Laser Induced Nuclear Excitations,

1

• Laser Induced Nuclear Excitations,
• past, present and future of isomer research using
  Compton backscattering (LICB) facilities
• K Spohr
• Scottish University Physics Alliance (SUPA)
• University of the West of Scotland
• University of Strathclyde

2
Nuclear excitation research with lasers

• Pumping of isomeric state with laser generated
  bremsstrahlung
• Direct population with plasma
• Isomer production via nuclear reaction e.g. (?,n)
• Joint Laser/ELINAC systems
• Laser induced inverse Compton backscattering
  (LICB)
• Elevation of incident laser photon energy into
  keV regime
• Resonant pumping of isomeric states?
• Photon intensity higher than almost all
  synchrotron systems
• Photo nuclear reactions with LICB created
  quasi-monoenergetic ?-beams
• Recently discovered forms of nuclear excitations
• Coherent ensembles of excitations in nuclei
  (excitons)
• Quantum beats, Nuclear Lighthouse effect
• Enhancement of decay width, hence resonant
  absorption probability?
• Nuclear lasing?

3
Laser Driven Isomer research of SUPA group

• Unsuccessful attempt of resonant photon
  population of isomer with laser induced
  bremsstrahlung at the Astra (RAL)
• 181Ta, low-lying, long-lived, state (t1/2 6?s),
  E? 6.23keV (E1 9/2-?7/2)
• Spohr KM et al., J Mod Opt 53 (16), 2633 (2006)
• Before 2006 a series of unsuccessful attempts
  and doubtful claims
• Resume Dont use lasers to produce just
  ordinary bremsstrahlung
• Creation of nuclear isomers with lasers plasma
  triggered reaction
• Photo induced reaction 70Zn(?,n)69mZn,
• K Spohr et al., New J Phys 10, 043037 (2008), K
  Spohr et al. Physics Best of 2008, IoP
  Collection
• Experiment was dedicated to the study of laser
  induced photo-proton reactions

4
Setup of ASTRA Experiment (2005)

• Laser Plasma parameters
• YAG laser with ? 800nm
• I1017 Wcm-2 tpulse200fs
• kT 10 keV
• tplasma lt 10ps ltlt t1/2181Ta)

5
Detector response after shot
?
6
Laser induced production of nuclear isomers

• Aim to produce laser induced photo-proton
  reactions and measure ?int(?, p)
• Established by Stoyer et al., Rev Sci Inst 72,
  767 (2001)
• nuclear activation technique
• ?-rays allow determination of reaction channels
  and yield
• Table-top Laser systems as tool for nuclear
  research
• high intensity and hot bremstrahlung spectra
• Quasi-Maxwellian distribution T?Te, McCall, J
  Phys D15, 823 (1982)
• ?-radiation for achievable temperatures covers
  GDR
• kT 3 MeV
• Result Photo induced reaction 70Zn(?,n)69mZn

7
Experimental Set-up
Activation
Liesfeld et al., J Phys D 79,1047
(2004) Schwoerer et al., Phys Rev Lett 86, 2317
(2001)
8
IOQ Laser Facility (Jena) Target Chamber
9
Relativistic electrons from 10TW Laser-Gas
Interaction
Simulated e- density
Simulated e- momentum distribution for different
depths
PIC simulations, Pukov A and Meyer-ter-Veen, PRL
76 (21) 3975 (1996)
Energy of e- (measured)
Spatial confinement of e- (measured, E18 MeV)
2106
1106
250 mrad
0
from Guiletti A et al., PRL 101 105002 (2008)
10
Simulated ?-distribution after Ta-radiator and
Mo-probe (GEANT4)
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kT measurement via Ta-activation
fitted kT2.73(22)MeV
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Laser induced population of 69mZn

• only 8 min of laser activation
• total of 5000 pulses _at_ 10Hz
• I 3-5x1019 Wcm-2,

438.6 keV
13.76h
9/2
Conclusively proven that nuclear isomers can be
produced and measured using university-style
table-top Laser systems
56.40m
1/2-
69Zn
ß-
t1/2exp15.2(19)h
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sint-value of 70Zn(?,n)69mZn
TRK sum-rule
aligned acc. to branching
No errors given in Literature values
64Zn (g, 2n)62Cu
70Zn (g, n)69mZn
67Zn (g, p)66Cu
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)
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• LICB systems will become game-changers in nuclear
  excitation research
• SUPA Cobald/Alice team at Daresbury (Priebe,
  Seddon) SUPA beamline at the ELBE/150TW laser
  system _at_FZ-Rossendorf

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Motivation Nuclear Physics with LICB

• LICB systems (high powered laser combined with an
  electron accelerator) will provide unique quality
  features for new research with ?-radiation
• Finest g-source available
• High photon intensities predicted1021photons/s/mm
  2/mrad2/0.1?E/E
• shortest steerable pulse duration 350 fs
• A game-changing tool for investigations of
  isomeric states
• Observations of coherent ensembles of ?
  excitations in nuclei? (excitons)
• Newly discovered quantum phenomena (quantum
  beats), coherent nuclear physics, Nuclear
  Lighthouse effect
• Enhancement of decay width, hence resonant
  absorption probability

NEW UNIQUE
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The principle of Laser Induced Compton
Backscattering
Superconducting Elinac
energy recovery of e-beam
Cobald/ERLP _at_ Daresbury ELBE/150TW system has
similarities
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Laser/e-beam collision geometry
For given ? , the energy E? is a defined function
of the scattering angle ? ? 1800 (head on) ?
900 (transverse)
Thompson Scattering

• normalised vector potential of the laser field
• electromagnetic energy gained across laser
  wavelength compared to electron rest-mass
• 0 (classical Compton scattering), gt 1 non-linear

from Schoenlein RW et al., Science 274, 236
(1996)
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Cobald _at_ Daresbury
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Studying selected 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 Mößbauer isomers 161Dy,
  149Sm, 57Fe,181Ta,
• Questions feasibility, efficiency (yields), ..
  ,population inversion?

2gt
LICB
E1
1gt
Baldwin G and Solem J, Rev Mod Phys 69 (4), 1085
(1995)
161Dy
Einstein A, Phys Zeit 18, 121 (1917)
20
Spatial distribution of photon-energy
from Priebe G et al., Laser and Particle Beams 26
(4), 201 (2008)
21
Photon-energy vs scattered angle
57Fe (14.4keV,98ns)
? p
161Dy (25.7keV,29ns)
181Ta (6.2keV,6.0µs)
149Sm (22.5keV,7.3ns)
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Photon brightness vs photon energy
Simulation
161Dy (25.7keV,29ns)
149Sm (22.5keV,7.3ns)
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Spatial distribution of photons
very thin hollow cylinder target to absorb
monoenergetic photons
Backscattering angular distribution each colour
is a 1 keV energy band with 20-21 keV on outside
and 30-31 keV at centre
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Selected ELINAC features of LICB systems

• Challenges
• XHV conditions plt10-13 bar ? quality of e-beam
  pulse
• Synchronisation of e-beam and laser pulse
• Focussing and shot-to-shot reproducibility

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Selected Laser features of COBALD
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LICB experiment on resonant photoabsorption of
Mößbauer nuclei

• Nuclear Lighthouse Effect (NLE)
• Röhlsberger R et al., Phys Rev Lett 87 (4),
  047601 (2001)
• Resonant scattering of 149Sm achieved with
  synchrotron radiation
• Photon flux 2 x 1010 photons/s
• Massively enhanced transition rate (exciton
  state)
• Target spins within ultra-fast rotating spindle
  35kHz
• mapping of time into spatial domain possible, as
  population of state is short compared to life
  time and target rotates ultra fast 0.1mrad/ns
• Beam-line 3m after interaction area
• APD diode used

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Nuclear Lighthouse Effect
Rotors up to 70kHz air-driven MAS spindle
e.g.149Sm
fast rotor
Avalanche Photo Diode
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Nuclear Lighthouse Effect using LICB

• Is a direct laser induced population of nuclear
  isomers achievable and can the NLE be observed
  using LICB?
• Not just a mere repeat of Röhlsbergers
  experiment on 149Sm
• Do we observe changes for higher flux values per
  pulse using LICB systems?
• Presumably an ensemble of coupled nucleons in the
  same state of excitation (excitons) could be
  created
• Ideal field to study collective coherent
  ?-phenomena
• Excited states migrate through the probe with
  slow speeds (polaritons)
• Influence of Nuclear Lighthouse effect using
  LICB?
• Pronounced quantum beating as different hyperfine
  components interfere?
• Decay rate is increases, hence the absorption
  line-width
• Even more so, if an ensemble of interacting
  polaritons is created
• Theory of excitons is related with general
  phenomena of open quantum systems (coupling to
  the continuum)
• Shell-Model in complex plane Michel N,
  Nazarewicz W, J Phys G36 (1), 013101 (2009)

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Resonant photoabsorption of Mößbauer nuclei

• Proposal probing resonant excitation of 161Dy
  via LICB
• Measurements for cooled and un-cooled probe
• Corresponds to resonant and fully-non resonant
• Probe of thickness 2mm, ?res(25.6keV)
  10-8-10-7
• Exciton/Polariton creation can massively enhance
  resonant absorption, hence resonant yield
• Good, detectable yield for 3-5 days investigation
• 5 x 105 ?-rays in 5 days
• Efficiency for APD is high
• Low Energy Germanium Detectors have high
  efficiency at estimated rates ?total(25.6keV)0.8
  -0.9
• Good generic time resolution for low energy ? of
  ? ns
• ?Extraction of isomeric decay can be done with
  software gate
• ?Minimisation of prompt non-resonant background

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Resonant photoabsorption of Mößbauer nuclei

• At first probing resonant absorption
• Resonant excitation of 161Dy (E?25.6keV, t1/2
  29ns)
• Promising high yield (dtarget2mm) 5 x 105
  ?-rays in 5 days
• Cooling required (TDebye 210K)

Complex Shvydko et al., Eur Phys Lett 56 (2),
305 (2001)
Simpler APD and/or LEPs
6m
cryo
LEP
LEP
APD
LICB
Al2O3
APDs
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Resonant photoabsorption of Mößbauer nuclei

• Alternative target probes
• Much lower yields than 161Dy
• Photon flux, self-absorption and conversion
  coefficients
• 181Ta (E?6.2keV, t1/2 6.0µs), 57Fe
  (14.4keV,98ns)
• 181Ta
• ASTRA experiment (2005), with 1010 higher flux!
  
• No cooling required for 57Fe (TDebye 490K)

LEP
LEP
APD?
LICB
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Resonant photoabsorption of Mößbauer nuclei

• High-power laser driven resonant population of
  nuclear isomers could be demonstrated
• Direct, ?-induced population of states
• Big step towards the control of nuclear states
  (switch on)
• LICB as tool for high precision measurements
  involving isomers?
• Evaluation of related cross-section
• Creation of ensembles of interacting coherent
  polariton states?
• Radiative width increases dramatically, hence the
  probability of resonant absorption,
  Npolaritons(10-1000)
• Avalanche like growth of linewidth?
• gt more and more photons of the wider ?-beam
  may join condensate
• New, theoretical challenging field of coherent
  phenomena
• Collective properties of polariton condensates
• Coherent gamma-ray spectroscopy

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A final thought on controlled amplification
?
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Final thoughts induced nuclear emission (lasing)

• Induced emission at resonance
• Purely radiative decay into stable g.s.
• Problem population inversion, many nuclei in
  g.s. level
• Never feasible for 1gt within bulk target,
  without delicate preparation
• Decay from unstable state 2gt into unstable state
  1gt
• Becomes feasible if higher energetic 2gt is more
  populated than 1gt
• achievable by appropriate reaction for T2 gt T1
• favourable pumping reaction to enhance
  population of 2gt
• favourable branching for population of the two
  states
• And decay times are somewhat similar, as for T2
  gtgt T1, ?s is small
• Resembles Four-level pumping scheme of optical
  lasers

35
Final thought induced nuclear emission (lasing)
Pumping nuclear reaction e.g. photonuclear type
High energetic levels
2gt
LICB g
1gt
g.s.
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Final thought induced nuclear emission (lasing)

• Stedile generated population inversion via NRF in
  stable 103Rh
• 2gt 357 keV, (5/2-) t107 ps and 1gt 295
  keV, (3/2-) t9.8 ps
• Generating an inversion on a nuclear transition
  - Photopumping of 103Rh, Stedile F, Hyper
  Inter. 143 (1-4), 133 (2002)
• Idea Use bremsstrahlung radiation to create
  population inversion via NRF with high yields
  (high rep rate) and then impinge
  quasi-monochromatic matching ?-energy from LICB
  on target!
• For the moment yields by far to small (however
  one may not need so much, if a potential lasing
  effect would be strong!)
• 103Rh not ideal, E?62 keV too high , better
  isotopes need to be identified and measured,
• New Idea investigate the possible increase of
  radiative width due to production of polaritons
  with high photon-fluxes

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Summary

• Laser driven studies on nuclear isomers are
  starting to deliver
• Efficient production of isomers via photonuclear
  reactions e.g. (?,n)
• Exploitation of laser produced electron
  bremsstrahlung is not that suited for an
  efficient direct pumping of isomers!
• Laser Induced Compton Backscattering systems
  promise to become the real game-changer
• trigger research into coherent nuclear phenomena
  with isomers
• LICB systems have the potential to add a new
  unique quality to nuclear research
• Cobald can become a leading facility for such
  investigations

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Collaborators

• K. Spohr
• J.J. Melone
• R. Chapman
• A. Andreiev
• S. Pain
• M. Shaw
• R Samir
• M. Hassan
• KWD. Ledingham
• P. McKenna
• W. Galster (in memoriam)
• G. Priebe
• E. Seddon
• R. Sauerbrey
• U. Schramm
• ..

Thanks for your attention