isospin

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16.451 Lecture 14 Neutron beta decay 26 Oct.
2004
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isospin
?
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Neutron beta decay
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Reaction
light particles or leptons, produced in
association.

• Neutrino or little neutral one postulated
  in 1931 by Pauli (q 0, m? 0, s ½ )
• Only associated with the weak interaction
  very difficult to detect
• First detected by Reines Cowan, 1959 ?
  Nobel prize 1995

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Related processes
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? decay in a nucleus, where energetically
favourable, eg 25Al?25Mg decay
Notice the electron and anti-neutrino appear
together the positron and neutrino appear
together.... This suggests a new conserved
quantity called lepton number, Le
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Detection of anti-neutrinos
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Reines Cowan Experiment (1953)

• Very low rate experiment gt 1013 incident
  antineutrinos/sec but only 3 events/hr!
• ? 5 months of data taking!
• No computer data acquisition! For each event,
  an automatically-triggered camera
• system took a photograph of analog oscilloscope
  traces!
• Delayed coincidence detection of both neutron
  and positron suppressed background
• Auxiliary measurements to determine detection
  efficiencies, etc.
• Absolute cross section measured was 1 x 10-43
  cm2 (10-19 b), in agreement with theory!!

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Schematic of the experiment
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delayed neutrons have to slow down...
positron annihilation (instantaneous)
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2 water tanks, sandwiched between 3 scintillation
counters to detect ?s
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vertical height 2 m surrounded in Pb
shielding to reduce ? background...
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No computers! electronic coincidence pulses
for e and n-capture ?s....
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Raw data oscilloscope photographs!
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scintillation light from e annihilation first,
neutron capture later ( 3 10 ?s)
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Data coincidence event rate as a function of
time delay
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distribution indicates slowing-down time
of neutrons in water 1/v Cd capture cross
section is large at low energy!
Bottom line first direct demonstration of the
existence of antineutrinos!
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More on lepton number
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There are actually three generations of leptons
that we know about (in order of increasing mass
e, ?, ? ) and each one has its own distinct
associated neutrino type with a separately
conserved lepton number....
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Case study state of the art neutron lifetime
measurement
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Outline of the method
decay rate
measure rate by counting decay protons in a given
time interval (dN/dt) and normalizing to the
neutron beam flux (N)
Ideally done with cold neutrons, e.g. from a
reactor, moderated in liquid hydrogen...
Issues 1. precise decay volume ? 2. proton
detection ? 3. beam normalization ? ...
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Neutron beam distribution definitely not
monoenergetic

• MeV neutrons from a reactor are moderated
  by scattering in a large tank of
• water (thermal) or liquid hydrogen
  (cold)
• after many scatterings, they come to thermal
  equilibrium with the moderator and
• are extracted down a beamline to the
  experiment
• velocity distribution is Maxwellian
  energies in the meV range (kT 26 meV _at_ 293K)

Krane, Fig 12.4
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Neutron detection at low energy
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• several light nuclei have enormous neutron
  capture cross sections at low energy
• (recall, cross sectional area of a
  nucleus, e.g. 10B is about 0.2 barns, lecture 4)
• key feature cross sections scale as
  1/velocity at low energy

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Put this together for detector counting rates as
shown
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neutrons must be captured to be detected!
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Experimental details (all in vacuum, at ILL
reactor, France)
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• use Penning trap to confine decay protons
• let them out of the trap after accumulation
  interval T
• measure the ratio Ndet/Ndecay as a function of
  trap length L ? slope gives ?

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Amazing results
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