Laser Induced Nuclear Physics
K.W.D.Ledingham
Dept of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland
and
AWE pic, Aldermaston, Reading, Berkshire RG7 4PR, UK
Abstract. The interaction of ultra-intense focused laser beams with solid targets is a
new field of research resulting in the production of exotic plasma conditions similar to
the conditions which exist in the interior of some stellar objects. The lasers generate
very high energy electrons and ions which can subsequently produce j-rays, positrons,
neutrons andpions. The paper will show that results obtained from these studies have
major implications to fundamental plasma physics and high energy accelerator
physics as well as important technological potential for the production of compact
sources of protons, neutrons, positrons and isotopes. One of the applications
considered at some length will be the production of protons and the possibility of
designing a table top laser to produce radioactive sources for positron emission
tomography (PET) as well as proton oncology. The exciting new physics which can be
carried out at laser intensities of Iff2'23 Wcm2 will also be briefly discussed.
PREAMBLE
This story started nearly fourteen years ago when two of us KWDL and Ravi
Singhal (Glasgow) were discussing with Joe Magill (Karlsruhe) the seminal
theoretical paper written by Boyer, Luc and Rhodes [1] which discussed the possibility
of using a high intensity focused laser beam (1021 Wcm"2 and 248 nm) to induce
fission in 238U.
Many probes had been used to induce fission, particularly neutrons both fast and
slow. Other nuclear probes [2], had also been used e.g. protons, deuterons, a particles
and heavy ions as well as non nuclear beams e.g. y-rays, electrons and muons. It is
perhaps not surprising for a sufficiently intense light source to induce fission
especially since a number of lasers had pulse intensities of a terawatt (1012 W) or even
apetawatt(1015W).
This initiated, for the research team at Glasgow working in collaboration with
Imperial College and the plasma physics group at the Rutherford Appleton Laboratory
and now many other groups around the world, an exciting new area of physics that has
been called laser induced nuclear physics.
CP634, Science of Super strong Field Inter actions, edited by K. Nakajima and M. Deguchi
© 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00
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INTRODUCTION
The mechanism of the interaction of charged particles with intense electromagnetic
fields has been considered for more than fifty years. This was one of the first
explanations put forward by the early workers to explain the origin and energies of
cosmic rays, e.g. [3]. Simply the idea is as follows: a charged particle in an intense
electromagnetic field is accelerated initially along the direction of the electric field.
The vxB force causes the particle s path to be bent into the direction of travel of the
wave. In large fields the particle s velocity rapidly approaches the velocity of light
and tends to travel with the EM wave gaining energy from it. In as trophy sical
situations the solar corona was thought to be one of the sources of the electromagnetic
waves. Charged particles which are known to exist throughout space become
entrained in a long wavelength wave and can be accelerated to energies as high as
1021eV. These astrophysical phenomena are the counterparts of the machines built on
the earth to accelerate particles to high energies.
In 1971 the possibility of accelerating electrons in focused laser fields was first
proposed by Feldman and Chiao [4]. They showed theoretically that an electron could
gain energies as high as 30 MeV after a single pass through the focus of a diffraction
limited laser beam of power 1012 W and wavelength IJLL Chan [5] similarly calculated
that an intense laser beam could be used as an energy booster for relativistic charged
particles showing that a 10 MeV electron can absorb 40 MeV from a laser beam of l|i
wavelength and an electric field of 3x1010 Vcm"1 in a distance of 1.3 mm. The
problem with these schemes is that it is still not possible to maintain the required
intensities over the necessary distances in vacuum, even with the highest energy laser
systems which exist today.
In the late seventies, this problem was overcome by the seminal work of Tajima and
Dawson [6] who realised that by focusing laser light into a plasma medium, much
higher accelerated energies could be generated than by focusing into the vacuum alone.
They proposed the construction of a laser-electron accelerator which could be created
when an intense laser pulse produced a wake of plasma oscillations (volumes of low
and high densities of electrons). Similar to a boat creating a bow wave or wake as it
moves through water, a bunch of high velocity electrons creates a wake of plasma
waves as it passes through a plasma. They demonstrated with computer simulations
that existing glass lasers of 1018 Wcm"2 directed on plasmas of densities 1018cm~3 could
yield electrons of 100 GeV energy per m of acceleration. It is known that
conventional accelerators are limited by electrical breakdown at fields of about 20
MV/m; at these fields the electrons are torn from the nuclei in the accelerator s support
structure. Thus these plasma particle accelerators promise fields more than 1000 times
stronger that those of the most powerful conventional accelerators, Dawson (1989).
The reason for the current and burgeoning interest in high intensity laser-plasma
interactions is not only their relevance to advanced concepts of plasma high energy
particle accelerators but also to a number of other fields e.g. laser induced nuclear
photo-physics, astrophysics and inertial confinement fusion (ICF) e.g. Takabe [7] and
Tabak et al [8]. Specifically the question of carrying out nuclear physics using a laser
source has been addressed by a number of authors using high repetition rate, fs lasers
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e.g., Shkolnikov et al [9] Umstadter [10] Schwoerer et al [11] and by using single shot
ultra-high intensity lasers like NOVA, LULI and VULCAN e.g. Ledingham and
Norreys [12], Ledingham et al [13], Cowan et al [14] and Gauthier [15].
BACKGROUND INFORMATION
Photo-nuclear Physics
In a nucleus, nucleons (protons and neutrons) are bound in shells similar to the
electrons in Bohr s picture of the atom. The most weakly bound nucleon in a typical
medium mass nucleus is about 10 MeV. If a nucleon absorbs energy from a beam of
incident y-rays of energy >10 MeV, it can escape from the nucleus. Neutrons can
escape much more readily than protons from a nucleus since protons in addition must
burrow through a positive Coulomb barrier before being liberated. Thus photon
induced nuclear physics generates large numbers of neutrons via (y,n) processes.
<j>
0
10
20
30
40
50
60
70
80
70
80
90
90
Proton Number
Figure 1. A plot of the stable isotopes in order of increasing
neutron and proton number.
When all the stable nuclei are arranged in order of increasing neutron and proton
number in the Segre chart as shown in Figure 1 a line of stability exists. It can be
seen that for light stable nuclei, the number of neutrons and protons are equal but as
the mass increases however, many more neutrons must be added to reduce the unstable
effect of the repulsive protons. Above the line of stability lies a sea of isotopes which
are neutron rich. These isotopes are unstable and have the ability to transform
themselves back to stability by P" radioactivity in which a neutron decays to a proton
plus an electron (with an accompanying neutrino).
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Below the line of stability we have a sea of proton rich isotopes which reach stability
when one of the protons transforms into a neutron plus a positron (fT radioactivity).
When y-rays of tens of MeV energy interact with solid targets, beams of neutrons are
produced with the formation of proton rich isotopes which decay by (3+ emission.
A nucleus which decays by emitting positrons can be detected in an unambiguous
way. A positron combines with an electron to produce two simultaneous 511 keV yrays emitted at 180°. These can be readily detected in coincidence by two radiation
counters like Nal scintillation detectors. A typical coincidence system is shown in
Figure 2.
3"x 3" Nal
Scintillator
3"x 3" Nal
) )Single Channel
Analyser
Amplifier
^—
ul
Figure 2. A 3 x3 Nal coincidence system for measuring positrons. A positron annihilates with an
electron to form two 511 keV y rays in time coincidence emitted at 180° to each other. The star
indicates the position of the radioactive source
When an intense laser beam interacts with a high Z target, the high electric fields
associated with the laser, accelerates electrons from the preformed plasma on the front
face of the target into the bulk which then decelerate emitting high energy
bremsstrahlung photons.
These are the photons which knock neutrons from
secondary targets resulting in proton rich isotopes decaying by positron emission.
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EXPERIMENTAL RESULTS
Proton Production
The procedure used the Chirped Pulse Amplification (CPA) beam line of the
VULCAN Nd:Glass laser [16] incident on a thin aluminium or CH foil target, at 45°
incidence with p-polarised light within an evacuated target chamber. This system
delivered pulses of energies up to 120 J, duration 0.9 —1.2 ps with a wavelength of
1.053 |im. When focused to a spot size of radius 6|Lim on the target using a f/4 off-axis
parabolic mirror at 45°, the laser yielded intensities of up to 1020 Wcm"2. The contrast
ratio between the peak and ASE energies was measured by a third order autocorrelator
and found to be 1: 10"6. This is sufficient to generate a pre-plasma on the target surface,
a few picoseconds before the arrival of the main pulse. The main pulse then interacts
with this pre-plasma, causing electrons, and subsequently protons, from hydrocarbon
impurities on the target surface, to be accelerated.
To characterise the proton beam, and to produce radioisotopes, different activation
samples were placed in the target chamber, behind the target, in the straight through
direction at a distance 10 mm from the target, and in front of the target, directed along
the target normal, at a distance 50 mm from the target, in the blow-off direction.
These samples were then transferred to a nuclear laboratory for analysis. A diagram of
the Vulcan laser and the target chamber inside which is the high Z target is shown in
Figure 3.
The energy spectrum, and spatial distribution of the accelerated protons were
determined by placing activation stacks in both the blow-off and straight through
directions. These consisted of 9 pieces of copper foil, 100 jiim thick, plus a 1mm piece.
Figure 3. The target chamber and some of the Nd glass disc amplifiers in the present
VULCAN laser
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Copper undergoes the nuclear reaction 63Cu(p,n)63Zn when bombarded with protons of
energies greater than 4.15 MeV. The 63Zn isotope produced is radioactive, and decays
via (3+ emission with a half life of 38.1 mins. The efficiency of the coincidence system
was measured using a calibrated 22Na source, and hence the absolute activity, i.e. the
number of (p,n) reactions in the copper pieces in the stack could be determined.
Knowing the cross section, the number of protons incident on each piece of copper in
the stack can be found, and hence a proton energy spectrum can be built up. Each
piece of copper in the stack represents an energy bin , because protons are slowed
down in copper. Figure 4 shows typical energy spectra of the emitted protons from the
front and rear of the target. It is clear that more energetic protons are observed in the
straight through direction, having energies up to a diagnostic limit of 37 MeV. The
first copper foils in each stack are shown as insets in Figure 4 The white circles
observed are the result of aluminium debris from the target, and correspond to the
dimensions of the proton beam, and hence provide spatial information about the
proton beam.
10
12
Behind the target —
\ straight through
j
, direction
g
10 "
5 cm
In front of
garget — blowoff direction
io
c
5 cm
10
10
15
20
25
30
35
Proton Kinetic Energy (MeV)
Figure 4 Proton energy spectra obtained from copper activation stacks in front of and behind the
target. Insets show the first copper pieces in each stack, containing spatial information
The protons were used to produce radio-active sources of U C by (p,n) reactions of
activity >105Bq per pulse. For commercial viability, PET source activities some 1000
times greater than this are required. Potentially this could be carried out using tabletop OPCPA lasers with high repetition rates [17]. These lasers could also provide
proton energies up to 150 MeV that could be used for proton oncology.
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REVIEW OF EXCITING NEW NUCLEAR AND PARTICLE
PHYSICS AT LASER INTENSITIES OF 1022 23Wcm2
Ultrahigh magnetic field generation
Tatarakis et al [18] have recently demonstrated the production of huge magnetic fields
using the VULCAN laser at intensities of up to 9 x 1019 Wcm"2onto thin solid targets.
Such fields are more than an order of magnitude larger than any previously observed
in the laboratory. At 1022 Wcm"2 simulations have suggested that magnetic fields
greater than 109 gauss may be possible. Magnetic fields generated in this way could
soon approach those needed for testing astrophysical models of neutron stars and
white dwarfs [19].
Direct interaction with the nucleus
A number of experiments have been carried out at different laboratories to produce
laser induced high energy gamma rays and protons. These beams have been used to
produce radioactive isotopes via (y, n) and (p,n) reactions. However as the laser
intensity is increased above 1022 Wcm"2, the oscillating electric field can make the
protons in the nucleus quiver in exactly the same way as the electrons in the plasma.
At 1022 Wcm"2 the direct interaction of the laser electric field and a high Z nucleus
causes energy shifts of the order of 250 eV. At 1024 Wcm"2 this has increased to 2.5
keV and at 1028 Wcm"2 shifts of the order of 250 keV occur. Such shifts could
radically alter nuclear energy levels and decay half-lives.
Fusion by direct laser acceleration of ions
When deuterium (D) and tritium (T) ions collide with sufficient energy they fuse. If
these ions are oscillating in the field of a laser beam they can gain sufficient energy
from the field to cause fusion to take place. At 1021 Wcm"2 the collision energy is on
average about 8 keV with a corresponding fusion cross section of ~ 10"4 barn.
However while the ion energies remain non relativistic the collision energies scales
directly with the laser intensity and hence at 1022 Wcm"2 the collision energy is about
80 keV. The peak of the DT fusion cross section (5 barns) occurs at about 100 keV.
Most fusion reactions take place in very large scale facilities associated with the
inertial confinement programme but now it will be possible to study fusion reactions
in this novel way for the first time for a range of low Z nuclei with small OPCPA laser
systems.
Particle Physics
As the intensity of the laser increases, the energy of the accelerated electrons also
increases. At 1020 Wcm"2, electron energies up to 100 MeV have been measured. This
is very much greater than the expected ponderomotive energy of about 10 MeV and
hence there are accelerating mechanisms like plasma wave acceleration that can result
in greatly increased electron energies. At 1022 Wcm"2 the radiation pressure on
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electrons in a thin target can reach values greater than 1 Tbar and for very thin targets,
resulting in low plasma densities, electron energies in excess of 100 GeV are possible.
It is even possible that electron energies as high as 100 TeV can be generated by lasers
at intensities of 1026 Wcm'2 and even 10 PeV at intensities of 1028 Wcm"2.
EPILOGUE
Since we embarked on this exciting journey of laser induced nuclear physics and its
applications, we have never ceased to be amazed at the world-wide interest it has
generated. At this time, we feel that among the most important applications for this
technology is the generation of proton and neutron beams which I have not managed
to cover in this paper. However as with many scientific endeavours, the most
important results that come from the study of a new technology are likely to be totally
unexpected..
ACKNOWLEDGMENTS
We would like to acknowledge the hard work and dedication of all the members of our
team without whom none of this would have been possible and the support of the
engineering and target area staff of the Central Laser Facility. In addition we would
like to thank Prof. Steve Rose for help with the calculations which has lead to the
potential experiments described in the latter part of the article dealing with super
intense laser beams.
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