Chemical Engineering 412

Transcription

1 Chemical Engineering 412 Introductory Nuclear Engineering Lecture 26 Radiation Detection & Measurement II

2 Spiritual Thought 2 I would not hold the position in the Church I hold today had I not followed the counsel of a prophet, had I not prayed about that decision, had I not come to an appreciation of an important truth: the wisdom of God ofttimes appears as foolishness to men. But the greatest single lesson we can learn in mortality is that when God speaks and His children obey, they will always be right. President Thomas S. Monson

3 Types of gas-filled detectors Three types of gas-filled detectors in common use: Ionization chambers Proportional counters Geiger-Müller (GM) counters Type determined primarily by the voltage applied between the two electrodes Ionization chambers have wider range of physical shape (parallel plates, concentric cylinders, etc.) Proportional counters and GM counters must have thin wire anode (why)

4 Voltage influence on Sensitivity E = VV oo rr ln bb/aa b a

5 Ionization chambers If gas is air and walls of chamber are of a material whose effective atomic number is similar to air, the amount of current produced is proportional to the exposure rate Air-filled ion chambers are used in portable survey meters, for performing QA testing of diagnostic and therapeutic x-ray machines, and are the detectors in most x-ray machine phototimers Low intrinsic efficiencies because of low densities of gases and low atomic numbers of most gases

6 Compensated Ion Chamber

7 Ionization Chambers

8 Proportional counters Must contain a gas with specific properties Commonly used in standards laboratories, health physics laboratories, and for physics research Seldom used in medical centers

9 Proportional Counter

10 Proportional Counter

11

12 Compensated Ion Chamber

13 Ionization Chambers

14 Proportional counters Must contain a gas with specific properties Commonly used in standards laboratories, health physics laboratories, and for physics research Seldom used in medical centers

15 Proportional Counter

16 GM counters GM counters also must contain gases with specific properties Gas amplification produces billions of ion pairs after an interaction signal from detector requires little amplification Often used for inexpensive survey meters In general, GM survey meters are inefficient detectors of x-rays and gamma rays Over-response to low energy x-rays partially corrected by placing a thin layer of higher atomic number material around the detector

17 GM Counter

18 Typical GM Counter

19 GM counters (cont.) GM detectors suffer from extremely long dead times seldom used when accurate measurements are required of count rates greater than a few hundred counts per second Portable GM survey meter may become paralyzed in a very high radiation field should always use ionization chamber instruments for measuring such fields

20 Scintillation detectors Scintillators are used in conventional film-screen radiography, many digital radiographic receptors, fluoroscopy, scintillation cameras, most CT scanners, and PET scanners Scintillation detectors consist of a scintillator and a device, such as a PMT, that converts the light into an electrical signal

21 Scintillation Mechanism - Inorganic

22 Fluorescence Decay Signal NN tt = NN 00 eeeeee λλλλ nn tt = 00 tt λλnn tt ddddd λλ = 11 tt

23 Example 23 Problem 8.3 in book. λ=4.35e6/sec n/n= nn NN = λ 0 tt oo ee λtt t o = 1 λ 1 nn NN =53μs

24 Absorption Efficiency

25 Scintillation Mechanism - Organic

26 Inorganic vs. Organic Inorganic Depends on crystalline structure and hence must be crystalline or poly-crystalline Involve high Z elements (compared to organics), especially as activator/dopants, and are therefore effective gamma absorbers Generally hydroscopic and fragile Organic Depend on molecular rather than crystalline structure, and hence more easily fabricated. Less sensitive generally and especially to gamma radiation Low Z-numbers produce less backscatter Good for electron and beta detection and for fast neutron detection (fast neutrons produce fast H and C that ionize as they slow).

27 Scintillators Desirable properties: High conversion efficiency Decay times of excited states should be short Material transparent to its own emissions Color of emitted light should match spectral sensitivity of the light receptor For x-ray and gamma-ray detectors, µ should be large high detection efficiencies Rugged, unaffected by moisture, and inexpensive to manufacture

28 Scintillators (cont.) Amount of light emitted after an interaction increases with energy deposited by the interaction May be operated in pulse mode as spectrometers High conversion efficiency produces superior energy resolution

29 Materials Sodium iodide activated with thallium [NaI(Tl)], coupled to PMTs and operated in pulse mode, is used for most nuclear medicine applications Fragile and hygroscopic Bismuth germanate (BGO) is coupled to PMTs and used in pulse mode as detectors in most PET scanners

30 Semi-conductor Detectors Type Efficiency (Z) Density (g/cm 3 ) Resolution (Band Gap ev) Ionization Energy (ev/e-h) Convenience Notes Si(Li) Very Low (14) 2.33 High Low LN operation Ge(Li) Low (32) 5.33 High Very Low LN always GaAs Low (31/33) modest CdTe Moderate (48/52) low polarizes slowly with time Cd 5 Zn 45 Te 50 HgI 2 Moderate (48/52) Moderate (80/53) high no cooling necessary, stable low polarizes with time

31 Photons Spectrometer An incident photon can deposit its full energy by: A photoelectric interaction (A) One or more Compton scatters followed by a photoelectric interaction (B) A photon will deposit only a fraction of its energy if it interacts by Compton scattering and the scattered photon escapes the detector (C) Energy deposited depends on scattering angle, with larger angle scatters depositing larger energies

32

33 Possible Interactions

34 Interactions (cont.) Detectors normally shielded to reduce effects of natural background radiation and nearby radiation sources An x-ray or gamma-ray may interact in the shield of the detector and deposit energy in the detector: Compton scatter in the shield, with the scattered photon striking the detector (E) A characteristic x-ray from the shield may interact with the detector (F)

35 Spectrum of Cesium-137 Cs-137 decays by beta particle emission to Ba- 137m, leaving the Ba-137m nucleus in an excited state The Ba-137m nucleus attains its ground state by the emission of a 662-keV gamma ray 90% of the time In 10% of decays, a conversion electron is emitted instead, followed by a ~32-keV K-shell characteristic x-ray

36 Photon Interactions: 36

37

38 Pulse height spectrum of Cs-137 A. Photopeak corresponding to interactions in which the energy of an incident 662- kev photon is entirely absorbed in the crystal B. Compton continuum caused by 662-keV photons that scatter in the crystal, with the scattered photon escaping the crystal C. The Compton edge is the upper limit of the Compton continuum

39

40 Pulse height spectrum (cont.) D. Backscatter peak caused by 662-keV photons that scatter from the shielding around the detector into the detector E. Barium x-ray photopeak caused by absorption of barium K-shell x-rays (31 to 37 kev) F. Photopeak caused by lead K-shell x-rays (72 to 88 kev) from the shield