Neutron Skins and Halo Neutron Distributions with Real Photon Probes

1
Neutron Skins and Halo Neutron Distributions with
Real Photon Probes

• Claire Tarbert
• University of Edinburgh
• Dan Watts, Derek Branford, Klaus Foehl, Nick
  Harrington

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Neutron distributions with electromagnetic probes
Elastic Electron Scattering

• Photons are excellent nuclear probes
• - electromagnetic interaction is well
  understood.
• - probe the entire nuclear volume.
• Electron scattering (virtual photons) has been
  most successful method of nuclear shape and size
  determination but only samples charge
  distribution.
• How can we use photon probes to gain information
  on neutron distributions?

Nuclear Charge Radii
r(r)
r fm
3
Neutron distributions with electromagnetic probes

• Traditionally use strong probes to probe matter
  radius e.g. p, a scattering
• Theory predicts a neutron skin for n-rich nuclei
  (208Pb 0.1 0.3 fm)
• How can we use photon probes to gain information
  on neutron distributions?

• Will describe 2 different techniques which
  exploit the
• electromagnetic probe
• Coherent p0 photoproduction
• Measuring the neutron skin thickness of 208Pb
  using coherent p0 photoproduction with the
  Crystal Ball _at_ MAMI.
• p photoproduction
• Study of the halo nucleus 6He using the
  6Li(g,p)6He reaction with Ge6 _at_ MAMI.

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Coherent p0 Photoproduction
p0
Photon
Nucleus
Coherent A(g,p0)A Incoherent A(g,p0)A
Nucleus

• Takes place with same probability on n and p.
• Cross section contains information on matter
  distribution
• F2m(q) is Fourier Transform of Matter Density
  as a function of radius.
• r.m.s matter radius
• No initial state interactions.
• At low energies, final state interactions are
  small modelled easily.
• Very accurate 0.035fm!

ds/dW A2(q/kg)P32Fm(q)2sin2qp
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Motivation Nuclear Matter Radii
Relativistic Mean Field Calc.
Nuclear Theories
rn - rp
A (Mass Number)
Astrophysics (NPA624 349 (1997)) extrapolation
to neutron or proton rich isotopes
Atomic PNC (PLB 464 p177 (1999))
Antiprotonic atoms (PRL 87 082501(2001))
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208Pb and Neutron Stars
208Pb
Neutron Skin
Neutron stars (PRL 86 5647 (2001))
Radius of neutron star of 0.5Msun (km)

• Skin thickness on 208Pb gives information about
  compressibility of matter.
• Calibrates symmetry energy as a function of
  density at low densities.
• Large neutron skin gt large crust on neutron
  star.

Crab Pulsar
Possible Equations of State for Neutron Stars
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A2 Collaboration _at_ MAinz MIcrotron(MAMI)
g

• Glasgow Tagger
• Eg (40-800) MeV
• Resolution 2 MeV
• Up to 108 g/s

e-

• MAMI-B
• LINAC 3 race-track microtrons
• Ee- (180 855) MeV
• 100 duty factor cw electron beam
• Up to 100mA

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Crystal Ball and TAPS

• TAPS Two Arm Photon Spectrometer
• 512 BaF2 Crystals
• 512 Plastic Scintillators
• Forward Wall

• Crystal Ball Photon Spectrometer
• 672 NaI Crystals
• Involved in first measurements of J/y particle

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Charged Particle Detection
2 Multi Wire Proportional Chambers for charged
particle tracking
Barrel of 24 plastic scintillators for charged
particle ID
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p0 Detection
Invariant Mass Spectra

• p0 too short lived to detect directly.
• p0 -gt gg (98.8)
• Reconstruct invariant mass of 2 photons.

p0 Mesons
Mp0 134.98MeV
p0 -gt gg
h Mesons
Mh 547.75MeV
Invariant mass of 2 photons
Mgg (2E1E2(1-cosy))1/2
DEp Ep(g1,g2) Ep(Eg) Ep(g1,g2) detected
pion energy (cm) Ep(Eg) calculated pion energy
(cm) Incoherent p0s always less energetic than
coherent equivalent.
Nuclear decay gs.
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Separation of Coherent Events
DEp Ep(g1,g2) Ep(Eg) Ep(g1,g2) detected
pion energy (cm) Ep(Eg) calculated pion energy
(cm)
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Cross Sections
Far better than any previous measurements. X-sect
ions not yet normalised still to include a
slowly varying detection efficiency.
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Cross Sections
14
Comparison to Theory
DREN calculation by Kamalov
Relativistic Mean Field Calc.
(NPA624 349 (1997))
Data -- DREN
Rn -Rp (fm)
Mass No.

• Compare one energy bin to theoretical
• model
• rm 5.78fm
• (cf rc 5.45fm)

• rm rc 0.33fm

Diffraction from a circular disc gt rm
5.85fm
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Coherent p0 Summary

• Coherent p0 photoproduction provides a very
  accurate means of accessing the matter
  distributions of nuclei.
• An accurate measurement of the rms matter radii
  of 208Pb will have an impact on a wide range of
  physics.
• New measurement using the Crystal Ball _at_ MAMI is
  significantly better than any previous
  measurements and preliminary result puts neutron
  skin thickness of 208Pb at (0.33 0.05)fm
• Finalised result should have uncertainty of only
  0.035fm !

16
Investigating Halo Nuclei using Edinburgh Ge6
Array

• Derek Branford, Klaus Foehl
• Edinburgh University
• Dan Watts, Nick Harrington

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Physics motivation Halo Nuclei
Physics motivation halo nuclei

• BE last nucleon (n or p) close to separation
  energy.
• Core with a low density extended tail of n- or
  p-matter e.g. 6He alpha 2neutrons.
• Halos are interesting as (i) they allow pure
  (neutron) or proton matter to be studied (ii)
  play an important role in determining
  astrophysical reaction rates and (iii) are a
  unique test of cluster models and three-body
  theories

Photopion reactions - complimentary to ion
reactions also allow excited state halos to be
accessed e.g. 6Li(g,p)6He
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The GE6 - Array

• Ge detectors ? good charged particle energy
  resolution and particle identification. Excellent
  alternative to expensive magnetic spectrometer
  systems

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GE6 Double sided Silicon strip detectors
BB2s 2.5cm x 2.5cm (24 x 24 strips)
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Ge detector
Some of the BB2 strips in position
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GE6 Particle identification

• DE/E technique. Obtain DE from Si strip
  detector(s) and from Ge(sum)
• Ge versus Ge if gt1 Ge fired (also useful to
  reject nuclear scattered events see later)
• Identify p from afterpulses arising from
  delayed muon decays.

p
Si strip
n
m
p?m nm (t1/226 ns)
m ? e ne nm (t1/22.2 ms)
e
n
Michel spectrum of positron energies
n
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Particle ID using DSSD GE6
x y distribution on Ge frontface
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Proof of principle measurement_at_Mainz
Run period 6 days on 6Li(g,p)6He Calibrations
and X-section normalisation based on p(g,p)n
reaction (CH2 target) Strip detectors were all
Micron BB2s
Ge resolution measured with gs 15 keV
(Edinburgh) 30 keV (Mainz)
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6Li(g,p)6He (ground state) reaction at Eg 200
MeV
GE6_at_MAMI results
Blue Present Data Red Shaw et al. Phys. Rev.
C43 1800 (1991) Green Shoda et al. Phys. Lett.
B101 124 (1981)
Young - halo
Young Surrey PhD Thesis (2004) Uses cluster
model for Y Kartaglidis PRC 61 024319
(2000) Uses WS and HO Y
Kartaglidis - halo
Kartaglidis no halo
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Angular Distribution for the 6He 2 Excited State
at 1.8 MeV
GE6_at_MAMI results
Red squares Present data Black triangles
Shoda et al. Phys. Lett. B101 124 (1981)
CK Curve Cohen and Kurath Calcn. presented in
Shoda et al. Sask C Curve Bergstrom Phys.
Rev. C21 2496 (1980)
Large angle cross sections comparable to g.s.
results Suggests 1st excited state also a
halo state.
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GE6 Summary and Future programme

• Proof of principle experiment has shown that
  charged pion photoproduction is a viable means of
  probing the wave function of halo nucleons.
• In 2006/2007 Ge6 will be one of the primary
  detector systems at MAXLAB_at_Lund
• Upgraded Lund facility has DEg 200-500 keV
  (FWHM)
• Halo measurements
• 6Li(g,p)6He, 11B(g,p)11Be
  and 17O(g,p-)17F

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Conclusion

• The photon is an excellent nuclear probe
• - no initial state interactions.
• - probes entire nuclear volume, not just
  surface.
• Can be used as a very clean probe of neutron
  distributions via coherent p0 photoproduction and
  charged p photoproduction.
• Experiments presented give a flavour of the
  possibilities of these techniques.

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Neutron Skin of 208Pb with the Crystal Ball _at_ MAMI

• Claire Tarbert, Dan Watts
• Edinburgh University
• Derek Branford, Klaus Foehl

40
Fits to DEp
qp (30-32)o
Completed first iteration of fits to pion missing
energy.
s (MeV)
Eg (MeV)
Crystal Ball
TAPS 2001 data
41
GE6 element gain matching

• Kinematically overdetermined
• p(g,p)n reaction
• Measure Eg, qp ? Calculate Ep
• Source of energy tagged p

n
g
p
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DE/E for Ge versus Ge
43
Estimation of Losses due to Hadronic Interactions
in the Ge
Deduced from the p(g,p)n Results
Comparison GE6 with known p(g,p)n cross section
Pions can interact with the Ge nuclei scatter,
knockout protons etc. Cross section rises as
pions acquire sufficient energy to excite the D
44
GE6 Trigger logic
Trigger split into two levels controlled by
Lecroy LRS4508 Memory lookup unit First Level
Pulse from Crystal one of any GE6 detector gt 300
mV Second Level (i) Energy in start detector
and sum used to
reject most high energy electrons
(ii) Hit
in Tagger OR
(iii) Afterpulse signal between
1 and 8 ms after
initial signal
45
First test of GE6 at PSI Muon afterpulse

• p?m nm

TDC spectrum (combined for crystals 4-6)
e ne nm
Michel spectrum -positron energy
Fit with exponential Half life
2.20.2ms Recover muon lifetime
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Dummy animation
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Charged particle detection in GE6

• To stop high energy particles need large
  thickness of Germanium (GE6 has 13 cm)
• In contrast to g, charged particles deposit
  energy all along their path ? measure energy
  deposited in all material passed through
• Conventional Ge technology use thick 1/2mm rear
  contact on the crystals ?
• Prototype detectors commissioned using thin
  contacts (lt1 micron) from
• Eurisys Mesures
• Detector Systems GmbH
• Considerable effort but in the end has been
  successful !
• Eurisys won contract to build 4 more

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Charged particle detection in Ge

• Employ Planar crystals Little dependence of
  pulse shape on entry point. Easier signal
  analysis.
• Band gap small at voltages required to fully
  deplete the
• crystal 103 Vcm-1? Need to cool with liquid N2 to
  avoid large leakage current

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HpGe Tagger TDC Spectrum
Hp-Ge tagger TDC spectrum
Imply timing resolution Ge 3 - 4ns
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GE6 Double sided Silicon strip detectors

• Give (i) position information and (ii) energy
  signal for use in DE/E
• Donor impurities in perpendicular strips on the p
  and n sides ? 576 pixels
• Electrons drift to the anode (horizontal n-type)
• Holes drift towards cathode (vertical p-type)
• Use two detectors to get track information

Micron BB2 Si Strip
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Photo of TIGRE ???
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First test of GE6 at PSI Energy resolution
_________________
MeV
Single crystal resolution (FWHM) lt 366 keV 8
keV (from proton events which stop in first
crystal) p resolution consistent with spread of
momenta in the beam 1.4 MeV at 120 MeV (FWHM)
54
First test of GE6 at PSI
Test of prototype Ge detector characteristics 590
MeV proton beam passed through 12C target to
produce secondary beam (cocktail of alpha,
protons, pions, muons) Beam focussed by
quadropole magnets
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Physics motivation halo nuclei
I. Tanihata et al. Phys. Rev. Lett. 24 2676 (1985)
Models of 6He
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Physics motivation halo nuclei

• Although most halo nucleus studies have been made
  using secondary beams of radioactive ions, it was
  noted quite early in their study that a few halos
  can be accessed by photonuclear reactions
• (e.g. 6Li(g,p)6He, 11B(g,p)11Be and
  17O(g,p-)17F).
• What are the advantages?
• The use of a photoproduction reaction provides an
  alternative method of studying halo nuclei using
  the very precise e/m probe.
• Initial (ISI) and final state interactions (FSI)
  are relatively small.
• Photopion reactions also allow excited state
  halos to be accessed, which is generally not
  possible using secondary beams of radioactive
  halo ions as they decay to their ground states
  before reaching the downstream target.

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Physics motivation halo nuclei
DWBA Models
Experimental results are usually compared to DWBA
calculations in momentum space. Using 6Li a
1p-proton is converted to a halo neutron. Hence
the method is very sensitive to the halo nucleon
momentum distribution (i.e. its wavefunction).
For the 6Li(g,p)6He reaction the g.s. wf of
6Li is well known from electron scattering
measurements. Also at a gamma energy of 200MeV
the outgoing p-particles are sufficiently low in
energy to be reasonably well described by plane
waves (small FSI).
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GE6 electronic readout
Timing Filter Amplifier (pulse length
400ns) Ortec 855 Amplifier CAEN V462 Gate
generator Delay box
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DWBA Results Compared to Published 6Li(g,p)6He
Reaction Data
DWBA calculations made for the 6Li(g,p)6He
reaction compared to the Bremsstrahlung end-point
data of Shaw et al. Phys. Rev. C43 1800
(1991) Notes Cross section label on this
published figure should have read nb/sr.
Momentum transferred to the hit proton increases
with qp.
Solid line Woods-Saxon wavefunctions
(Halo). Dashed line Harmonic Oscillator
wavefunctions (No Halo).
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The Mainz Facility
Three cascaded microtrons delivering 850 MeV
electrons the Glasgow photon tagging facility
Schematic diagram of the
tagger
Schematic diagram of a microtron Microtron 1
gives 14 Mev Microtron 2 gives 160 MeV Microtron
3 gives 850 MeV Direct-Current Beam
( 100 Duty Factor) Resolution 20keV
Eg Ee Etagged-e
Main tagger resolution 2 MeV Tagger microscope
170 -220 MeV had nominal resolution 400 keV
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More Recent DWBA Calculations for the
6Li(g,p)6He Reaction
Calculations of Karatiglides at Eg 170 MeV
also show larger cross section at large angles
due to the halo. (Private Communication
1998). Confirmed at Eg 200 MeV by Young
(Surrey PhD 2004)
Conclusion Halo gives rise to enhanced cross
section at large angles!! Large angle cross
section sensitive to halo nucleon(s) wavefunction.
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The New Mainz Measurement of the 6Li(g,p)6He
Reaction at 200MeV

• The plan was to make a new measurement using
  tagged photons
• at the 850 MeV Mainz microtron MAMI-B.
• However, the tagger has a resolution of 2 Mev and
  the 1st excited (Jp 2) state of 6He is at 1.8
  MeV.
• Hence, we used a Tagging Microscope in addition
  to the Main Tagger to cover the range 170 -220
  MeV.
• Resolution 500 keV.
• To detect the p-particles we required high
  resolution, large solid angle detectors.
• Decided to build a new array of Ge detectors
  (Ge6-Array) for detecting
• p-particles.

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Tests with p beams at PSI
_________________
MeV
Resolution equal to p beam resolution of 1 MeV
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Missing Energy Spectra for p(g,p)n Reaction
Em Eg Ep ER Q
Resolution with Main Tagger 2 MeV Resolution
with Tagging Microscope 1.4 MeV ?
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Borromean Nuclei
Physics motivation halo nuclei
6He nucleus equivalent to an alpha-particle
plus 2 neutrons. Remove a neutron or the alpha
and the system falls apart. 5He unstable 2n
unstable
Borromean Rings Remove any 1 and the system
falls apart.
Precludes radiative capture reactions such
as 5He(n,g)6He
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Missing Energy Results from the 6Li(g,p)6He
Measurement
Average Eg 200 MeV
Qp 100O
Qp150O
Em Eg Ep ER Q
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Conclusions and Future Work

• Our backward angle cross section results
  approximately a factor of 5 larger than the 1
  previous measurement of Shaw et al.
• Would like to assume new results more reliable.
• Experimental results larger than present theory.
• Could imply (i) Halo more pronounced than
  thought, or (ii) Theory needs to be improved
  (e.g. to include two body currents).
• Plans for the future include a new high
  statistics, higher resolution measurement of
  6Li(g,p)6He at MAX-lab, Lund Sweden (2006).
• New calculations from Surrey University for the
  g.s. and 1st excited state.
• Study of the 11B(g,p)11Be reaction to
  investigate the halo nucleus 11Be.

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The Edinburgh Team for This Experiment

• Nick Harrington PhD 2005
• Klaus Foehl
• Ewan Roche
• Dan Watts
• Gordon Turnbull Technician
• Derek Branford

69
Acknowledgments
J. Brudvik, J. Goetz, B.M.K. Nefkens, S.N.
Prakhow, A. Starostin, I. Suarez, University of
California, Los Angeles, Ca, USA J. Ahrens, H.J.
Arends, D. Drechsel, D. Krambrich, M. Rost, S.
Scherer, A. Thomas, L. Tiator, D. von Harrach,
Th. Walcher, Institut für Kernphysik, University
of Mainz, Germany R. Beck, M. Lang, A. Nikolaev,
S. Schumann, M. Unverzagt, Helmoltz-Institut für
Strahlen und Kernphysik, Universität Bonn,
Germany S. Altieri, A. Braghieri, P. Pedroni, T.
Pinelli, INFN Sezione di Pavia, Pavia,
Italy J.R.M. Annand, R. Codling, E. Downie, D.I.
Glazier, J. Kellie, K. Livingston, J.C. McGeorge,
I.J.D. MacGregor, R.O. Owens, D. Protopopescu, G.
Rosner, Department of Physics and Astronomy,
University of Glasgow, Glasgow, UK C. Bennhold,
W. Briscoe, George Washington University,
Washington, USA S. Cherepnya, L. Filkov, V.
Kashevarow, Lebedev Physical Institut, Moscow,
Russia B. Boillat, B. Krusche, F. Zehr, Institut
für Physik, University of Basel, Base 1, Ch P.
Drexler, F. Hjelm, M. Kotulla, K. Makonyi, V.
Metag, R. Novotny, M. Thiel, D. Trnka, Il.
Physikalisches Institut, University of Giessen,
Germany D. Branford, K. Föhl, C.M. Tarbert, D.P.
Watts, School of Physics, University of
Edinburgh, Edinburgh, UK V. Lisin, R.
Kondratiev, A. Polonski, Institute for Nuclear
Research, Moscow, Russia J.W. Price, California
State University, Dominguez Hills, CA, USA D.
Hornidge, Mount Allison University, Sackville,
Canada P. Grabmayr, T. Hehl, Physikalisches
Institut Universität Tübingen, Tübingen,
Germany Yu.A. Usov, S.B. Gerasimov, JINR, Dubna,
Russia H. Staudemaier, Universitat Karlsruhe,
Karlsruhe, Germany D.M. Manley, Kent State
University, Ohio, USA M. Korolija, I. Supek, D.
Mekterovic, Rudjer Boskovic Institute, Zagreb,
Croatia D. Sober, Catholic University,
Washington DC, USA
70
Strips Edinburgh-RAL 2-3 ms
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Cluster Models of 6He
72
Proof of principle experiment at MAMI Study of
Halo Nucleus 6He using the 6Li(g,p)6He Reaction

• Double Sided Strip Detectors
• BB2s 2.5cm x 2.5cm 24 x 24 strips
• Amplifiers
• HpGe TFA Pulse length 400ns
• Strips Edinburgh-RAL 2-3 ms