Spring, 2009Phys 521A1 Neutrons Neutrons interact only strongly; they also decay, but not quickly; τ(n) ~ 886 s Cross-section depends on energy –slow neutrons can have large capture cross-sections; resultant isotopes can be stable, meta-stable or unstable –fast (>MeV) neutrons travel long distances, lose energy via elastic collisions with nuclei (  moderator must include large fraction of hydrogen for efficient slowing of n; look for paraffin cylinders suspended from fences around CERN…) Neutron albedo can be significant source of background in accelerator experiments

Spring, 2009Phys 521A2 Low-energy hadrons In addition to EM interactions (for charged hadrons), hadrons ( π +, K +, K 0, p, n ) can interact strongly with nuclei –Elastic collisions (nuclear recoils); energy transfer mostly to H. Nuclear recoils produce local ionization which can be detected in an active medium –Inelastic collisions can produce secondaries (mostly π + and π 0, but also p and n), split the target nucleus, or leave the nucleus in an excited state, which may be meta-stable –Low-energy neutrons can escape detection (previous slide)

Spring, 2009Phys 521A3 High energy hadrons Interact with nuclei, producing energetic secondaries, which further interact; result is a hadronic shower Longitudinal development governed by interaction length λ I = A/(N A σ nucl ), the inverse cross-section/gram Transverse development governed by rms momentum of partons in hadrons, nominally Λ QCD ~ 0.3 GeV (this can be thought of as the rms quark momentum within the hadron)

Spring, 2009Phys 521A4 Absorption lengths Contrast λ I with X 0 Heavy nuclei have longer λ I in gm/cm 2 λ I (cm)

Spring, 2009Phys 521A5 Hadronic shower – EM fraction Not all hadronic energy is prompt/local; neutrons can escape, excited nuclei can have significant ½ lives, neutrinos are produced in beta decays, etc. However, neutral pions immediately decay to 2 photons; fluctuations in charged/neutral pion production lead to varying electromagnetic fraction f EM Signal from EM component is more efficiently detected; so fluctuations in f EM worsen shower energy resolution: where the ε indicate the response to EM/HAD energy

Spring, 2009Phys 521A6 Muons For energies below ~100 GeV, muons are mip’s At TeV-scale energies, radiation dominates; energy loss is sharply peaked

Spring, 2009Phys 521A7 Neutrinos Neutrinos interact weakly; cross-sections are tiny Neutrinos of energy E incident on a target nucleon have σ = 0.7* E barns/GeV = 0.7* E cm 2 /GeV Both charged-current (via W ± ) and neutral-current (via Z) interactions take place off quarks and electrons –Charged current interactions easier to detect due to final state charged lepton –Neutral current can be seen only in recoil or scattering of target Flavor oscillations important over long distances –In vacuum governed by mixing angle (amplitude) and mass difference (oscillation frequency) –In matter only ν e component (at MeV energies) can undergo CC interactions; leads to M ikhaev S mirnov W olfenstein effect: changes mix of ν 1, ν 2, ν 3 eigenstates, mimics energy-dependent mixing

Spring, 2009Phys 521A8

Spring, 2009Phys 521A9 Source activity (decays/time) measured in Becquerel –1 Bq = 1 decay/second –1 Ci = 3.7*10 10 decays/second (Curie) Absorbed dose: energy deposit per unit absorber mass –1 Gy = 1 J/kg = 100 rad = 6.24*10 12 MeV/kg (Gray) Equivalent dose: sum of absorbed doses weighted by biological risk factors for specific radiation types –H T = ∑ R w R D T,R where D = absorbed dose (H T in Sievert; 1 Sv = 100 rem) Effective dose: sum of equivalent doses weighted by tissue factors (whole body dose) Radioactivity

Spring, 2009Phys 521A10 Radiation safety Radiation levels –Natural background (cosmic, radon gas, …) varies with location. Annual equiv. dose from mSv; range up to 50 mSv Safety: whole body effective dose limits (radiation workers; also see International Commission on Radiological Protection, ICRP) –EU/Switzerland: 20 mSv/year –Canada/U.S.: 50 mSv/year Fluence (particles/cm 2 ) to effective dose conversion; e.g., 1 GeV proton fluence of 1/cm 2 corresponds to about 3 μSv

Spring, 2009Phys 521A11 Particle Detection Techniques

Spring, 2009Phys 521A12 Particle detection techniques Ionizing radiation deposits energy in material What can be measured/counted to derive a signal? –Ions or electrons from ionization –Cherenkov light (blue to near UV) –De-excitation of atoms/molecules (~visible scintillation light) –Phase changes in superheated/cooled medium (seeded by ionization) Ideally want the directly measured quantity – a signal amplitude – to be proportional to number or energy of ionizing particles

Spring, 2009Phys 521A13 Signal and background Primary signals are usually small (~10s of e charges or photons) –need low-noise electronics, or –signal amplification prior to readout Background sources are varied –Ionizing particles from sources other than the one being studied –Noise (thermal, statistical, electronic, …) Speed of signal development varies greatly –From nanoseconds to seconds –Applications where the data rate is high (e.g., accelerator-based experiments with short time between collisions) cannot afford long signal integration times

Spring, 2009Phys 521A14 Outline of next sections Ionization yields Photo-sensitive devices Scintillators Charged particle detectors Calorimeters Neutrino detectors Dark matter detectors

Spring, 2009Phys 521A15

Spring, 2009Phys 521A16 Ionization yields in gases (10 4 gm /cm 2 )

Spring, 2009Phys 521A17 i.e., distribution is not Poisson

Spring, 2009Phys 521A18 Ionization multiplication Ionization produced by charged particles leaves electron-ion pairs Pairs re-combine unless separated by an electric field Electro-negative molecules cause attachment, reduce number of electrons freed by ionization Strong electric fields accelerate e - to energies (10s of eV) sufficient to cause further ionization (multiplication) Detectors based on this will be discussed later