Laser driven nuclear reaction
studies: recent successes and
new unique opportunities for
future research
Klaus M Spohr
SUPA collaboration
University of the West of Scotland
&
University of Strathclyde
Overview
• Laser driven Nuclear Physics: Introductory remarks
– Development of high power laser systems
– History and milestone achievements in laser-nuclear research
• Laser induced nuclear photo-proton reaction studies
– Motivation for measuring sint(γ,p) of Mg-,Ti-, Zn- and Mo-isotopes
– Methodology and setup of IOQ Jena multi TW laser system
– Results for sint(γ,p) in Mg-,Ti-, Zn- and Mo-isotopes
– Conclusions, opportunities and future challenges
• Photonuclear reaction studies using laser induced Compton backscattering (LICB): proposed studies & vision
– The principle of ‘Laser Induced Compton Backscattering’
– Motivation
– LICB experiment on resonant photoabsorption of Mößbauer nuclei
– Some final thoughts: induced nuclear emission (lasing)
– Summary
Introductory remarks
Development of high power laser systems
HiPER
NIF
Development of high power laser systems
• Chirped Pulse Amplification (CPA) by Strickland and Mourou allowed
intensities I > 1015 Wcm-2
– CPA (Strickland D and Mourou G, Opt. Comm. 56 (3) 219 (1985))
• Ultrashort laserpulse up to the petawatt level with the laser pulse being
stretched out temporally and spectrally prior to amplification
– CPA is the current state-of-the-art technique for all of the highest power lasers
>100 TW
• Currently strongest civilian systems
– NIF @ LLNL, I > 1022 Wcm-2
– 2009 ASTRA/GEMINI @ CLF RAL I ~1022 Wcm-2
– Vulcan @ CLF RAL, I > 1021 Wcm-2
• HiPER project I ~ 5 ×1024 Wcm-2 (2020)
‘game-changers’
NYT, March 2009
• Concept of Chirped Pulse Amplification (CPA)
Chirped Pulse Amplification
History and milestone achievements in laser-nuclear research
• 1996 Femtosecond quasi-monoenergetic keV-pulses (atomic physics)
– Schoenlein R et al., Science 274 236 (1996)
• 2000 Laser-induced nuclear fission of 238U
– Ledingham K et al.,PRL 84 899 (2000), Cowan T et al.,PRL 84 903 (2000)
• 2003 Laser-induced fusion evaporation reactions
– McKenna P et al., PRL 91 (7) 075006 (2003)
• 2006 GeV electron beams from a centimetre-scale 40 TW laser
accelerator
– Leemans W et al., Nature Physics 2 (10) 696 (2006)
• 2007/08 Proton acceleration to 60 MeV and proton focussing
– Robson L et al., Nature Physics 3 (1) 58 (2007), Schollmeier M et al., PRL 101 055004 (2008)
• 2007 High order harmonic keV radiation (HOHG) of high brightness
– Dromey B et al., PRL 99 085001 (2007)
• 2009 Highest density of antimatter in solids (e+) (20 MeV) via Bethe-Heitler process n(e+)=1016cm-3
– Chen H et al., PRL 102 105001 (2009)
Laser induced nuclear photo-proton reaction studies
The measurement of integral photonuclear cross-sections such
as sint(γ, p) using nuclear activation is ideally suited for
modern high power multi-TW laser systems
• Table-top Laser systems as competitive tool for nuclear
studies
– High intensity and hot bremsstrahlung spectra kT>2 MeV
– Bremsstrahlung spectra spans over GDR regime ~8 - 35 MeV
– Multi-TW Ti:Sapphire Laser system at the IOQ Jena, Germany
Motivation for measuring sint(γ,p) of Mg-,Ti-, Zn- and Mo-isotopes
• Nuclear Theory: cross-sections for p-emission and capture in
plasma conditions, need to extend astrophysical data sets for low-Z
isotopes: 25Mg,48,49Ti, 68Zn and 97,98Mo (feasible with method)
– Hauser-Feshbach code
• Applied: GDR-regime of interest for technological R&D work
– nuclear power, shielding, radiation transport, radiotherapy, reactor
development (transmutation studies) & medical applications
– IAEA: encourages experiments esp. to retrieve reaction data in the
region of the Giant Dipole Resonance (GDR) esp. for ~40 isotopes
• Limitations and ‘old’ age of measurements
– Values of sint(g, p) for only 40 different stable isotopes are published
– Ratios: sint(g, n)/sint(g, p) for Z=12-42 needed!
– Accuracy of old measurements
• Probing and enhancement of nuclear models and reaction codes
– EMPIRE, GNASH
Methodology and setup of IOQ Jena multi-TW laser system
• Harvest the bremsstrahlung radiation of high-intensity laser generated
relativistic electrons to induce reaction
– Quasi-Maxwellian distribution: Tγ Te , McCall G,J Phys D 15 823 (1982)
– Energy distribution of γ-radiation for temperatures achievable with multi-
TW lasers extends over the full GDR-region
• Measure the activity of the decay of the radioactive daughter nucleus
• Characteristic g-rays of decay: intensity of photopeaks allows yield
determination of original daughter products, hence sint(g, p)
– Efficient Ge-detector system
– Adjustments for: branching, detection efficiency (system, geometry), self-
absorption, abundances; irradiation-, handling- and decay-time;
contaminating reaction channels, target impurities and electro-
disintegration
• Introduced for laser nuclear physics by Stoyer et al.
– Stoyer M et al., Rev Sci Inst 72 767 (2001)
Activation
Liesfeld B et al., J Phys D 79 1047 (2004)
Schwörer H et al., PRL 86 2317 (2001)
Schematic setup of IOQ Jena multi-TW laser system
E0laser pulse ~ 600 mJ
tlaser pulse ~ 80 fs
λ = 800 nm
P ~ 7.5 TW
ne per pulse ~ 2 - 5 ×109
(Elaser / Ee) ~ 0.6 - 1.5%
~5m
Activation
targets: MgF2,Ti,Zn,Mo
thicknesses~2-4 mm
p~ 80 bar
Setup of target chamber
Target chamber
Target & radiator
holder
f/2-mirror
He gas-jet nozzle Laser-room
university-scale system!
Relativistic electrons from 10TW Laser-Gas Interaction
Spatial confinement of e-
(measured, E=18 MeV)
Simulated e- density Simulated e- momentum distribution for different depths
Energy of e- (measured)
from Guiletti A et al., PRL 101 105002 (2008)
2×106
1×106
0250 mrad
PIC simulations, Pukov A and Meyer-ter-Veen, PRL 76 (21) 3975 (1996)
Simulated γ-distribution after Ta-radiator and Mo-target
(GEANT4)
Experimental considerations & limitations
• Extraction & detection limit: 5 min ≲ t1/2 ≲ 200 days
• kT measurement necessary with activation method
• Bulk targets with natural abundances
• Analysis:
– Uncertainty from kT & Ge-efficiency
– Activity is weighted with photon distribution and needs to be referenced
to well known (g, n) channel in probe
– To extract sint(g, p) the three parameters determining the GDR have to
be assumed based on models: sres, res(i), Eres(I)
kT measurement via Ta-activation
fitted kT=2.73(22)MeV
Activation spectra: MgF2-, Ti- and Zn-probes
~only 8 min of laser activation, 5000 pulses with 3-5 x1019 Wcm-2
Observed photo-proton channels
Measured and simulated photo-reaction channels in 25Mg
sint (g, p) deduced from
EMPIRE calculation is fully
reproducing measured sint (g, p)
Experimental data &
EMPIRE calculations
Results for sint(γ,p) in Mg-,Ti-, Zn- and Mo-isotopes
• TRK Dipole Sum Rule:
• TRK=Thomas Reiche Kuhn Sum rule 1925
– A standard benchmark for E1-strengths
• Aligned with data from Wyckoff J et al., PR 137 576 (1965)
─ Values of sint up to 35 MeV relative to the classical dipole sum rule show a
monotonic increase with atomic weight
– Correction factor ~ 0.85 - 1.25
)(60~35
0MeVmb
A
NZ s
Agreement with TRK sum-rule
Agreement with TRK sum-rule
only s(g,n)
Total σint values ~ σint (g,sn) + σint (g,p) show good to excellentagreement with the TRK-sum rule. For 97Mo no σint (g,n) known
0
500
1000
1500
2000
2500
25Mg 48Ti 49Ti 68Zn 69Zn 98Mo
Isotope
s in
t [M
eV
mb
]σ(γ,sn) from Lit. + σ(γ,p) Exp.
TRK (folded with Wyckoff)
only σint(g,n) known
0
50
100
150
200
250
1 2 3
Reaction
sin
t [M
eV
mb
]this work
Literature
Agreement with known σint-values
64Zn (g, 2n)62Cu 70Zn (g, n)69mZn
Aligned acc. to branching
No errors given in
Literature values
Ivanchenko V et al.,
P.ZHETF 11 452 (1966)
Carlos P et al.,
NP A258 365 (1976)
Goryachev A et al.,
Yad.Fiz. 8 121 (1982)
All three measured σint-values for different targets and particle channelsthat can be benchmarked with known data show good agreement
Known σint(g,n)/ σint(g,p) ratios (IAEA)
Experiment added 15%
towards all existing data!
σin
t (g,
n)/
σin
t (g,
p)
this work
• We measured a total of 6 new sint (g,p) values and
hence deduced six new sint (g,n)/sint (g,p) ratios for light
nuclei
– First time laser driven research adds new data to nuclear physics
– almost 15% of previously published data
– Spohr K et al., New J Phys 10 043037 (2008) & New J Phys Best
of 2008 collection
• Conclusively proven that nuclear reactions can be
produced and cross-section can be measured using
table-top Laser systems
– data agrees with TRK-sum rule
– data agrees with EMPIRE calculations
– data agrees with three previously known data-sets
• A good base for a more extensive research investigation
Conclusions, opportunities and future challenges
• Opportunity for extended campaign:
– Determination of >110 new sint (g, p) measurable with university-
scale multi-TW laser systems is possible
• ~90 lifetimes 5 min < t1/2 < 300 days (feasible)
• Challenge ~25 lifetimes t1/2 < 5 min
• Challenges:
– Use of isotopic enriched targets
– Rapid transport mechanism (@ e.g. ELBE)
– On-line measurement of prompt g-radiation
– Deflection of electrons, separation with small Halbach magnets
– Lowering the uncertainties of kT measurement
– Multi-Ge-system in coincidence
– Particle detectors in coincidence, radiation resistant detectors
Conclusions, opportunities and future challenges
• Conjoined ELBE/Laser @ FZ-Dresden Rossendorf
– SUPA has allocated beam-time quota
– Elinac (40MeV) and 150 TW laser system (mid-2009)
• We could use both systems and compare
• 150 TW laser ~ E(e-) = ~80-90 MeV endpoint
– Proposal to study sint (g,p) reaction of stable p-nuclei:
• p-nuclei are neutron deficient (except 176Lu) nuclei that are shielded by
their isobaric neighbours from production via the r-process and can not
be produced by the s-process either
• p-nuclei of astrophysical interest: 96Ru, 120Te, 130Ba, 156Dy, 162Er, 168Yb
and 176Lu are feasible to study, yield improvement with new system ~103
• Understand formation of p-nuclei and support Hauser-Feshbach
calculations
• Higher power will give laser competitive edge over Elinacs
Conclusions, opportunities and future challenges
Using different kT for evaluation of s(E)
Unfold cross-sectionby using differentkT values, 1 MeV <kT <10 MeV
K. Spohr
J.J. Melone
R. Chapman
M. Shaw
K. Ledingham
W. Galster
L. Robson
P. McKenna
T. McCanny
K-U. Amthor
B. Liesfeld
R. Sauerbrey
H. Schwoerer
J. Yang
Collaborators
Photonuclear reaction studies
using laser induced Compton
backscattering (LICB): proposed
studies & vision
The future strategy: harvest the
unique capabilities
of newly developed laser driven
accelerator systems
Photonuclear reaction studies using laser induced
Compton backscattering (LICB): proposed studies & vision
• Laser Inverse Compton Backscattering (LICB)
– Cobald/ERLP system @ Daresbury (May 2009) & ELBE(40MeV)/150TW Laser (Spring 2010)
– High brightness, ultra short, energy tunable source for low-lying quasi coherent, polarised gamma-radiation <30 keV (2009)
– Proof of concept: Schoenlein R et al., Science 274 236 (1996)
– Brightness:
– Total Photons:
– Brightness comparable to proposed 4th generation light sources (only approched by SPring8 in the moment)
– Comparison with synchrotrons
• + Much smaller (cheaper) systems
• + Better time resolution
• - lower repetition rate 10 Hz and hence total yield 10 to 100 lower
– Cobald/ERLP: Priebe G, Spohr K et al., Las Part Beams 26 (4) 201 (2008)
Photonuclear reaction studies using laser induced
Compton backscattering (LICB): proposed studies & vision
• Motivation:
– ‘Finest’ g-source available
• Highest intensity & shortest steerable pulse duration
– A new class of (g, g’) reactions allowing precise study of nuclear
transitions (as of 2009: E<40 keV)
• Population of isomeric states
• Nuclear spectroscopy with high energy resolution
• Nuclear lifetime measurements (direct measurements of fs-lifetimes and below!) ,
spectroscopic information
– Coherent ensembles of gamma excitations in nuclei (excitons)
• New quantum phenomena (quantum beats), ‘coherent nuclear physics’
• Enhancement of decay width
– Resonance reactions to probe for existence of materials in probes
The principle of Laser Induced Compton Backscattering
Superconducting Elinac
Cobald/ERLP @ DaresburyELBE/150TW system is similar
Cobald @ Daresbury
LICB photons: the principle
• Laser light works like undulator on electrons
• Electrons are deflected ~ 104 more often than in
conventional magnetic undulators
• LICB scattered photons remain partially
coherent
Principle of SCAPA
(SUPA2) project
from Priebe G, …, Spohr K et al., Laser and Particle Beams 26 (4) 201 (2008)
Laser/e-beam collision geometry
Normalised vector potential of the laser field
~undulator deflection parameter of static field
0
100
200
300
-2
0
2
0
10
20
30
0
100
200
300
Eγ
[kev
]
collision angle Φ [] scat
tere
d an
gle θ
[
]keV15
22
g
g
EE
transverse configuration
keV304 2 LEE gg
head on configuration
Spatial distribution of photon-energy
Photon-energy vs scattered angle
θ = π
Simulation
Motivation for studying isomers with LICB
• Laser inverse Compton Backscattering present an unique opportunity for
studies into the laser induced pumping of nuclear states
– Ideal cases: short lived Möβbauer isomers:161Dy, 57Fe,181Ta
– Questions: feasibility and efficiency, new coherent phenomena?
Baldwin G and Solem J, Rev Mod Phys 69 (4) 1085 (1995)
Einstein A, Phys Zeit 18 121 (1917)
161Dy
LICBE1
|2>
|1>
LICB experiment on resonant photoabsorption of Mößbauer nuclei
• High brightness of impacting pulsed photon-beam complicates detection of
resonant absorbed and re-emitted γ-radiation
– Target spins within ultra-fast rotating spindle, mapping of time into spatial domain
– Röhlsberger R et al., PRL 87 (4) 047601 (2001) Nuclear Lighthouse Effect
LICB - γ
fast rotor
e.g.161Dy
Avalanche Photo Diode
Rotors up to 70kHz
LICB experiment on resonant photoabsorption of Mößbauer nuclei
• Will reveal insight into new coherent nuclear physics phenomena– Ensemble (>1000) of coupled nucleons in the same state of excitation
(excitons)
– Exciton stationary or migrates through the probe with slow speeds (polaritons)
• E.g. Nuclear Lighthouse effect
• Quantum beats as different hyperfine components interfere
– Decay rate is increases, hence the absorption line-width
– Even more so, if an ensemble of polaritons is created
– Estimate: 161Dy ~ 2 x 108 detectable 25.7 keV g-rays in 5 days experiment, 181Ta ~5000 detectable 6.2 keV g-rays in 5 days
• Based on natural and measured line-widths, conversion coefficients, resonant excitation cross-section, mass attenuation coefficient, recoilless f-fraction of Mossbauer nuclei, brightness and spatial beam distribution
• Cobald functional May 2009, Future in STFC? ....
A word on controlled amplification
nuclear systems
• Cross section at resonance:
• Radiative width |2> -> |1> into stable g.s.:
– For lasing: problem population inversion, many nuclei in g.s. level
– Small for long lived isomers! Never feasible for bulk targets
• Decay from unstable state |2> into unstable state |1>:
– Inversion population if N(|2>) > N(|1>)
• Possible for T2 > T1
• Favourable ‘pumping reaction’ to enhance population of |2>
• Favourable branching b=1 and low conversion a small
– Decay times are somewhat similar, as for T2 >> T1, ss is small
– Resembles Four-level pumping scheme of optical lasers
Some final thoughts: induced nuclear emission (lasing)
Some final thoughts: induced nuclear emission (lasing)
Pumping nuclear reaction
e.g. photonuclear type
Laser induced!
High energetic levels
|2>
|1>
g.s.
LICB
g
Some final thoughts: induced nuclear emission (lasing)
• Stedile generated population inversion via NRF in stable 103Rh – |2> = 357 keV, (5/2-) t=107 ps and |1> = 295 keV, (3/2-) t=9.8 ps
– ‘Generating an inversion on a nuclear transition - Photopumping of 103Rh’, Stedile F, Hyper Inter. 143 (1-4) 133 (2002)
• A thought… for something new: Use bremsstrahlung radiation to create population inversion via NRF with high yields (high rep rate) and then impinge quasi-monochromatic matching γ-energy from LICB on target!– For the moment: yields by far to small
– Need effective high yield pump (high power and repitition laser accelerator) and LICB laser system
– Identification of favourable isotope
• 103Rh not ideal, Eγ=62 keV too high …,
– 2 high power laser systems: MBI Berlin (~2011)
Summary• Laser Nuclear physics offers promising research into new phenomena at the
interface of two very diverse areas of research
– Will it become a mainstream field in Nuclear Physics in the UK?
– As of March 2009: 4 researchers (Strathclyde & UWS)
• Laser driven nuclear physics has just begun to deliver the goods for applied
and fundamental new nuclear research
– Integrated cross-section measurements
– Ability to reach high kT values and hence to derive effective photodisintegration
rates in high temperature plasma
• Though facing tremendous challenges, British-led efforts could be crucial for
the exploitation of new systems using Laser Inverse Compton backscattering
– LICB systems have the potential to add a new unique quality to nuclear research
– Study of resonant photoabsorption is ideally suited to harvest these systems
– Something really new and challenging
In Memory of Dr Wilfred Galster 1948-2009
The next generation IOQ Jena multi-TW laser system
Single diode pump station of
150TW-system (5 total) @IOQ
Start: 04/2008
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
0 1 2 3 4 5 6 7 8 9 10
Energy(MeV)
Flu
x(M
eV
/cm
^2
)
kT=2.91MeV kT=5MeV kT=7MeV kT=4MeV
Simulated bremsstrahlung-spectra in Ta-radiator (MCNPX)
kT measurement
• First to be determined: kTg
– By activation method
– Very well known cross-sections allow best-fit of kTg
– Two independent measurements
• 181Ta(γ,n)180Ta and 181Ta(γ,3n)178Ta
– Intensities: I103keV with t1/2=8.15 hrs and I426keV with t1/2=2.36 hrs
– N (178Ta) / N (180Ta) = 3.06(48) x 10-4, with Ge-detector
– Uncertainty: I103keV, literature value, influence of electro-disintegrationreactions (e,e’) was ~20% (no e--rejection)
– Measured kT=2.73(33) MeV
• 12C(γ,n)11C and 63Cu(γ,n)62Cu
– Both β+ giving rise to 511 keV annihilation
• Influence of electro-disintegration (e,e’) dramatically reduced
– N (11C) / N (62Cu) = 8.58(13) x 10-3, with NaI coincidence system
– Uncertainty: low intensity in 11C, ~ 2 counts/s
– Measured kT=3.09(23) MeV
• Accepted result: kT=2.90(23) MeV
Measured and simulated photo-reaction channels in 66,67Zn
Experimental data &
EMPIRE calculations
sint (g, p) deduced from EMPIRE
calculations are reproducing
measured sint (g, p)
-20
-15
-10
-5
0
5
10
15
20
-30 -20 -10 0 10 20 30
Y (
mra
d)
X (mrad)
Backscattering angular distribution; each colour is a 1 keV energy band with 20-
21 keV on outside and 30-31 keV at centre
Spatial distribution of photons
Photon brightness vs photon energy
Simulation