Spring, 2009 Phys 521A 1

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, 2009 Phys 521A 2

Low-energy hadrons

• In addition to EM interactions (for charged hadrons), hadrons (π+, K+, K0, 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 π0, 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, 2009 Phys 521A 3

High energy hadrons

• Interact with nuclei, producing energetic secondaries, which further interact; result is a hadronic shower

• Longitudinal development governed by interaction length λI = A/(NAσnucl), the inverse cross-section/gram

• Transverse development governed by rms momentum of partons in hadrons, nominally ΛQCD ~ 0.3 GeV (this can be thought of as the rms quark momentum within the hadron)

Spring, 2009 Phys 521A 4

Absorption lengths

• Contrast λI with X0

• Heavy nuclei have longer λI in gm/cm2

λI

(cm)

732

42

46

17

9.9

11

83

Spring, 2009 Phys 521A 5

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 fEM

• Signal from EM component is more efficiently detected; so fluctuations in fEM worsen shower energy resolution:

where the ε indicate the response to EM/HAD energy

HADEMEMEMtrueSH ffEE 1

Spring, 2009 Phys 521A 6

Muons

• For energies below ~100 GeV, muons are mip’s

• At TeV-scale energies, radiation dominates; energy loss is sharply peaked

Spring, 2009 Phys 521A 7

Neutrinos

• Neutrinos interact weakly; cross-sections are tiny• Neutrinos of energy E incident on a target nucleon have

σ = 0.7*10-14 E barns/GeV = 0.7*10-38 E cm2/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 νe component (at MeV energies) can undergo CC

interactions; leads to Mikhaev Smirnov Wolfenstein effect: changes mix of ν1, ν2, ν3 eigenstates, mimics energy-dependent mixing

Spring, 2009 Phys 521A 8

Spring, 2009 Phys 521A 9

• Source activity (decays/time) measured in Becquerel– 1 Bq = 1 decay/second– 1 Ci = 3.7*1010 decays/second (Curie)

• Absorbed dose: energy deposit per unit absorber mass– 1 Gy = 1 J/kg = 100 rad = 6.24*1012 MeV/kg (Gray)

• Equivalent dose: sum of absorbed doses weighted by biological risk factors for specific radiation types– HT = ∑R wR DT,R where D = absorbed dose (HT in Sievert; 1 Sv =

100 rem)

• Effective dose: sum of equivalentdoses weighted by tissue factors(whole body dose)

Radioactivity

Spring, 2009 Phys 521A 10

Radiation safety

• Radiation levels– Natural background (cosmic, radon gas, …) varies with location.

Annual equiv. dose from 0.4-4 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/cm2) to effective dose conversion;e.g., 1 GeV proton fluence of 1/cm2 corresponds to about3 μSv

Spring, 2009 Phys 521A 11

Particle Detection Techniques

Spring, 2009 Phys 521A 12

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, 2009 Phys 521A 13

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, 2009 Phys 521A 14

Outline of next sections

• Ionization yields• Photo-sensitive devices• Scintillators• Charged particle detectors• Calorimeters• Neutrino detectors• Dark matter detectors

Spring, 2009 Phys 521A 15

Spring, 2009 Phys 521A 16

Ionization yields in gases

(104gm/cm2)

11.0

5.5

5.7

5.6

8.1

Spring, 2009 Phys 521A 17

i.e., distribution is not Poisson

Spring, 2009 Phys 521A 18

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