Induced Gamma Emission, IGE

and

The Gamma-ray Laser


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The Gamma-ray Laser

PHOTO

In 1961 Prof. Lev Rivlin of Moscow first introduced the concept and plan for a gamma-ray laser. A concrete design was introduced at the Center for Quantum Electronics in 1982. Experimental work began in earnest in 1986.

During 1991-1992 the Center constructed a dedicated facility for the pumping of nuclear materials. A 4MeV High Intensity Linac was procured and emplaced.

Typical data here proves the high intensity showing the delayed Induced Gamma Emission (IGE) from Hf-179 exposed to a single 30 sec. burst from the linac.

Now, research is quite advanced and there are numerous published articles available in the literature.

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CONCEPTS of Induced Gamma Emission, IGE

The advance of technology makes ever increasing demands upon high powers. At present the highest are EXAWATT POWERS (10^18 Watts), which are encountered in concepts for advanced propulsion technologies, directed energy beams, sterilization countermeasures against biological weapons, photothermal countermeasures against chemical weapons, full-scale nuclear simulators, and laser fusion reactors for commercially competetitive power generation. Many of these advanced applications involve delivery of the final output in the form of X rays and there is a promising solution for those cases.

X rays are the most energetic forms taken by the photons of light and there are new technologies which offer the prospect for directly multiplying the numbers and energies of X ray photons from media of even microscopic dimensions. Introduced under various names, these concepts are found in discussions of gamma-ray laser stimulation and Induced Gamma Emission (IGE.) They all make use of the idea of releasing as electromagnetic waves the energies naturally stored in little-known isomeric materials. If scored by the energy storage divided by the characteristic times for energy released, these species could be rightfully termed exawatt materials.

The nucleus is the smallest part of an atom which in turn is the smallest structural unit of physical matter. Thus, quantum mechanics teaches that the motions of the charged particles found within the nucleus will represent the highest velocities of circulation possible in a sample of any material. This is a fundamental precept that means that the very highest density of (non-nuclear) energy storage will be found in the motions of those charges in nuclei. Just as in the case of atoms, in a nucleus the movement of charges can absorb photons of electromagnetic waves which in this case are X rays and make a transition to an excited state of higher energy.

Because of the high energy densities and great velocities, the charges usually reradiate absorbed energies in times too short to be measured (<10^- 18 sec.) However, in rare cases quantum mechanical selection rules forbid the coupling of the particle motion to the electromagnetic field and then the high energies are stored for tens and even thousands of years in those special nuclei. Such long-lived, high energy states of excitation are termed isomeric levels and the materials are simply known as isomers. Such isomers are the natural materials for INDUCED GAMMA EMISSION, IGE. If such emission has some coherence then the IGE DEVICE producing it will be a Gamma-ray Laser.

PHOTO

The sequence for triggering the release of the energy stored in an isomeric state is described by an integrated cross section for the pumping of that level with a continuum of X rays. The population is transferred through a gateway state where the selection rules that would otherwise limit the process do not apply. The normal decay from the gateway is accompanied by the emission of immediate fluorescence that leads to the principal state from which the sustained output of power will be emitted. Such processes fall into the general catagory of (gamma, gamma') reactions shown schematically in Fig. 1. Population can be accumulated in the output level by continuing to run the pump cycle for a time comparable to the lifetime of the output state much like charging a capacitor.

All (gamma, gamma') reactions occurring at energies below the threshold for particle evaporation excite discrete pump bands, or gateways, as shown in Fig. 1. Although only one gateway appears in the figure, there could be more. Each would be excited at a different pump energy but all would branch to some extent into the same fluorescence level, f. The j-th gateway is shown in Fig. 1 as typical.

PHOTO

In experimental work of the last five years the bremsstrahlung from five accelerators in different experimental environments was used to verify the model for the triggering of exawatt materials and to cross-check the accelerator intensities. During these experiments samples with typical masses of grams were exposed to the bremsstrahlung from the five accelerators for times ranging from seconds to hours for the continuously operating machines and to single flashes from the pulsed devices. Results were in close agreement with the predictions of the model used with literature values of parameters. The most detailed confirmation of theory was obtained with the reaction Sr- 87(gamma, gamma')Sr-87m as shown in Fig. 2. It served to completely confirm the model and to validate its use.

The isomers of many of the exawatt materials belong to the class of nuclei deformed from the normally spherical shape. For those systems there is a quantum number of dominant importance, K which is the projection of individual nucleonic angular momenta upon the axis of elongation. To this is added the collective rotation of the nucleus to obtain the total angular momentum J. The resulting system of energy levels resembles that of a diatomic molecule.

In most cases an isomeric state has a large lifetime because its value of K differs considerably from those of lower levels to which it would otherwise be radiatively connected. As a consequence, bandwidth funneling processes such as shown in Fig. 1 that start from isomeric levels must span substantial changes in K and component transitions have been expected to have large, and hence unlikely, multipolarities.

From this perspective the isomer, Ta-180m was initially unattractive as it had one of the the largest changes of angular momentum between isomer and ground state, 8 units. However, because a macroscopic sample was readily available, Ta- 180m became the first isomeric material to be "optically" pumped (with x rays) to a fluorescent level.

This particular isomer, Ta-180m carries a dual distinction. It is the rarest stable isotope occurring in nature and it is the only naturally occurring exawatt material. The actual ground state of Ta-180 is 1+ with a halflife of 8.1 hours while the tantalum nucleus of mass 180 occurring with 0.012% natural abundance is the 9- isomer, Ta-180m. It has an adopted excitation energy of 75.3 keV and a halflife in excess of 1.2 x 10^15 years.

In an experiment conducted in 1987, 1.2 mg of Ta-180m was exposed to the bremsstrahlung from a 6 MeV linac and a large fluorescence yield was obtained. This was the first time a (gamma, gamma') reaction had been excited from an isomeric target as needed for triggering the release of exawatt materials and was the first evidence of the existence of giant pumping resonances. Simply the observation of fluorescence from a milligram sized target proved that an unexpected reaction channel had opened. Usually grams of material have been required in this type of experiment. Analyses of the data indicated that the partial width for the dumping of Ta-180m was around 0.5 eV.

PHOTO

To determine the transition energy, Ej from the Ta-180m isomer to the gateway level, a series of irradiations was made at the S-DALINAC facility using fourteen different endpoints in the range from 2.0 to 6.0 MeV. The existence of an activation edge was clearly seen in the data shown in Fig. 3. The fitting of such data to the model by adjusting trial values of parameters provided the integrated cross sections for the dumping of Ta-180m isomeric populations into freely radiating states. Reported values were 12,000 and 35,000 in the usual units of 10^-29 cm^2 keV for gateways at 2.8 and 3.6 MeV, respectively. These are enormous values exceeding anything previously reported for transfer through a gateway by two orders of magnitude. In fact they are 10,000 times larger than the values usually measured for nuclei.

FIG

A survey of 19 isotopes conducted with the four U.S. accelerators over a fairly coarse mesh of bremsstrahlung endpoints confirmed the existence of giant resonances for transferring K in the region of masses near 180 as shown in Fig. 4. Activation edges continued to support the identifications of integrated cross sections for pumping and dumping of isomers that were of the order of 10,000 times greater than usual values.

The conclusion of our studies at the Center for Quantum Electronics is that the best material for INDUCED GAMMA EMISSION - IGE is the 31-year isomer of Hafnium-178 because it should be the easiest to trigger.

FIG

The energy level diagram for Hf-178 is shown in Fig. 5 with prominent fluorescent transitions indicated. The transfer band for dumping is plotted where the systematics of Fig. 4 would indicate. The remaining physics issues being resolved by research at the state-of-the-art in various institutions are: 1) the energy of the trigger photon, 2) the cross section for triggering with those photons, and 3) the precise nature of the output spectra of x-rays. While the results are important in detailling the particular form of a practical technology using metastable nuclear states, there are unlikely to be major surprises or reversals.

If the Hf-178 isomer does not pump successfully, it will break a 100% successful trend for the occurrance of trigger levels throughout this mass range. Isotopes from Er-167 to Ir- 191 have proven the pervasive occurrance, location and structure of the important nuclear states. The confidence in finding them for the best material, the 31-year isomer of Hf-178, rests on interpolations of data solidly established for neighboring nuclei on "both sides" of its mass value; as opposed to the alternative of requiring the success of long extrapolations from remote points of reference, poorly established. Now, the significance of the resolution of the remaining physics is of much less importance that the practical issues of production and early availability of test samples.

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FROM QUANTUM ELECTRONICS TO QUANTUM NUCLEONICS

C. B. Collins

Center for Quantum Electronics, University of Texas at Dallas

P. O. Box 830688, Richardson

Texas, 75083-0688, USA

L. A. Rivlin

Moscow State Institute of Radio Engineering, Electronics, and Automation

78 Vernadsky Ave.

Moscow, 117454, RUSSIA

Introduction

The movement of charged particles in confined volumes leads to the emission of electromagnetic radiation. Electrons in antennas emit radio waves and microwaves, while electrons moving in molecules and atoms radiate photons of infrared, light, or x-rays. At small scales the motions of charges are quantized and such electromagnetic radiations are emitted as photons (the simplest of the bosons) during transitions between the discrete levels of energy storage that are allowed in the confined volumes. It was the genius of Prof. Prokhorov together with Prof. Basov and Prof. Townes to first realize that the emission of bosons could be stimulated on the laboratory table and so to make possible the laser as we know it.

Within the smaller domain of the nucleus, two types of charges are capable of independent "movement;" the concentrations of positive charge associated with the protons, and the neutron "holes" in the average charge of the nuclear fluid which must act as localizations of relatively negative charge. The transitions between the quantized states of excitation of either (or both) lead to the emission or absorption of electromagnetic radiation known as gamma rays. However, the lowest energies of gamma radiation overlap the highest energies of x-rays and, once emitted; for those energies there is no distinction between photons arising from atoms and those emitted from nuclei.

The same rules of electromagnetic radiation apply to all systems, so in principle, excited states of nuclei could be stimulated to emit their stored energy coherently. A gamma-ray laser would be only the most straightforward result. Unfortunately, because of the unfamiliar perspectives arising from the strongly interdisciplinary nature of the challenge in stimulating the emission of gamma rays, it is customary to feel that despite abundant evidence to the contrary, nuclei are somehow "shielded" by the electrons and cannot be "seen" by radiation emitted by non-nuclear sources. Originally, results of investigations along the traditional lines of nuclear physics were quite negative in their conclusions, despite the fact they were reporting that ratios of cross sections for resonant nuclear to non-resonant (electronic) interactions of photons with matter actually ranged from 100 to 10,000 across the table of the elements. Nevertheless, from the first proposal for a gamma-ray laser as detailed in an early review it has been clear that an interdisciplinary perspective drawing from quantum electronics, materials science, and nuclear physics could advance the quest for the control of the interactions of photons with nuclei without the need for nuclear reactions involving fission, fusion or energetic material particles.

In all cases the cross section for the interaction of electromagnetic radiation with matter is described by the Breit-Wigner cross section (which is of the order of the square of the wavelength) reduced by effects of recoil, Doppler broadening, and reductions in the lifetimes of the excitation of the material states caused by environmental effects. In the familiar domain of optical frequencies, the latter effects dominate to such an extent that the actual cross section for the stimulated emission of a photon by Nd ions in YAG is over an order of magnitude smaller than comparable interaction of 14.4 keV photons with Fe-57 nuclei in a foil of iron under Mossbauer conditions which prevent recoil and thermal motion of the nuclei. The electrons in the iron do not shield the nucleus because the peak cross sections for interaction of the electrons with the radiation are greatly reduced by broadening, as in the case of the YAG. While the Mossbauer effect is the most obvious ally in the attempts to control the interaction of photons with nuclear states, there are many others which hold even greater promise.

Ab initio, it would seem that nuclear transitions have considerable advantages over atomic and molecular transitions for the stimulation of the emission of radiation at comparable photon energies. When broadening processes are taken into account, the former have larger interaction cross sections in real environments, naturally higher densities for the storage of energies associated with the "movements" of the radiating charges, and more conveniently longer transition times because more modes for coupling the radiation fields to the radiators are practicable than for the atomic case (which is usually limited to electric dipoles.) To us there seems a rich opportunity to exploit these natural advantages to --extend the basic principle of induced boson emission, successfully applied in optical laser physics, to a new class of quantum oscillators, namely, nuclei and antiparticles; --open up opportunities for using in modern science and technology a new energy range of coherent photons, namely, keV and even MeV; --introduce into practice a new type of nuclear reaction, namely, the chain reaction of induced radiative transitions.

We believe that progress in this field will give birth to a new branch of science and technology - QUANTUM NUCLEONICS - that should extrapolate quantum electronics and nonlinear optics into a new range of high photon energies and new quantum amplifying media.


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