Lesson 17 Detectors

Introduction When radiation interacts with matter, result is the production of energetic electrons. (Neutrons lead to secondary processes that involve charged species) Want to collect these electrons to determine the occurrence of radiation striking the detector, the energy of the radiation, and the time of arrival of the radiation.

Detector characteristics Sensitivity of the detector Energy Resolution of the detector Time resolution of the detector or itgs pulse resolving time Detector efficiency

Summary of detector types Gas Ionization Ionization in a Solid (Semiconductor detectors) Solid Scintillators Liquid Scintillators Nuclear Emulsions

Detectors based on gas ionization Ion chambers 35 eV/ion pair  >10 5 ion pairs created. Collect this charge using a capacitor, V=Q/C NO AMPLIFICATION OF THE PRIMARY IONIZATION

Uses of Ion Chambers High radiation fields (reactors) measuring output currents. Need for exact measurement of ionization (health physics) Tracking devices

Gas amplification If the electric fields are strong enough, the ions can be accelerated and when they strike the gas molecules, they can cause further ionization.

The Result

Proportional counters Gas amplification creates output pulse whose magnitude is linearly proportional to energy deposit in the gas. Gas amplification factors are Will distinguish between alpha and beta radiation

Practical aspects gas flow typical gas: P10, 90% Ar, 10% methane Sensitive to , , X-rays, charged particles Fast response, dead time ~  s

Geiger- Müller Counters When the gas amplification factor reaches 10 8, the size of the output pulse is a constant, independent of the initial energy deposit. In this region, the Geiger- Müller region, the detector behaves like a spark plug with a single large discharge. Large dead times, µs, result No information about the energy of the radiation is obtained or its time characteristics. Need for quencher in counter gas, finite lifetime of detectors which are sealed tubes. Simple cheap electronics

Semiconductor Radiation Detectors “Solid state ionization chambers” Most common semiconductor used is Si. One also uses Ge for detection of photons. Need very pure materials--use tricks to achieve this

Semiconductor physics

p-n junction Create a region around the p-n junction where there is no excess of either n or p carriers. This region is called the “depletion region”.

Advantages of Si detectors Compact, ranges of charged particles are µ Energy needed to create +- pair is 3.6 eV instead of 35eV. Superior resolution. Pulse timing ~ 100ns.

Ge detectors Ge is used in place of Si for detecting gamma rays. Energy to create +- pair = 2.9 eV instead of 3.6 eV Z=32 vs Z=14 Downside, forbidden gap is 0.66eV, thermal excitation is possible, solve by cooling detector to LN2 temperatures. Historical oddity: Ge(Li) vs Ge

Types of Si detectors Surface barrier, PIN diodes, Si(Li) Surface barrier construction

Details of SB detectors Superior resolution Can be made “ruggedized” or for low backgrounds Used in particle telescopes, dE/dx, E stacks Delicate and expensive

PIN diodes Cheap p-I-n sandwich strip detectors

Si(Li) detectors Ultra-pure region created by chemical compensation, i.e., drifting a Li layer into p type material. Advantage= large depleted region (mm) Used for  -detection. Advantages, compact, large stopping power (solid), superior resolution (1-2 keV) Expensive Cooled to reduce noise

Ge detectors Detectors of choice for detecting  -rays Superior resolution

Scintillation detectors Energy deposit  light  signal Mechanism (organic scintillators) Note that absorption and re-emission have different spectra

Organic scintillators Types: solid, liquid (organic scintillator in organic liquid), solid solution(organic scintillator in plastic) fast response (~ ns) sensitive (used for) heavy charged particles and electrons. made into various shapes and sizes

Liquid Scintillators Dissolve radioactive material in the scintillator Have primary fluor (PPO) and wave length shifter (POPOP)> Used to count low energy  Quenching

Inorganic scintillators (NaI (Tl)) Emission of light by activator center

NaI(Tl) Workhorse gamma ray detector Usual size 3” x 3” 230 ns decay time for light output Other common inorganic scintillators are BaF 2, BGO

NaI detector operation

Nuclear electronics

Nuclear statistics

Distribution functions Most general distribution describing radioactive decay is called the Binomial Distribution n=# trials, p is probability of success

Poisson distribution If p small ( p <<1), approximate binomial distribution by Poisson distribution P(x) = (x m ) x exp(-x m )/x! where x m = pn Note that the Poisson distribution is asymmetric

Example of use of statistics Consider data of Table 18.2 mean = 1898 standard deviation, , = 44.2 where For Poisson distribution

Gaussian (normal) distribution

Interval distribution Counts occur in “bunches”!!

Table Uncertainties for some common operations OperationAnswerUncertainty AdditionA+B(  A2+  B2)1/2 SubtractionA-B(  A2+  B2)1/2 MultiplicationA*BA*B((  A/A)2+(  B/B)2)1/2 DivisionA/BA/B((  A/A)2+(  B/B)2)1/2

Simple statistics

Uncertainties for some common operations OperationAnswerUncertainty AdditionA+B(σ A 2 +σ B 2 ) 1/2 SubtractionA-B(σ A 2 +σ B 2 ) 1/2 Multiplication A*B A*B((σ A /A) 2 +(σ B /B) 2 ) 1/2 Division A/B A/B((σ A /A) 2 +(σ B /B) 2 ) 1/2