Chapter 12

Thermal and Radiation

Measurement

Photons: IR (thermal), Visible, UV, soft X-ray, Hard X-ray, Gamma ray Charged particles: Low energy electrons, high energy electrons, heavy ions Neutrons: Cold neutrons, Thermal neutrons, Fast neutrons

Major detectors for continuous measurements: 1.Ion chamber (by current) 2.Faraday cup (by current) 3.Thermopile (by temperature) 4. Charge Coupling Device (CCD); Imaging plate Detectors can be operated in pulse mode: 1. Gas proportional counter 2. Electron multiplier tube or MCP 3. Scintillation detector + PMT(Photo-Multiplier Tube 4. Semiconductor detector (P-N junction in reversed

bias) Current type: for high intensity flux only How low the current can be measured today? Pulse type: for low counting rate only How is the highest counting rate today?

12 Nuclear Radiation Measurement

Detection of Nuclear Radiation

pulse mode and current mode

12.7 Detection of Nuclear Radiation

pulse mode and current mode

Pulse height Vmax= Q/c The Q in each pulse can be measured

Solar simulator AM0.0 above atmosphere AM1.0: perpendicular ( at sea level) AM1.5: 45 degrees(at sea level)

Solar constant 1395 w/m2

For solar radiation

Radiometer with thermopile sensor

Thermopile: thermocouples in series

Radiometer with thermistor

Compensate Temperature drift By the differential input Focussing intensity Noise reduction By lockin chopper frequency Calibrate before taking data

Reflection and Transmission measurement

積分球

To avoid loss of Diffuse scattering

For charged particles measurement High vacuum technology is needed

For ionization radiations

Photons- Produced electrons by photoelectric

effects, Compton effect, and pair production.

High energy electrons or charged particles-

accelerated to produce more electronics.

Neutrons—slow neutron, by nuclear reactions

and high energy charged particle produced.

Neutrons-fast neutron, by recoil protons.

1 Gy: 1 J /Kg of absorption

Detectors:

Dimension: zero, one, two, three dimensional

Large area detectors

Films,imaging plate, multiwire detector, CCD

detector, flat panel detectors, pixel detector

Specifications:

Spatial resolution, energy resolution, time

resolution, dynamic range of counting rate,

dynamic range of energy, dynamics of area,

sensitivity, efficiency, detector dead time,

radiation damage, cost.

12 Nuclear Radiation Measurement

12.6 Nuclear Radiation

Four Major Radiation Categories

12.6 Nuclear Radiation

-rays

3. The three major interactions are: the photo-electric effect (P.E.),

the Compton scattering (C.E.) and the pair production (P.P.).

γ

12 Nuclear Radiation Measurement

12.8-12.9 Gas-Filler Detectors

Generation of ion pairs e.g., a 3.5 MeV alpha particle may generate ~100,000 ion pairs in the air.

12 Nuclear Radiation Measurement

12.8-12.9 Gas-Filler Detectors

2. ionization chamber, proportional counter, and Geiger-Mueller (G-M) counter

12 Nuclear Radiation Measurement

12.8-12.9 Gas-Filler Detectors

2. ionization chamber, proportional counter, and Geiger-Mueller (G-M) counter— Gas Multiplication

Gas proportional counter

12 Nuclear Radiation Measurement

12.10-12.11 Scintillation Detectors

2. Inorganic Scintillators– e.g., NaI(Tl), CsI(Tl), LiI(Eu)…

12 Nuclear Radiation Measurement

12.10-12.11 Scintillation Detectors

1. Organic Scintillators : e.g., anthrancene and stilbene -- fluorescence and phosphorescence

12 Nuclear Radiation Measurement

12.10-12.11 Scintillation Detectors

3. Light Guides and Photo-Multiplier (P-M) Tubes

12 Nuclear Radiation Measurement

12.X Semiconductor Detectors

Junction Properties

12 Nuclear Radiation Measurement

12.X Semiconductor Detectors

High Purity Germanium (HPGe) Detectors

12.X Semiconductor Detectors Peak-efficiency Calibration

MCA: Multichannel analyzer The pulse height (Q/c) after ADC became an address of memory The counts is added by one at specified address and stored back

12.X Semiconductor Detectors

HPGe Gamma-ray Spectra

12 Nuclear Radiation Measurement

Thermal Neutron Detection

Q= 200 MeV for fission products

Q=0.763 MeV

27

Neutron interaction cross section

Neutron Reactions of Interests in Neutron Detection

Fast neutron (2 MeV) from fission needs to slow down to thermal energy (25 meV) To gain much higher fission cross section (>100 times)

10B, Cd very high absorption cross section for thermal neutrons 4He very low absorption cross section for gamma rays

The neutron detector at nuclear reactor: CIC compensated ion chamber Self power detector Measure the neutron flux profile in the reactor By activation analysis Put gold wire in the reactor 197Au(n, g) 198 Au T1/2=2.7 days, gamma ray 410 keV

31

CIC (Compensated Ion Chamber)

Reactor Instrumentation

32

PWR In-core Detector

Self-Power Neutron Detector (SPND)

Reactor Instrumentation

103Rh capture neutron and beta decay I = CsANf

12 Nuclear Radiation Measurement

Fast Neutron Detection:

Counting:

By proton recoil: fast proton as charged particle

By moderator: neutron slowing down, then nuclear reaction

produce charged particles

Thermal or cold Neutron energy measurement:

Time of flight

Crystal monochromator

Larmor procession

25 meV (kT at 300 K) neutron, 2200 m/s, wavelength 0.18 nm

Triple axis neutron spectrometer 2dsinq=l Energy resolution - meV For measuring the Inelastic neutron scattering spectrum due to vibration of molecules or crystal lattice vibration— Phonon dispersion

Larmor precession + Scattering

Spin-echo spectroscopy -neV (diffusion)

Larmor precession

)( BPdt

Pd

g

Only for polarized neutrons

use Two identical magnets In series.

Neutron Spin-echo Spectrometer Several neV energy resolution

Detectors:

Dimension: zero, one, two, three dimensional

Large area detectors

Films,imaging plate, multiwire detector, CCD

detector, flat panel detectors, pixel detector

Specifications:

Spatial resolution, energy resolution, time

resolution, dynamic range of counting rate,

dynamic range of energy, dynamics of area,

sensitivity, efficiency, detector ded time, radiation

damage, cost.

To measure photon energy

Using only pulse type

Multi-wire detector

Charge Coupling Device (CCD)

Imaging Plate

S.O.Kasap , J. Mater Sci.(2000)

Flat panel X-ray detector using amorphous Se under high voltage

Textures Scintillator

Picture of bone in Hand is easiest Good contrast Not moving For angioraphy: Blood vessel and muscle with the same density No contrast: Solution: 1. with dye injection 2. phase contrast

Three dimensional detector: tomography

Density contrast

Phase contrast

三維 x光全像攝影術

TOMOGRAPHY

空間解析度可達 100 nm

胡宇光博士提供

Imaging

By X-ray, gamma ray, neutron, good penetration By optical light, not for metal or non-transpant one By IR: some of them is good , Si wafer BY 200 keV electron: only very thin sample

12 Nuclear Radiation Measurement

12.13 Statistics of Counting – Example 12.5

sT=(21552)1/2/tT

sb=(1850)1/2/tb

ss=(sT2+sb

2)1/2

Statistical error of counting N is √N

N √N 100% error

1 1 100 %

100 10 10 %

10000 100 1%

If you need a precision improved by M times, you have to collect M2 of counts

If the Gaussian or Poisson distribution is ture. The more counts you collected, the more precision you can obtain

However, subtract the background is needed.

12.X Semiconductor Detectors

HPGe Gamma-ray Spectra

Background

What is the minimal detection limit?

Error propagation

{Signal+background} measurement (NT) subtract background (NB)

NS=NT-NB

Assume all Gaussian errors: ss2 =st

2 + sb2

NT=101± 10, NB=100 ± 10 , => NS= 1 ± 14 (1400%)

Accumulate 100 times data acquisition time:

NT=10100 ± 100, NB10000 ± 100 => NS = 100 ± 140 (140%)

Acquired 10000 times

NT=1010000 ±1000, NB =1000000 ±1000 => NS =10000 ±1400 (14%)

We need a stronger source and low background detectors

59

Detector dead time (pulse counting)

n-m=nmtd

n: real incident radiation M: the recorded radiation

60

Detector dead time loss

Chapter 12

Nuclear Radiation Measurement

Any questions?