N EUTRONS AND THEIR INTERACTION WITH MATTER Auteur - Date 1 I N S T I T U T M A X V O N L A U E - P A U L L A N G E V I N
T HE CONTEXT Programme Neutron talks by • Teresa Fernandez (yesterday) • ME • Ulli Koester (neutron prod.), Giovanna Fragneto (soft matter) • Juan Rodriguez-Carvajal (diffraction), Eddy Lelievre (instrumentation) • Andrew Wildes (spectroscopy), Mechtilde Enderle (inelastic scattering) • Gerry Lander (history) • Oliver Zimmer (nuclear and particle physics) 2 05/09/2017
N EUTRONS AND THEIR INTERACTION WITH MATTER Overview • History – neutrons and nuclear reactions • Production – reactors and spallation sources • Properties – as a particle and a probe • Instruments – exploiting the probe to do science 3 05/09/2017
A BIT OF HISTORY The neutron • 1932: J. Chadwick, after work by others, discovers the ‘neutron’, a ) c 2 + T ) c 2 + T ( ( m He + m B He = m N + m n N + T neutral but massive particle n m n = 1 . 0067 ± 0 . 0012 a . m . u 4 05/09/2017
A BIT OF HISTORY The nuclear reaction • 1938: O. Hahn, F. Strassmann & L. Meitner discovered the fission of 235 U nuclei through thermal neutron capture • 1939: H. v. Halban, F. Joliot & L. Kowarski showed that 235 U nuclei fission produced 2.4 n 0 on average – chain reaction • 1942: E. Fermi & al. demonstrated first self-sustained chain reaction reactor 5 05/09/2017
N OBEL PRIZES , NEUTRONS AND THE ILL Chadwick, Shull & Brockhouse 6 05/09/2017
N OBEL PRIZES , NEUTRONS AND THE ILL 7 05/09/2017
N OBEL PRIZES , NEUTRONS AND THE ILL Haldane (1977 – 1981), Kosterlitz and Thouless for topological phase transitions and phases of matter (Electronic structure and excitation of 1D quantum liquids and spin chains) 8 05/09/2017
N EUTRON SOURCES Fission reactors • Nuclear fission chain reaction with excess neutrons (1n 2.5n) • Slow neutrons split U-235 nuclei • Fission neutrons have MeV energies and need to be moderated (thermalized) to meV energies by scattering from water • Thermalisation @ RT thermal neutrons, @ 25K cold neutrons and @ 2400 K hot neutrons • ILL – flux 1.5 x 10 15 n/cm 2 /s 9 05/09/2017
N EUTRON SOURCES Spallation sources • Neutrons can be produced by bombarding heavy metal targets • 2 GeV protons (90% speed-of- light) produce spallation – evaporation of ~30 neutrons 10 05/09/2017
N EUTRON SOURCES ESS Effective thermal neutron flux n/cm 2 -s 10 20 SNS ISIS ILL HFIR NRU MTR 10 15 IPNS NRX HFBR ZINP-P / FRM-II WNR KENS SINQ X-10 ZINP-P 10 10 CP-2 CP-1 Berkeley 37-inch cyclotron 10 5 350 mCi Steady State Sources Ra-Be source Pulsed Sources 1 Chadwick 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 (Updated from Neutron Scattering , K. Skold and D. L. Price, eds., Academic Press, 1986) 11 05/09/2017
C ONTINUOUS OR PULSED BEAMS Integrated vs peak flux – ESS will have a time-integrated flux comparable to ILL 12 05/09/2017
N VS X ESRF (hard X-rays) 13 05/09/2017
T HE NEUTRON As a particle • free neutrons are unstable: β -decay proton, electron, anti-neutrino life time: 886 ± 1 sec • wave-particle duality: neutrons have particle-like and wave-like properties • mass: m n = 1.675 x 10 -27 kg = 1.00866 amu. (unified atomic mass unit) • charge = 0 • spin =1/2 • magnetic dipole moment: μ n = -1.9 μ N, μ p = 2.8 μ N, μ e ~ 10 3 μ n, • velocity (v), kinetic energy (E), temperature (T), wavevector (k), wavelength (λ) 14 05/09/2017
T HE NEUTRON As a particle • velocity (v), kinetic energy (E), temperature (T), wavevector (k), wavelength (λ) 2 π (h/ λ 2 2 E m v / 2 k T hk/ 2 / 2 m ) / 2 m n B n n • Neutron energy determines velocity and therefore time-of-flight ( tof ) over a given distance i.e. tof energy determination L o μ λ 253 sec tof A L m v 15 05/09/2017
T HE NEUTRON As a probe 16 05/09/2017
T HE NEUTRON As a probe • Wavelengths on the scale of inter-atomic distances: Å - nm wavelengths to measure Å - m m distances/sizes n l = 2dsin • Energies comparable to structural and magnetic excitations: meV neutrons to meaure neV – meV energies • Neutral particle – gentle probe, highly penetrating (e.g. 30 cm of Al), no radiation damage • Magnetic moment (nuclear spin) probes magnetism of unpaired electrons (N.B. m e ~ 1000x m N ) 17 05/09/2017
T HE NEUTRON As a probe – interacting with matter – scattering from at atom • Neutron flux at reactor core • 1.5 x 10 15 n/cm 2 /s • Flux at an instrument sample position spherical waves • 10 8 n/cm 2 /s emitted by scattering centres 10 -6 n/nm 2 /s interference pattern in front of detector source 1 n/nm 2 / m s 10 -6 - 10 -3 n/nm 3 (depending on v ) • On these time and length scales, neutrons are being scattered one at a plane waves in scattering system time • Need wave-particle duality of neutrons 18 05/09/2017
T HE NEUTRON As a probe – interacting with matter – (elastic) scattering from a single fixed nucleus - b e i k f . r • Nuclear size << neutron r wavelength point-like s-wave scattering • b is the scattering length (‘power’) in fm • #neutrons scattered per second per unit solid angle : 2 r 2 d d s /d = b 2 ( ) = 2 p h 2 ( ) b d r V r • s is the cross-section: 4 p b 2 (in m r barns) 19 05/09/2017
T HE NEUTRON As a probe – interacting with matter – scattering from a set of nuclei d σ i Q . R R , j k b b e j k d Ω j k spherical waves emitted by Q k k f i scattering centres interference pattern in front of detector • Q is called momentum source transfer • Q -dependence (eg angle) gives info about atomic plane waves in positions scattering system 20 05/09/2017
T HE NEUTRON As a probe – interacting with matter – scattering from a set of identical nuclei – coherent and incoherent scattering • Set of N similar atoms/ions – spins/isotopes are uncorrelated at N different sites d σ 2 2 iQ R R 2 b e j k b b d Ω j,k • b depends on spin/isotope • Average is ‹b› 2 σ π σ πb 2 4 b 4 coh coh coh • Incoherent scattering gives a Q 2 2 σ πb σ π 2 4 4 b b incoh incoh inc independent background • But it can be useful to probe the dynamics of single particles (later) 21 05/09/2017
T HE NEUTRON As a probe – interacting with matter – scattering from a set of identical nuclei – coherent and incoherent scattering • If single isotope and zero nuclear spin, no incoherent scattering 1 b I 1 b Ib • If single isotope and non-zero nuclear 2 I 1 spin I I I 1 2 • nucleus+neutron spin: I+1/2 and I-1/2 2 2 b b b b 2 scattering length b + and b - 2 I 1 • To reduce incoherent scattering (background): – use isotope substitution – use zero nuclear spin isotopes – polarise nuclei and neutrons 22 05/09/2017
T HE NEUTRON Scattering lengths 23 05/09/2017
T HE NEUTRON Scattering lengths can be positive or negative (nuclear physics) • Positive b (most nuclei): phase change • Negative b : no phase change at scattering point 24 05/09/2017
T HE NEUTRON Scattering lengths can be positive or negative Contrast matching : = + 25 05/09/2017
T HE NEUTRON As a probe – interacting with matter - absorption • Absorption – neutron capture • Several strong absorbers: He, Li, B, Cd, Gd ,… • Isotope dependent – choose to your advantage 26 05/09/2017
T HE NEUTRON As a probe – interacting with matter - absorption - Neutron detection • How to detect a weakly interacting, neutral particle? • With a neutron absorber and measure the resulting signal 27 05/09/2017
T HE NEUTRON Scattering and absorption cause attenuation of a neutron beam imaging 28 05/09/2017
T HE NEUTRON Scattering and absorption cause attenuation of a neutron beam imaging 29 05/09/2017
T HE NEUTRON As a probe – interacting with matter - summary • Interaction with nuclei: – short range interaction angle independent scattering (no form factor) – scattering length can be positive or negative ( contrast variation) – depends on isotope ( selectivity) and nuclear spin – Coherent and incoherent scattering – strength and weakness – Scattering contrast different from X-rays, favours light atoms • A gentle probe - meV neutron beam does not cause radiation damage like a ~10 keV photon beam (what about XFEL!) • Magnetic moment probes magnetism of unpaired electrons 30 05/09/2017
I NSTRUMENTS & SCIENCE Time and length scales ? 31 05/09/2017
T HE ILL’ S INSTRUMENT SUITE 32
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