Fundamentals of Neutron Physics M. Scott Dewey National Institute of Standards and Technology 11/10/2011 Radiation Metrology Workshop, Buenos Aires, Argentina
Acknowledgements for slides Geoff Greene, Pieter Mumm, Dan Neuman,
Short History Lesson
Ernest Rutherford 1920 Noting that atomic number (Z) does not correspond to atomic weight, Rutherford suggests that, in addition to bare protons, the nucleus contains some tightly bound proton- electron pairs or neutrons. 1930 Bothe and Becker discovered a penetrating, neutral radiation when alpha particles hit a Be target. 9 Be 12 C n Walter Bothe 1931 Mme Curie shows that they are not gamma rays and they have sufficient momentum to eject p s from paraffin.? Irene Curie
1932 Chadwick replaced the paraffin with a variety of other targets and, by measuring the recoil energies of the ejected particles, was able to determine the mass of the neutral particle M = 1.15 ± ~10% J. Chadwick, Proc. Roy. Soc., A 136 692 (1932) Chadwick claimed this was Rutherford s Neutron
1933 Bainbridge makes precision measurements of the atomic masses of the proton and the deuteron using the mass spectrograph 1934 Chadwick and Goldhaber make the first precision measurement of the neutron mass by looking at the photo-disassociation of the deuteron h d p n Using 2.62MeV gammas from Thorium and determining the recoil energy of the protons they were able to determine*: M n 1.0080 0.0005 KEY OBSERVATION: M n > M p + M e 1. The neutron cannot be a bound proton-electron pair 2. It is energetically possible for a neutron to decay to e - + p + *Chadwick and Goldhaber, Nature, 134 237 (1934)
Sources of neutrons Neutron generators D + D n + 3 He yields 2.5 MeV neutrons D + T n + 4 He yields 14.1 MeV neutrons Radioactive sources 252 Cf, half-life 2.6 years Am-Be, half-life 432.2 years
Spellation Neutron Source
NIST Center for Neutron Research
Why Neutrons? Neutrons are very sensitive to hydrogen! H and D scatter very differently Appropriate wavelength and energy => geometry of key motions D C O Si Magnetic moment H Weak neutron nucleus interaction => penetrating => easily modeled
NIST Develops Neutron Instrumentation & Makes it Available to the Scientific Community Source Liquid hydrogen cold neutron source Cold neutron instrumentation Thermal neutron instrumentation Neutron guides www.ncnr.nist.gov
The NCNR Has 25 Operating Beam Instruments Tailored to Specific Needs BT8 Resid. Stress Diff. BT9 3-Axis Spec. BT7 3-Axis Spec. NG0 MACS NG1 AND/R NG1 Vert. Refl. NG1 Depth Pofiling NG2 Backscattering Spec. NG3 30 m SANS BT1 Powd. Diff. Thermal Column BT5 USANS NG-5 SPINS NG5 Spin-Echo Spec. NG4 Disk Chopper TOF Spec. BT2 Neutron Imaging Facil. BT4 FANS NG7 Hor. Refl. NG7 Prompt NG6 Neutron Physics NG7 Interferometer NG7 30 m SANS Diffraction Instruments Spectrometers Other Neutron Methods www.ncnr.nist.gov
Emission rate of a neutron source
Concept of the Mn bath
Role of the manganese Time (hours)
Where do the neutrons go?
NIST Manganese Baths
Why Study Neutrons? The neutron exhibits much of the richness of nuclear physics, but is vastly simpler, and thus more interpretable, than nuclei. The neutron can be used to probe Strong, Weak, EM and Gravitational phenomena as well as serving as probe for new interactions. Neutron decay is the archetype for all nuclear beta decay and is a key process in astrophysics. The neutron is well suited as a laboratory for tests of physics beyond the Standard Model.
The Neutron is complicated enough to be interesting But is simple enough to be understandable.
Some Neutron Properties Mechanical Properties Mass Gravitational Mass (equivalence principle test) Spin Electromagnetic Properties Charge (or limit on neutrality) Internal Charge Distribution Magnetic Dipole Moment Electric Dipole Moment Neutron Decay Neutron Mean Lifetime Correlations in Neutron Decay Exotic Decay modes Miscellaneous Quantum Numbers: Intrinsic Parity (P), Isospin (I), Baryon Number (B), Strangeness (S),
Neutron Decay
1930 Pauli proposes the neutrino to explain apparent energy and angular momentum non-conservation in beta decay Wolfgang Pauli 1934 Fermi takes the neutrino idea seriously and develops his theory of beta decay Enrico Fermi 1935 The β decay of the neutron is predicted by Chadwick and Goldhaber based on their observation that M n >M p +M e. Based on their ΔM, the neutron lifetime is estimated at ~½ hr. 1948 Snell and Miller observe neutron decay at Oak Ridge 1951 Robson makes the first measurement of the neutron lifetime
Fermi s View of Neutron Decay: Modern View of Neutron Decay:
Neutron Lifetime (s) Neutron Lifetime (The need for a new measurement) Big Bang Nucleosynthesis Thermal Equilibrium (T > 1 MeV) 900 PDG tau = (885.7 +/- 0.8) s New result from Serebrov et al. 890 After Freezeout n/p decreases due to neutron decay e- 880 Nucleosynthesis (T~0.1 MeV) Light elements are formed. 870 beam UCN bottle e- 1990 1995 Year 2000 2005 almost all neutrons present are 4 He Neutron lifetime dominates the theoretical uncertainty of 4 He abundance.
Measuring the Neutron Lifetime Step 1. Get One Neutron Bottle
Measuring the Neutron Lifetime Step 1. Fill Neutron Bottle neutrons
Decay Rate Measuring the Neutron Lifetime Step 3. Let neutron decays for time t~τ n Time n p e n p e p p p n p e n n p p e p p p e n p e p p p
Measuring the Neutron Lifetime Step 4. Pour neutrons out and count t n N t N 0 e
Some Neutron Bottles Mampe et al, PRL 63 (1989) Serebrov et al, Phys Lett B605 (2005) Acrylic lightguide Magnet form Racetrack coil Cupronickel tube Trapping region Solenoid Beam stop TPB-coated acrylic tube Neutron shielding Collimator Huffman, et al, Nature, 403 (2000)
NIST Lifetime Apparatus