Induced nuclear reactions: processes – systems – and elements – Nuclear transmutation
Reexamination Certificate
1998-05-29
2001-06-26
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Nuclear transmutation
C376S158000, C376S189000
Reexamination Certificate
active
06252921
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the release of energy stored in excited nuclear matter and neutrons and, more particularly, to the use of long-lived, superdeformed states in nuclei, which have sufficient stored excitation energy, as neutron and energy sources, and to the release of the energy stored in conventional nuclear spin isomers, which can be de-excited using neutron sources. This invention was made in part with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
BACKGROUND OF THE INVENTION
Nuclear isomers are long-lived nuclear excited states that have the same atomic and mass numbers as those of the ground state. Isomers were discovered in 1921 and are currently believed to exist as a consequence of a low excitation energy and a high spin quantum number, I. This combination of low excitation energy and high spin makes the decay of nuclear isomers by &ggr;-ray emission or by conversion-electron emission much slower than for conventional excited states. Because high spin is partly responsible for their long lifetime, such nuclear isomers are frequently called spin isomers (or K-isomers). A nuclear isomer having a lifetime of 44 years, such as
178m
Hf, combines a high spin (I=16) with a relatively high excitation energy (2.4 MeV), which is still much lower than the neutron separation energy (S
n
=7.6 MeV) for this species.
Another type of nuclear isomer,
242m
Am, was found in 1962, in attempts to synthesize very heavy elements. See, “Nuclear Fission,” A. Michaudon, in Advances in Nuclear Physics, M. Baranger and E. Vogt, Eds. (Plenum Press, 1973), Vol. 6, p. 1. This isomer does not have high spin, has a relatively high excitation energy (2.9 MeV), and was observed to decay by fission, as opposed to &ggr;-ray emission. Since this species is not a spin isomer, these observations were subsequently interpreted to be properties of highly deformed matter. Potential energy surfaces (PES) for actinide nuclei are known to possess a second well for large deformations in addition to a first well at ground-state deformation (see, e.g., Michaudon, supra). The
242m
Am isomer is interpreted as being in a superdeformed (SD) nuclear state when characterized by this second well and, for this reason, is called a shape isomer. The inner barrier in the PES between the first and second wells retards &ggr;-ray decay of shape isomers which, therefore, preferentially decay by fission through the outer barrier. About 25 shape isomers, also called fission isomers, because they decay principally by fission, have been discovered to date for actinide nuclei generally grouped between uranium and curium in the Periodic Table.
Since 1986, many SD rotational bands have been observed for nuclei having masses between A≈150 and A≈190 (generated using heavy-ion-induced reactions). The results of PES calculations strongly suggest the existence of a second well in the PES for these nuclei. See, e.g., “Superdeformed Nuclei,” Robert V. F. Janssens and Teng Lek Khoo, Annu. Rev. Nucl. Part. Sci. 41, 321 (1991). Therefore, shape isomers, similar to fission isomers but with smaller atomic numbers, are likely to exist in the A≈150 and A≈190 mass regions, but their decay by fission is inhibited by their outer fission barrier, which is much higher than for actinide nuclei. Although less likely, shape isomers may also exist in other mass regions. Most postulated shape isomers are expected to have an excitation energy E
exc
smaller than S
n
and would decay by &ggr;-ray emission in a similar manner to spin isomers.
Some shape isomers may have an energy E
exc
greater than S
n
, however. This property, which makes the decay of these isomers by spontaneous neutron emission possible, is supported by PES calculations for nuclei in the A=200 region. For example, some mercury isotopes have shown deep second wells with E
exc
of the order of 10 MeV. Other nuclei may present similar properties. See, e.g., “Super-Deformation and Shape Isomerism: Mapping the Isthmus,” by S. J. Krieger et al., Nucl. Phys. A542, 43 (1992) and “Isomères De Forme Dans Les Noyaux Pairs-Pairs: Première Sélection De Candidats Dans La Région De Masses A<208,” by Michel Girod et al., Centre d'Études de Bruyères-le-Châtel Note No. CEA-N-2560 (May, 1998). The neutron decay of these shape isomers should make it possible for them to be useful for both neutron and energy sources. Shape isomers having E
exc
<S
n
may also be of interest. For convenience, (N,Z)
n,is
shape isomers that have neutron number N, proton number Z, and E
exc
>S
n
(E
exc
<S
n
) are called N-isomers (n-isomers) in what follows.
An incident neutron interacting with an isomeric state may be inelastically scattered with an outgoing energy greater than the incident energy because the initial isomeric state of the nucleus can make a transition to a lower-energy state during the interaction. This type of neutron acceleration (called superinelastic scattering), which cannot occur for target nuclei in their ground state, is however predicted by theory and has been experimentally verified for a few spin isomers. For these isomers, neutron acceleration is limited by the small angular momentum carried by the incident neutron, which can therefore cause only low-energy transitions from the isomeric state to other excited states having lower energy, but high spin. In most isomers, states reached in the residual nucleus after neutron acceleration also have a relatively high excitation energy and decay by prompt &ggr;-ray emission, thus liberating most of the energy initially stored in the spin isomer. Superinelastic scattering is also possible with N-isomers with possible greater neutron acceleration than with K-isomers because transitions of the N-isomer to lower-energy states are not limited by the same spin and energy considerations. In addition, the high excitation energy of the N-isomer makes the reaction (n,2n) and neutron multiplication possible, even for incident neutrons with low energies. The exact properties of these reactions depend on the intrinsic properties of the N-isomers (excitation energy and shape of the PES) and on the incident energy of the neutron.
N-isomers have lifetime, yield, and neutron-energy spectrum properties that could make them useful as neutron sources. N-isomers might also be used as neutron multipliers [through the use of (n,2n) reactions] and as “neutron accelerators” (through superinelastic scattering). Such neutron sources, depending on their specific properties and on their availability, could supplement existing neutron sources which rely on radioactive substances mixed with materials with a low neutron-emission threshold (like beryllium), or on fission (like
252
Cf). As an example, a source containing 1 g of N-isomers having A≈190 and a lifetime of 1 yr. would emit neutrons at a rate of about 10
14
n/s and low-energy &ggr;-rays at a similar rate. By comparison, a
252
Cf fission source emits at most about 10
10
n/s (for a quantity of 5 mg), which is 4 orders of magnitude below the above intensity quoted for N-isomers. Large quantities of
252
Cf are unavailable because these nuclei are generated from a long neutron-irradiation chain, which involves a sequence of ten neutron captures with four intervening &bgr;-decays after the process is started with the irradiation of
242
Pu in the high neutron flux of a fission reactor. It is anticipated that the formation of N-isomers would be simpler than for the formation of
252
Cf and that larger quantities of N-isomers are possible to produce than can presently be obtained for
252
Cf. Radioactive neutron sources based on (a,n) reactions induced by &agr;-ray emitters can produce up to about 10
8
n/s and are therefore less intense by about 2 orders of magnitude than
252
Cf sources. The energy of the neutrons emitted by N-isomers is difficult to predict, because it partly depends on the energy difference E
exc
-S
n
. There
Carone Michael J.
Freund Samuel M.
Mun Kyongtack K.
The Regents of the University of California
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