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Expansion of Research on the Tuning and
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Final Technical Report
7/16/85 - 7/15/86
• PERFORMING ORO. REPORT HUMMER 1
1. ESSYKTSY AITMant NuMicko
Carl B. Collins
». ACRFORHINCl OR# ANUATIOH HANK AND AOORftS
University of Texas at Dallas
Richardson, TX 75083-9688
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NR SDRE-102 412
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This contract supported a critical phase of an ongoing project having
the objective of determining the feasibility and means of realizing a gamma-
ray laser. Previous work had served to establish that a gamma-ray laser is
feasible if a certain combination of elementary properties can be found in a
real nuclear material. Traditional technology could not support the evalu¬
ation of these laser-like properties of nuclei and new methodologies had to
Continued on attachment.
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SECURITY CL ASSlFlCATtOlTor THIS PAGE (Whmt Dotm WZl-rod)
RESEARCH REPORTS DIVISION
NAVAL postgraduate school
MONTEREY, CALIFORNIA 93040
QUANTUM ELECTRONICS AND APPLICATIONS
THE UNIVERSITY OF TEXAS AT DALLAS
BOX 688 RICHARDSON, TEXAS 75080
be developed. Described here are highly significant successes in the
development and demonstration of prototypes of two new devices needed
for the screening of candidate laser materials in a realistic time
frame, a flash x-ray device for pumping test materials and a Nuclear
Raman Spectrometer to facilitate the search for certain arrangements
of nuclear energy levels.
FINAL TECHNICAL REPORT
for the period
16 July 1985
through 15 July 1906
Office of Naval Research
Task No. NR SDRE-102
EXPANSION OF RESEARCH ON THE
TUNING AND STIMULATION OF NUCLEAR RADIATION
Principal Investigator: Carl B. Collins
The University of Texas at Dallas
Center for Quantum Electronics
P.0. Box 830688, Richardson TX 75083-0688
Reproduction in whole, or in part, is permitted for any
of the United States Government.
♦This document has been approved for public release and
distribution is unlimited.
TABLE OF CONTENTS
A Flash X-Ray Source of Intense
Nanosecond-_fulses Produced at High
Repetition Rates .9
Nuclear Raman Spectrometer .15
The universal concerns of those aspiring to develop a new
type of high-power pulsed laser are to maximize the storage time
of the pump energy and to minimize the bandwidth of the laser
transition. Successes in either direction are generally rewarded
by lessened demands upon both the peak level of input power and
total fluence needed in the pump source to achieve threshold in
the laser medium. By definition, a gamma-ray laser must draw its
energy from nuclear excitation and so there exists the unique
opportunity to exploit the Mossbauer effect to insure that the
minimum bandwidth possible for the laser transition is actually
achieved. Provided the macroscopic host in which the emitting
nuclei are diluted is sufficiently rigid and provided tempera¬
tures are sufficiently reduced, nuclear transitions at energies
in the range from 1-100 keV can occur with widths corresponding
to the natural lifetimes of the levels. Over the past six years,
our research group has described'*' ^ several viable means
through which this excitation energy might be coupled at will to
the radiation fields while maintaining the natural Mossbauer
width. In such cases the cross section for stimulated emission
could reach 10 cm , ten times more favorable than the value
for the stimulation of 1.06 /jm from Nd^ + in YAG.
At the nuclear level the storage of excitation energies in
the Mossbauer range can approach tera-Joules (10 J) per liter
for thousands of years. The stimulated release of this energy
would occur at the rate at which resonant electromagnetic
radiation passed through the laser medium and could lead to
output powers as great as 3 x 10 Watts/liter. This is an
astronomical level of intensity and has not been approached to
within five orders of magnitude on earth by any means previously.
The peak power from a one liter device would represent 0.03% of
the total power output from the sun. At this level a relatively
small laser rod of 5 mm in diameter and 1 cm long would emit a
pulse of power comparable in amplitude to the total power
developed by a 100 megaton nuclear device at the peak of its
burn. Of course the output from the 100 megaton detonation would
persist for a much longer duration, but with the laser the same
total power would be delivered in a narrow output beam.
Unfortunately, the quest for a gamma ray laser has been one
of the longest unfruitful efforts in the field of laser science.
Virtually all of the sustained pioneering work was done by
Baldwin and Solem’s groups in the US and by Gol’danskii ’ s in
the USSR and focused upon the single photon, brute force approach
to pumping. Their work dealt extensively with concepts involving
the use of a neutron flux for pumping the laser medium, either in
real-time or as a preparatory step to be followed by a rapid
separation of isotopes within their natural lifetimes. All
proposals were concluded to require infeasibly high levels of
particle fluxes to pump the inversions, exceeding even those
available from nuclear explosions, and to require neutron
moderators having virtually infinite thermal capacities. By 1980
all conceivable variants of the single photon approach had been
characterized as hopeless. In 1981 this "traditional" approach
to a gamma ray laser was virtually abandoned with Baldwin’s
publication of the monumental review 15 of all classical efforts.
The involvement of our UTD Center for Quantum Electronics
dates back to 1978 and can be conveniently divided into three
periods of accomplishments. First was an NSF sponsored period of
international theoretical effort from 1978 to 1981. It arose
from a long program of previous activity focused upon fundamental
interactions of coherent radiation with matter that had been
conducted within NSF’s International Cooperative Science Program.
Concerned at first with the problem of the correct gauge and
basis sets to use in describing multiphoton process, we began to
consider the impact of the work upon areas other than the usual
atomic and molecular. As a result, the modernized concept of
coherent pumping with optical radiation was introduced in a
sequence of papers concerned with nonlinear processes mediated
by virtual states of nuclear excitation and included the stimu¬
lated anti-Stokes scattering of intense but conventional laser
radiation. The theoretical treatment served to estimate matrix
elements for a new class of two-photon Mossbauer transitions
making possible, in principle, the frequency upconversion of
optical laser photons to gamma ray energies.
In 1981 the implications of this theoretical renaissance to
the prospects for a gamma rav laser based on several variants of
upconversion were reviewed in an article appearing the following
year. If strengthened by recent infusions of dressed state
theory, ’ that article still provides the most convenient
review of the basic concepts and requirements for a viable gamma
ray laser scheme.
The second phase of our y-ray laser program at (JTD began in
1981 with the initiation of a modest effort supported by ONR that
was aimed at providing some experimental verification of the
theoretical models. Successful beyond any reasonable expecta¬
tion, it produced a small but important group of experiments that
have been recently reported. 9,11 " 13 These experiments to date
13 That the matrix elements used in the previous research
phase to obtain the favorable estimates of the threshold
for laser output were correctly estimated;
2) That extremely large ferromagnetic enhancements of the
effective powers applied in the coherent pumping scheme
can be obtained;
3) That enhancements may also result from the modulation
of the polarization of ferroelectric media in which
coherently pumped nuclei could be diluted.
The conclusion from these experiments is that the gamma-ray laser
is definitely feasible if a sufficiently ideal isotope exists
in reality. This is the single most critical issue to the
development of a gamma-ray laser--the identity of the most nearly
ideal candidate for upconversion.
Despite the many applications of beautiful and involved
techniques of nuclear spectroscopy, the current data base is
inadequate in both coverage and resolution either to answer the
question of whether an acceptable isotope exists or to guide in
the selection of a possible candidate medium for a gamma-ray
laser. Two new techniques for the measurement of nuclear
properties with laser-grade precision resulted from the second
phase of our research into the feasibility of a gamma-ray laser.
The contract supporting the achievements being currently
reported initiated a third phase of the project. The objective
of this phase was to develop and to demonstrate prototypes of the
hardware needed to instrument the two new measurement techniques
that will be essential for the screening of candidate materials
for a gamma-ray laser. Reported here will be the construction
and evaluation of a flash x-ray device for pumping test materials
and a Nuclear Raman Spectrometer to facilitate the search for
certain necessary arrangements of nuclear levels.
By involving two distinct steps, the schemes we have proposed
for pumping a gamma-ray laser avoid the severe relationships
between storage times and spontaneous powers wasted at threshold
that were imposed on the single-step processes. Replacement
power that is required falls within a technically accessible
range avoiding damage to the laser medium.
These two-step, upconversion processes for optically pumping
nuclear reactions can be divided into two categories that cor¬
respond to the type of pumping employed: coherent and inco¬
herent, as shown in Fig. 1. The critical concept here is that
either transfers the stored population to a state at the head of
a cascade leading to the upper laser level. To be effective
the pumping processes cannot transfer too many quanta of angular
momenta from the fields and the cascade provides a mechanism for
further changes that may be necessary to reach the laser levels.
Then the ultimate viability of these pump schemes will depend
upon: (1) spectroscopic studies locating a suitable configura¬
tion of nuclear energy levels, and C2] "kinetic" studies
Schematic diagram showing the energetically excited levels
of a typical nucleus of interest to the development of a gamma-
ray laser. Lifetimes of the stored energies in the isomeric level
produced by the initial capture can range from days to hundreds
of years. The first phase of the two-step process for the
stimulated release of the stored energy is shown in the figure by
the solid arrows. Both correspond to the use of longer
wavelength radiation to lift a nucleus from the storage level to
a higher level of excitation that has a much shorter lifetime.
The arrow marked (1) illustrates the incoherent pumping of the
storage level through the absorption of an x-ray that is resonant
with the energy separation between the storage level and the next
higher level of proper symmetry. The arrow marked Cl’) repre¬
sents the alternative process of coherent pumping through the
non-resonant absorption of a photon from the radiation field in
order to create a virtual or dressed state of excitation shown by
the dashed level in the figure. In either case the gamma-ray
output ultimately results from the upper laser level populated by
a cascade occurring as a second step, as shown in the figure by
either of the double arrows, (2) and C2’).
providing an efficient path of cascading from the intermediate or
dressed state to the upper laser level.
Both the coherent and incoherent schemes for pumping a
gamma-ray laser make stringent demands upon the arrangement of
nuclear energy levels in the potential laser material. Both also
depend upon the successful arrangement of an input source of
radiation either to mix the properties of the storage level with
those of some other state to release the metastability or to
simply transfer the populations from the storage level to the
For the examination of the nuclear properties we introduced
the techniques of Modulated Nuclear Radiation (MNR). This
methodology is the nuclear analog of the optical double resonance
studies which yielded much of the laser-grade database upon which
rest the newer visible and UV lasers. A database of comparable
quality for nuclear "kinetics" will be required for the screening
of the candidate materials, and this will require a great amount
of input radiation into implementations of the MNR process.
Essential to the success of this technique is the accessibility
of a source of pulses of x-rays of nanoseconds duration that can
emit a total of 10 keV/keV of linewidth in a reasonably brief
working period. Either laser plasmas or large e~beam machines
can do this in a single shot, each of which requires about an
hour of laboratory time to prepare; but costs are very high. As
a result, none of these traditional light sources for the
subAngstrom region could be used to complete an evaluation of the
29 most attractive materials before the turn of the century. In
the following section we report recent successes with a prototype
flash x-ray device producing 35 mW of average power from 10 nsec
pulses at energies near 8 keV, a level of performance approaching
that of a large synchrotron light source. University of Texas
System attorneys have considered this device to be patentable, and
appropriate actions have been taken.
To facilitate the search for the scattering states
energetically near isomeric levels, we had described a new
technique of Nuclear Raman Spectroscopy that now promises to
extend both the resolution and the tuning range available for
high resolution nuclear spectroscopy of the type needed in the
search for potentially resonant intermediate states needed for
applications of the dressing process.
The physical effect upon which this technique depends is
that new transition frequencies are generated at the sums and
differences between the frequency of the gamma transition and
those of an integral number of photons dressing the nuclear
states. By varying the frequency of the mixing radiation the
energy of the sum frequency line can be swept, just as in the
analogous processes implemented in the optical range and this
provides the bases for the Nuclear Raman Spectroscopy (NRS)
technique. It could be used to conduct Mossbauer spectroscopy
with much higher resolution than previously achieved. This would
be possible because no mechanical tuning needs to be used. In
case microwave or infrared photons were used, the tuning range of
Mossbauer spectroscopy would be dramatically increased. Viabili¬
ty of this NRS technique has now been fully demonstrated up to
radiofrequencies. The most recent results are described in the
A Flash X-Rav Source of Intense Na nosecond Pulses Produced
at-iiiflfr JIsils-S. inon Rates
Central to the problem of using our MNR technique to eval¬
uate candidate materials for a rray laser is the accessibility
to a source of pulsed X-rays that can emit at least 10 keV/keV
in a reasonably brief working period. We mentioned above that
laser plasmas can do this in a single shot, each of which requires
about an hour of laboratory time to prepare.
This section reports one of the achievements^ rea¬
lized in our Center for Quantum Electronics at UTD from this
contract work. We have succeeded in demonstrating a compact
flash X-ray device producing 35 mW of average power isotropically
from 6 nsec pulses at energies near 8 keV. At this level we
produce a photon flux in the K~lines that is only one order of
magnitude lower than the design objective of the Advanced
(Synchrotron) Light Source (ALS). In less than a minute our
device delivers an integrated fluence comparable to that obtained
in a similar line from a multi-kiloJoule, laser plasma shot. Of
course, each of the larger devices has its unique advantages,
more collimated beams from the synchrotrons and richer line
spectra from the laser plasmas; but unless those capabilities are
essential to a particular application, the laboratory scale
system we report here may offer attractive support for many
experiments in other applications, as well. Since it can be
scaled to much higher energy K-lines and much larger sizes, it
represents a proof-of-feasibi1ity of the large scale device,
V"SHEATHED IN KAPTON
VCAST IN EPOXY
CAST X-RAY TUBE
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1 _i MATERIALS
Schematic drawing of the high repetition rate,
device characterized in this work.
needed for the irradiation of candidate laser materials that must
be pumped at energies beyond those available from laser plasmas.
As shown in Fig. 2, this first flash x-ray device consisted
of three critical subassemblies: 1) a low impedance x-ray tube,
2) a Blumlein power source and 3) a commutation system capable of
operation at high repetition rates. The latter two components
differed little from drivers we had developed for short pulse
nitrogen ion lasers. For the present application the Blumlein
was constructed from massive copper plates, potted with epoxy on
outer surfaces to reduce corona and separated by layered Kapton
(polyimide) dielectrics of 0.635 mm total thickness. The line
impedance and transit times were calculated to be of the order of
1.50 and 5.3 nsec, respectively. Capacitances of the switched
and storage sides of the line were measured to be 3.5 and 3.2 nF,
respectively. In operation the middle conductor was charged to a
positive high voltage which could be varied to 25 KV and commuta¬
tion was effected by an EG&G 3202 hydrogen thyratron mounted in a
grounded grid configuration. The average available input power
was sufficient to support operation to a 100 Hz repetition rate.
With emphasis on obtaining a singularly low inductance input
to electrodes designed to give a filamentary source of radiation,
the x-ray tube was constructed from cast materials, selected to
minimize erosion and maximize heat transfer. Copper foil strips
0.05 mm thick and 10 cm wide were fastened to the electrode mounts
and then passed through the cast material forming the base of the
x-ray head before it hardened. After emerging from the base, the
foils were joined to the outermost copper plates of the Blumlein.
In this way any transverse constriction of the path of the dis¬
charge current between the Blumlem and the electrodes was avoided.
The anode was simply a rod of copper partially buried in the
cast material forming the base while the cathode was demountable.
It consisted of a strip of 0.381 mm thick graphite further ground
with a blade-like edge 10 cm wide and separated from the anode by
a variable distance chosen to optimize performance. The electri¬
cal length of the strip was selected to give a resistance that
was comparable but below the line impedance in order to assist in
damping the ringing of the discharge current at times subsequent
to the initial pulse which produced the x-rays.
The discharge space was enclosed by a pressure shell, also
fabricated from cast materials with an integral window of 0.076
mm thick Kapton plastic film. The window aperture was covered
with a graphite plate 0.127 mm thick to eliminate the emission of
visible and UV light. Even with the cast construction and ready
access to internal electrode spacings, operating pressures below
3.0 mTorr were routinely maintained with a small mechanical pump.
Precise measurements of time-resolved voltages and currents
were rendered difficult by the extremely low impedance of the
Blumlein and by the commutation of the thyratron in a grounded
grid configuration on this particular decade of time scales, 1-10
nsec. Operation generally produced the switching waveforms ex-
pected of such Blumleins with the observation that the slower
commutation of the thyratron prevented a full doubling of the
applied d.c. voltage. The discharge seemed to develop at a value
of voltage across the x-ray tube that was roughly 50% larger than
the original charge voltage of the Blumlein.
Outputs were detected with a block of iast scintillator
plastic equivalent to NE114 with a nominal 7.0 nsec decay time.
The resulting light output was measured with a faster photomulti¬
plier with 1.5 nsec resolution and recorded with a Tektronix
7912AD transient digitizer. Numerical deconvolution was subse¬
quently employed to remove these instrumental time constants.
The resulting data is shown in the inset to Fig. 3 to have a
temporal width (FWHM) of about 10 nsec.
Measurements of absolute intensities were made by comparing
the time integrated fluorescence from the plastic detector when
illuminated with geometrically attenuated x-ravs from the flash
source directly with the level of excitation produced by a
radioactive source of known characteristics. This technique was
used to determine the dependence of the total energies in the x~
ray pulses as functions of the important experimental variables.
Results are shown in Fig. 3 and suggest a rather surprisingly
weak dependence of the output on anode-cathode separation over a
substantial range of the smallest gaps. Moreover, an examination
of the pulse durations at the different separations showed no
variation that could be detected within the limits of resolution
permitted by the deconvolution noise apparent in the inset to
Linder single pulse conditions the x~ray fluence delivered to
an external target could be visually examined for uniformity by
allowing it to fall upon a fluoroscopy screen of the type used
in radiography. A uniform pattern, sharply delineated by the
edges of the output aperture could be readily seen to result from
ELECTRODE SPACING (mm)
Total pulse energies emitted near 1.5 A under single shot
conditions C2Hz) as functions of the critical experimental
parameters: charge voltage and electrode spacing. While
obtaining the variations shown, the other parameter was fixed at
0.127 mm spacing and 24 kV, respectively. Shown in the inset is
the time resolved intensity from a typical pulse at 24 kV and
0.127 mm gap.
the great majority of discharges. At higher repetition rates the
x-ray pulse energies were found to increase markedly, in direct
contrast to the behavior generally observed in other applications
when such Blumleins were used for the deposition of energy into
large volumes of gaseous laser media. Figure 4 shows that the
resulting dependence of the average x-ray power emitted upon
repetition rate seems to be greater than linear because of this
enhancement of pulse energies at the higher values.
Attenuations measured with a combination of K~edge filters
and known thicknesses of aluminum foil indicated that at least
25% of the total x-ray energy lay in the Cu K lines, in agree*
ment with previous observations and expectations. ’ If it is
assumed that these lines have their customary width of around
6eV, best values of output energy correspond to about 4.7 x 10
keV/keV when expressed in terms customary for reporting laser
plasma yields. Thus, in less than a minute of experimental time
at 100 Hz repetition, over 2 x 10 keV/keV could be emitted.
This would exceed the dose available from a shot of a large laser
plasma system at these X-ray wavelengths corresponding to 8 keV.
Nuclear Raman Spectrometer
During this contract period a fully integrated Nuclear Raman
Spectrometer was designed, constructed and has now begun to
operate. Built from an Apple computer and a Wavetek frequency
synthesizer it provides for the conduct of swept frequency
spectroscopy of nuclear levels without the need to employ any
mechanical effects for tuning that are required in conventional
implementations of Mossbauer spectroscopy. The successes of the
REPETITION RATE (Hz)
Average powers emitted as X-rays near 1.5 A as a function of the
repetition rate for a charge voltage of 20 kV. The linear
approximation corresponds to the emission of a pulse energy
independent of repetition rate.
Schematic representation of the experimental aggaratus for
demonstrating the mixing of nuclear states in Fe.
new NRS technique for nuclear spectroscopy indicate that a much
higher resolution, by perhaps six orders of magnitude, can be
achieved through a further upgrade of the apparatus. If the
range of tunability does extend to the ferromagnetic spin
resonance (FSR] frequency, then it will be possible to construct
a swept frequency device capable of continuously tuning over a
range of 10 linewidths, an enormous improvement in the state-
of-the-art of nuclear spectroscopy.
Construction details and operation of -a prototype NRS system
were reported in the literature earlier this year. That
device is shown schematically in Fig. 5. It was first intended
to be the nuclear analog of a spectrophotometer in which the
dispersive element is replaced with a constant acceleration motor
which moves a narrow line source of gamma radiation in order to
sweep the wavelength by the resulting Doppler shift. The radio¬
frequency power was applied to a coil containing the absorption
foil. It was pulsed for a duration equal to the time required
for the full sweep of the range of velocities being examined.
The output from a proportional counter monitoring the transmitted
y intensity was gated into a multichannel scalar only during
The absorption spectrum of ground state Fe nuclei has long
been considered a benchmark for techniques of Mossbauer
spectroscopy. In pure polycrystal1 me iron foil the magnitude of
the bulk magnetization is the same at all nuclei but varies
widely in direction for nuclei in different magnetic domains. As
a result, the standard 14.4 keV transition to the first excited
state is split by the hyperfine energies resulting from different
orientations of the spins of the states with respect to the local
magnetization vector. Figure 6 shows typical data with the six
allowed transitions being identified by the quantum numbers for
spin projection with the order for the Mj being ground state
followed by excited state.
Also shown in Fig. 6 are the sidebands to these six basic
transitions produced by dressing the nuclear states with
radiofrequency (rfj photons from the applied field. For scale
the energy of one rf photon is shown by the arrow enabling the
sidebands in the figure to be readily identified. For example,
if the line results from either the excited state being dressed
upward in energy by one photon, or the ground state downward by
one, the sideband line will appear displaced to lower transition
energy (to the left) by the length of the arrow.
The data of Fig. 6 was taken with the dispersion being
obtained in the conventional manner with Doppler shifts. The
true Nuclear Raman Spectroscopy (NRS) is based upon the use of
one of these sum or difference frequency sidebands generated by
mixing nuclear states as a tunable spectroscopic probe. By
varying the frequency of the mixing radiation the energy of the
sum frequency line would be swept, just as in the analogous
processes implemented in the optical range. While the addition
of a tunable sideband to a nuclear source is more instinctively
attractive, the complementary experiment with an absorber proves
the same principles with less expense.
An extension of the previously described experiment was
used to demonstrate the possibility for such Nuclear Raman
a s i * t
0.96- jf t
200 400 600 800
Typical data obtained for pure Fe at 10.1 W of radio - 1requency
power at 61.7 MHz. The parent transitions are identified by (M-,
M,), the projections of the nuclear spins in the initial and fiAal
states, respectively, upon the axis of local magnetization M .
The change in transition energy resulting from one additional
radio-frequency photon is indicated.
Spectroscopy (NRS) with the first prototype system. Figure 7
illustrates the concepts involved. The unsplit source energy
corresponded to the wavelength shown in Fig. 7 by the heavy
vertical line. At the lower mixing frequency shown in the upper
data, the particular feature 5+ lies at longer wavelength than
the source, while at higher frequency it lies at a lower value.
It can be imagined that with a smooth change of frequencies
between the two values, a point would be reached at which the
transmission of y-photons from the source would be suddenly
reduced. The realization of this supposition is seen in Fig. 8 .
In addition to the absorption resonance resulting from the
expected, 5+ line, another smaller feature appeared. It
corresponded to 3 + +, the second order sideband to transition #3.
It hints at the greater sensitivity of the NRS technique.
In any case, the point of the experiment was not to
characterize the energy levels of Fe but to demonstrate the
effectiveness of this new NRS technique for nuclear spectroscopy
in general, based on multiphoton effects. However, with this
prototype system, frequencies could be changed only manually
with rather elaborate retuning of the radiofrequency system being
necessary for the acquisition of each point. Completed during
the present contracting period was a full integration of
component subassemblies that allows the frequencies to be
stepped through a range in response to commands issued from the
CApple) computer controller via the IEER-488 protocol. Dwell
time at each frequency could be set by software at run time.
Counts resulting from 7 -ray photons detected during the period of
Two spectra t
lies at w .
Gamma transmission intensity plotted as a function of rf photon
frequency. The larger absorption line corresponds to the first
order, sum-f r equency s^eband of the |l/2, -1/2 >*• | 3/2 , - 1/2 >
nuclear transition of Fe, while the smaller line corresponds
to the second-order, sum-f r equency sideband of the | 1 / 2 , 1 /2 >“* | 3/2 ,
mixing at a single frequency were stored at addresses identified
with that frequency.
Since no mechanical movements or Doppler shifts were
involved in any way, instrumental resolution was set by the
stability of the frequency synthesized, +100Hz. However, the
transform of the lifetime of the 14.4 keV excited state of Fe
is 2.3 MHz so no detail can be expected on a scale significantly
smaller than MHz. Figure 9 presents the first NRS spectrum ever
obtained that is densely packed with observations at contiguous
frequency intervals. Instrumental resolution of both 60 and 30
kHz are shown.
An unexpected richness of structure is shown in Fig. 9. The
feature to the right of center, magnified in the higher resolu¬
tion scan is the superposition of the (-1/2, -3/2) minus one
photon sideband with the (+1/2, +3/2) plus one photon sideband,
as expected. These two absorption peaks are not precisely coin¬
cident, rather being displaced by a frequency, v. such that hv^
is the chemical shift of energies between the radioactive source
and absorber. The appearance of two peaks is clear in the high-
resolution version and could be fit to two lineshape functions
from which the chemical shift could be extracted.
Some of the other features can be interpreted as expected
sidebands but some are more difficult to identify. While those
latter examples reproduce during remeasurement of the spectrum,
their relative intensities seem quite sensitive to surface
conditions of the absorber material and may reflect further
shifts in energy caused by the proximity of a surface. Much more
work is needed for an unequivocal interpretation. Nevertheless,
such data attest to the extraordinary power and resolution of
the NRS system.
The achievements of this contracted work are very
significant to the critical path for development of a gamma~ray
laser. A gamma-ray laser is feasible if the needed sequence of
energy levels can be found in some real nucleus. Theory suffices
to identify 29 prime candidates among the 1886 distinguishable
nuclear materials, but further screening to eliminate inutile
members of this prime set of materials cannot be done with
Described in this report are the successful demonstrations
of two new devices which can be scaled to a capacity able to
evaluate the 29 candidates in a realistic time frame. One is
the flash x~rav device needed to mount the nuclear analog of
optical double resonance measurements to evaluate pumping
efficiencies and requirements. The ultimate signal which can be
made available to provide a basis for judgment of the merits of a
particular candidate material will be directly proportional to
the total keV/keV emitted by the x~ray source per hour of
laboratory time. The table top device reported here has already
achieved over 10 " keV/keV per hour in early tests. It was
designed to represent 1/10 scale of a single module of the
ultimate device which would be used in an array of 16 modules for
the screening of candidates. Thus it is entirely reasonable to
project an average spectral power of 2 x 10 21 keV/keV per hour in
the range of photon energies from a few to 115 keV.
The other device reported here is the Nuclear Raman
Spectrometer bringing the combination of extended resolution and
extended tuning range needed for the identification of nuclear
states useful in dressing isomeric levels. To dress an isomeric
state requires a certain arrangement of nuclear levels that would
make them indetectable to conventional techniques of nuclear
spectroscopy. Our method of NRS is the only means found to date
that can be used to search for this combination among the 29 best
candidates. The successes of the new NRS apparatus for nuclear
spectroscopy indicate that a much higher resolution, by perhaps
six orders of magnitude, can be achieved through a reasonable
upgrade of the apparatus. If the range of tunability does extend
to the ferromagnetic spin resonance (FSR) frequency, then it will
be possible to construct a swept frequency device capable of
continuously tuning over a range of 10 linewidths, an enormous
improvement in the state-of-the-art of nuclear spectroscopy.
C. B. Collins, S. Olariu, M. Petrascu, and I. Popescu,
Phys. Rev. Lett. 4_2, 1397 (1979).
C. B. Collins, S. Olariu, M. Petrascu, and I. Popescu,
Phys. Rev. C 20, 1942 (1979).
S. Olariu, I. Popescu, and C. B. Collins, Phys. Rev. C 23..
S. Olariu, I. Popescu, and C. B. Collins, Phys. Rev. C 23.
C. B. Collins in Proreedinas of the Int er.na,tj^Lnal_£ gn i er en ce
on Lasers '80 . edited by C. B. Collins (STS Press, McLean,
VA, 1981) p. 524.
C. B. Collins in Laser Techniques t or _ Ext rdine ,.Ul tx avi ole t
Rnectroscopy . edited by T. J. Mcllrath and R. R. Freeman
(AIP Conference Proceedings No. 90, New York, 1982) p. 454.
C B. Collins in Proceedings o f the International— Conteren ce
on Lasers ’8 1 . edited by C. B. Collins (STS Press, McLean,
VA, 1982) p. 291.
Lee, D. M. Shemwell, B.
D. DePaola, S
Olariu, and I .
Popescu, J. Appl.
, 4645 (1982).
B. Coll ins, J.
Op t. Soc.
. Am. B 1, 812
10. C. B. Collins and B. D. DePaola In Laser Techniques in the
Extreme Ultraviolet , edited by S. E. Harris and T. B.
Lucatorto (AIP Conference Proceedings No. 119, New York,
1984) p. 45.
11. C. B. Collins and B. D. DePaola, Optics Lett. JJ), 25 (1985).
12. B. D. DePaola, S. S. Wagal, and C. B. Collins, J. Opt. Soc
Am. B 2, 541 (1985).
13. B. D. DePaola, S. S. Wagal, and C. B. Collins, J. Opt. Soc
Am. B (pending).
14. C. B. Collins, M. Barb, S. Oiariu, and I. I. Popescu
15. G. C. Baldwin, J. C. Solem, and V. I. Goldanskii, Rev. Mod
Phys. 53, 687 (1981).
16. C. Cohen-Tannoud j i and S. Haroche, J. de Physique, 30., 125
17. S. Haroche, Ann. Phys. f>, 189 (1971).
18. C. B. Collins, F. Davanloo, and T. S. Bowen, Rev. Sci.
Instrum. 57_, 863 (1986).
19. C. B. Collins, IEEE J. Quantum Electron. QE~20 . 47 (1984).
20. L. C. Bradley, A. C. Mitchell, Q. Johnson, and I. D. Smith
Rev. Sci. Instrum. 55 . 25 (1984).
21. Q. Johnson, A. C. Mitchell, and I. D. Smith, Rev. Sci.
Instrum. 5.1.. 741 (1980).