (= 2.0 × 102) at 1.06/~m is two orders of magnitude greater than R (= 2.9) at 10.6 gm. It is concluded that 235U isomers were produced b y the NEET process in a laser produced uranium plasma, which was confirmed b y detecting the internal conversion electrons from the isomeric level.
References [1] M. Monta, Prog. Theor. Phys. 49 (1973) 1574. [2] K. Otozai, R. Arakawa and M. Morita, Prog. Theor. Phys. 50 (1973) 1771. [3] K. Okamoto, J. Nucl. Sci. Teeh. 14 (1977) 762. [4] M. Neve de Mevergnies, Phys. Lett. 32B (1970) 482. [5] M.R. Schmorak, Nuclear Data Sheets 21 (1977) 117.
61