Lesson 17

1. Lesson 17 Detectors

2. 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.

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

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

5. Detectors based on gas ionization • Ion chambers 35 eV/ion pair>105 ion pairs created. Collect this charge using a capacitor, V=Q/C NO AMPLIFICATION OF THE PRIMARY IONIZATION

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

7. 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.

8. The Result

9. Proportional counters • Gas amplification creates output pulse whose magnitude is linearly proportional to energy deposit in the gas. • Gas amplification factors are 103-104. • Will distinguish between alpha and beta radiation

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

11. Geiger- Müller Counters • When the gas amplification factor reaches 108, 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, 100-300µs, 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

12. 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

13. Semiconductor physics

14. 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”.

15. 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.

16. 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

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

18. 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

19. PIN diodes • Cheap • p-I-n sandwich • strip detectors

21. 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

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

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

24. 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

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

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

27. NaI(Tl) • Workhorse gamma ray detector • Usual size 3” x 3” • 230 ns decay time for light output • Other common inorganic scintillators are BaF2, BGO

28. NaI detector operation

29. Nuclear electronics

30. Nuclear statistics

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

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

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

37. Gaussian (normal) distribution

38. Interval distribution Counts occur in “bunches”!!

39. Table 18-3. Uncertainties for some common operationsOperation Answer UncertaintyAddition A+B (σA2+σB2)1/2Subtraction A-B (σA2+σB2)1/2Multiplication A*B A*B((σA/A)2+(σB/B)2)1/2Division A/B A/B((σA/A)2+(σB/B)2)1/2

40. Simple statistics

41. Uncertainties for some common operationsOperation Answer UncertaintyAddition A+B (σA2+σB2)1/2Subtraction A-B (σA2+σB2)1/2Multiplication A*B A*B((σA/A)2+(σB/B)2)1/2Division A/B A/B((σA/A)2+(σB/B)2)1/2