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Introduction ISIS accelerator and target general
1. FELIX QVI POTVIT RERVM COGNOSCERE CAVSAS Introduction to ISIS accelerator and target David Findlay Accelerator Division ISIS Department Rutherford Appleton Laboratory
8. Measurements are made on condensed matter samples on ISIS by neutron scattering Just as an object can be seen by suitably collecting scattered optical photons, so a condensed matter sample can “seen” by suitably collecting scattered neutrons Source Sample Detector neutrons
9. Neutron source is pulsed Neutron energies measured by time of flight t = 0 Source Detector time t E = ½ m (l/t)² length l
10. = /p (reduced de Broglie wavelength) pc = c/ = 1 Å = 0.159 Å c = 197 MeV.fm = 1970 eV.Å (e²/ c = 1/137) pc = 12.4 keV Neutrons: p²c² = 2mc²E, m = 938 MeV E(neutron) = 80 meV X-rays: pc = E E(X-ray) = 12 keV Cf. dynamics: typical energies ~meV
11. ISIS is accelerator -driven neutron source 800 MeV protons, 200 µA, 160 kW on tungsten target ~2×10 16 neutrons/second (mean) from spallation Uses three cascaded particle accelerators
19. Neutron generation on ISIS: 800 MeV protons, 200 µA, 160 kW on tungsten target ~2×10 16 neutrons/second (mean) from spallation
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29. Linac (70 MeV, H – ions) 4-section (-tank) drift tube linac Acceleration by 202.5 MHz RF, not DC Each tank highly ~10 m long, ~1 m diameter. Highly resonant; Q ~50000 Hide particles inside drift tubes while sign of oscillating accelerating field wrong
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36. Everything synchronised to magnetic field Biased sine wave — (660 + 400 cos ( t)) amps Megawatt resonant LC circuit
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40. All beam in synchrotron extracted in one turn = v/c = 0.84, 163 m circumference revolution time = 0.65 µs 4 µC ÷ 0.65 µs 6 A circulating current Extracted pulse ~0.4 µs long
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48. Target ~2.5×10 13 (4 µC) protons per pulse on to tungsten target (50 pps) ~15–20 neutrons / proton, ~4×10 14 neutrons / pulse Primary neutrons from spallation: evaporation spectrum (E ~1 MeV) + high energy tail
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51. Moderators But want m eV, not M eV Moderation — elastic nuclear scattering — low A Three moderators: liquid hydrogen (20°K), methane (100°K), water (43°C) Reflector Moderators Primary target Protons