Dr Paul Sellin Page 1 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Radiation Dosimetry and Detectors: Part 1 Dr Paul

Dr Paul SellinPage 1

Atkins Nuclear Radiation Physics CourseUniversity of Surrey, 3-6 October 2006

Radiation Dosimetry and Detectors: Part 1

Dr Paul Sellin



1. Interactions of Radiation with Matter

Basic definitions of nuclear radiation:

particles:

helium nucleus containing 2 protons and 2 neutrons:

Sometimes written as: or:

Note:

It is unusual to write the neutron number N and the electronic charge, and there numbers are often not shown

The alpha particle is a helium nucleus with no electrons the 2 electrons are removed, so the electronic charge is 2+

Mass of an = 4.0015 atomic mass units (amu)

= 4.0015 x 1.6604x10-27 kg = 6.644x10-27 kg

422

242He

atomic number Z = number of protons

atomic mass A = number of protons + neutrons

neutron number N

electronic charge



Alpha particle decay

Alpha particles can be emitted spontaneously by a range of heavy nuclei:

where X and Y are the parent and daughter nuclei.

A common example of an alpha emitter is 241Am (241-Americium), where the decay sequence is:

42

42

YX AZ

AZ

42

23793

24195 NpAm

See the free interactive Segre chart: http://atom.kaeri.re.kr/index.html

decay

decay

decay



Beta emission

particles come in two types: - which are electrons: charge = -1 + which are positron: charge = +1

Most laboratory sources are - sources

Beta decay similarly comes in two types:

- decay – emission of an electron (and an antineutrino)

+ decay – emission of a positron (and a neutrino)

The radioactive decay process for these two decay modes can be written as:

Note that the neutrino/anti-neutrino are not detectable using conventional radiation detectors.

Example of a ‘pure’ - emitter:

- decay: YX A

ZAZ 1

+ decay: YX A

ZAZ 1

ArCl 3618

3617



- decay of 36Cl

The maximum energy of the emitted particle is the decay Q value

In the detected spectrum, this is the ‘end point energy’



decay energy spectrum

The Q value of the beta decay reaction is shared between the electron and the antineutrino (in the cased of - emission) Because the antineutrino is not detected, the measured energy is only that of the electron Ee, which has a characteristic spectral shape:

When Ee = 0 the antineutrino ‘takes’ all the decay energy

When Ee = Q the antineutrino takes no a negligible energy – this is the endpoint energy of the pulse height spectrum

Calculated detected energy spectrum for Q=2.5 MeV



Photons: X-rays and -rays

The electromagnetic spectrum covers a wide range of photon energies, from ionising radiation to radio waves, and including visible light:

increasing photon energy



Nature of gamma rays

Gamma rays are photons emitted from excited nuclear states:

Excited nuclear states can be produced: in the daughter nucleus of a preceding alpha or beta decay from the excited nuclei resulting from fission from excited nuclei resulting from nuclear reactions

In the laboratory, -, + sources are often used as gamma ray sources, where the beta-decaying parent nucleus is long lived.

The -emitting daughter states decay very quickly (picoseconds): energy is the difference between the states involved in the transition long-lived -emitting states are called isomers

Typical gamma ray energies are ~ 100 keV – 10 MeV

NB: 241Am has a ‘famous’ low energy gamma ray at 59.5 MeV



Gamma spectra: 60Co and 137Cs

Decay schemes and end-point energies for the common gamma ray emitters 60Co, 137Cs:



1.+ decay which causes a positron to be emitted2. pair production from high energy gamma rays, which produces an electron/positron pair

Annihilation Radiation produces a very characteristic signature: two 511 keV photons are emitted back-to-back

X-rays and Annihilation radiation

Characteristic X-rays, produced from atomic transitions within the atom. The X-ray energy is characteristic of the element.

Typical characteristic X-ray energies are <1 keV to ~80 keV

keVkeVee 511511

Bremsstrahlung X-rays, these are X-rays generated from electron tubes, with a continuous spectrum of photon energies

Annihilation Radiation is an additional source of photons due to the annihilation of a positron with an electron. This can occur from:



Positron emitter 22Na: a positron and -ray source

22Na is a common + source:

90% +

10% EC

22Na (Z=11)

MeV

MeV

groundstate

22Ne (Z=10)

groundstate

gamma ray1.274 MeV (100%)

plus 0.511 MeV ann. rad.Note that 22Na is commonly used as

a laboratory ‘gamma’ source:• 0.511 MeV annihilation photons• 1.274 MeV gamma ray



Production of neutrons

Within the laboratory, neutrons are generated by two main mechanisms:

(a) Spontaneous fission sources

Many heavy nuclei have a significant decay probability for spontaneous fission - parent nucleus emits one or more neutrons per fission

Eg. 252Cf decays by: (1) decay to 248Cm

(2) spontaneous fission

Dominant decay mode is : 32x the fission rate

Neutron rate is still very large: 1g 252Cf produces 2.3x106 n/s

(b) Radioisotope (,n) sources

Mixture of an alpha emitting isotope with a suitable target material undergoes a nuclear reaction with the target to produce a fast neutron. Eg. 241Am/Be Common combination: Be has a large positive Q-value

Max neutron energy = Q + E = 11.2 MeV

Most alphas a stopped in the source material (typical range ~ 20m): 1 in 104 alphas react tp produce a neutron n yield: 80 per 106 primary alphas



Interactions of Radiation with Matter - Overview

Nuclear radiation can be grouped into 4 main types, based on their method of interaction with matter:

Charge particle radiations

Uncharged radiations

Heavy charged particles

(, protons, fission fragments)

Neutrons

Electrons Photons:

X-rays, gamma rays

charged particle interactions:

High stopping power

Scatter

non-charged interactions:

Scatter

Absorption

Reactions (neutrons)



Interactions of Heavy Charged Particles

Heavy charged particles interact with matter by Coulomb scattering from atomic electrons in the material: many interactions, each losing a small amount of energy the particle stops quickly, with a ‘short’ range the particle travel in a straight line – no scatter angles

Heavy charge particles mean: alpha particles, protons, fission fragments, heavy ions

Particle Range

The range depends on the Linear Stopping Power, S, which is defined as the energy loss per unit length travelled:

a high dE/dX means that the particle loses energy more rapidly shorter range a low dE/dx means that the particle loses energy more slowly longer range

BUT: dE/dx changes as the particle slows down, so estimating the range is difficult!

dx

dES



2 3 4 5

Alpha particle ranges*

Published range curves give estimates for particle ranges.

The range depends on: particle type (eg. ) particle energy material type

Example: find the range for 5.5 MeV alphas in Si.

(Density of Si = 2.33 g/cm3)

Note the quantity displayed on the y-axis: ‘range x density’ with units of mg/cm2

Divide by density, , to obtain range in cm



Alpha particle penetration

High energy loss (dE/dx) of alphas means they are easily stopped:

Range of 5.5 MeV in air: ~ 4cm stopped by almost all thin coatings, eg a sheet of paper particle sources must be ‘open’:

-emitting isotopes evaporated onto the surface of a metal disk even varnish or a similar layer will cause the ’s to lose a lot of energy

Detection of particles:

Requires a thin detector with good near-suface sensitivity

Detectors with a surface dead-layer are not suitableSilicon detectors or surface-sensitive scintillators are preferred

Contamination with -emitting nuclei (U, Th, Am): particles only penetrate micrometers into the skin – all dose is localised but the dose received is very high – due to ’s high energy (MeV)



Interactions of Electrons

Electrons (and positrons) interact through two mechanisms:

Coulomb Scattering with atomic electrons: because the electron is light, it is scattered by large angles overall path of the electron is the sum of many scatters – may not be a straight overall path energy loss dE/dx is less per collision – electron range is longer

Bremsstrahlung, or radiative scattering: de-acceleration of high energy (relativistic) electrons produces Bremsstrahlung X-rays only important for high energy electrons produced in X-ray tubes

Total linear stopping power S is given by the sum of these 2 terms:

radiativeCoulomb dX

dE

dX

dE

dx

dES

Only important for high energy electrons and high-Z materials



Electron range curves

Plot of electron range x density vs energy in silicon and sodium iodide scintillator

To a good approximation, most materials follow the same range x density function.



Interactions of X-rays and gamma rays

Photons (X-rays and gamma rays) interact in a material by:

(1) Absorption

(2) Scattering

A photon only interacts at a few discrete points in a material – or can pass through without interaction

Three primary photon interaction processes are possible: photoelectric absorption Compton scattering pair production

There are other photon interaction mechanisms which are generally less important and outside the scope of this course

The relative likelihood of each process occurring depends on:

1.the photon energy

2. the atomic number Z of the material



Summary plot of photon interactions

increasing photon energy makes Compton scatter and pair production more likely

increasing Z of the material makes

photoelectric absorption more likely



Attenuation Coefficient*

A beam of photons is exponentially attenuated as it passes though a piece of material: fgdgdg

where:

and is the linear attenuation coefficient (units of m-1)

Note that varies with: photon energy material (Z number)

xeII 0

Example:Calculate the % of 1 MeV gamma rays which are transmitted through a 7 mm thick aluminium sheet.Note: = 0.35 cm-2 for Al at 1 MeV



Mass Attenuation Coefficient

Photon attenuation is often expressed in terms of the mass attenuation coefficient:

mass attenuation coefficient =

where is the material density

The units of are cm2/g

If the mass attenuation coefficient is known, then the transmitted photon intensity I is given by:

To calculate I/I0 knowing the mass attenuation coefficient, the material density must also be known

x

eII

0



Total attenuation coefficient

The overall attenuation coefficient is given by the reciprocal sum of the attenuation coefficients for the 3 main processes:

Example figure shows the mass attenuation coefficient for lead:

PRODUCTIONPAIR

SCATTERCOMPTON

ABSORPTIONRICPHOTOELECT

TOTAL 1111



Interactions of Neutrons

Neutrons are classified as fast, epithermal, or thermal:

Fast Neutrons (eg. energies up to MeV) mainly interact by scattering, particularly of light nuclei proton recoil scattering is commonly used for detection: eg. in organic scintillators

Thermal/Epithermal Neutrons (eg no kinetic, only thermal, energy) mainly interact by reactions several common isotopes have very large cross-sections for thermal neutrons, commonly used in neutron detectors:10B: 10B(n,)7Li reaction Q=2.79 MeV s=3840 barns*6Li: 6Li(n,a)1H reaction Q=4.78 MeV s=940 barns3He: 3He(n,p)3H reaction Q=0.76 MeV s=5330 barns

*1 barn is a measure of cross section = 10-28 m-2



Detector Types (1): Silicon and Germanium

Detectors can be grouped into different families depending on type:

1. Semiconductor detectors: measure charge from the radiation event(a) Silicon (‘planar’ or ‘surface’ barrier) – high energy resolution, thin (300m – 2mm)

Ideal for high resolution , particle spectroscopy. Very poor detection efficiency for X-rays and gamma rays(b) Germanium (with ‘co-axial’ or ‘planar’ geometry) – large volume, high efficiency for X-rays and gamma rays

HPGe (high purity germanium) has the highest energy resolution

All Ge detectors need cooling: liquid nitrogen or a ‘cryo-cooler’

Less common detector types:

(c) Si(Li) Lithium-drifted silicon detector

Thick silicon detector, high resolution and good efficiency for X-rays

(d) CdZnTe/CdTe cadmium zinc telluride or cadmium telluride

A new detector technology for X-rays and gamma rays

Small volume, reasonably high resolution, operates at room temperature

(b)

(a)



Detector Types (2): Scintillators

2. Scintillator detectors: measure light from the radiation event, which is converted into a current pulse via a photocathode

Inorganic scintillators: good gamma efficiency, moderate/good energy resolution

Eg: Sodium Iodide (NaI), Barium Fluoride (BaF2)

Organic scintillators: commonly called ‘plastic’: large volume, cheap, very fast pulses poor energy resolution and low gamma efficiency

Eg. Plastic scintillator (BC501), liquid scintillator

New Lanthanum Chloride scintillators by St Gobain: BrilLanCe 350: 3.8% FWHM at 662 keV



Detector Types (3): Gas Detectors

3. Gas Detectors: (a) GM Tube (Geiger-Muller tube)

Very common radiation monitor

Produces pulses but no energy information – a counter

Efficient for beta and gamma particle

(b) Neutron detectors (3He, BF3 tubes)

Both helium-3 and boron have a very high detection efficiency for thermal neutrons

Fast neutrons are detected by surrounding the tube is a moderating plastic (eg. Bonner sphere)



Personal Dosimetry Detectors

Thermo-luminescent detector (TLD badge)

Plastic scintillator dose rate meter

Thermo Corporation Electronic Personal Dosimeter (EPD)



Instrumentation Chain

Spectroscopic detectors require instrumentation to convert the event energy into a pulse height spectrum: preamplifier, positioned close to the detector, produces small long-tailed pulses shaping amplifier, filters and amplifiers the signal, producing Gaussian pulses Multi-channel analyser (MCA) produces a pulse height spectrum

Portable digital MCA system: Canberra NaI

scintillator system

Amptek portable MCA

Conventional MCA card, embedded inside a PC

Standard setup for a Ge detector



Spectral Resolution

The resolution of a detector system defines how well close-spaced peaks can be detected: resolution = Full Width Half Maximum peak centroid small (‘high’) resolution narrow peaks

Note the same gamma spectrum taken with a CdTe detector (high resolution) and a NaI scintillator (low resolution)



Summary

The nature and origin of the major radiations have been summarised: are heavy charged particles, containing 2 protons and 2 neutrons are light charged particles, either electrons or positrons X-rays and gamma rays are photons, with low and high energies respectively Neutrons are uncharged heavy particles

The interaction of radiation with matter has been described: particles have a short straight paths in matter, typically 20-50m range particles travel are scattered in matter, travelling ~mm X-rays and gamma rays photons interact through a combination of photon absorption and scatter. Neutrons interact through either scatter or nuclear reactions

The major detector systems have been introduced: Silicon surface barrier detectors for a, b particles Germanium detectors for high resolution X-ray, gamma ray spectroscopy Scintillation detectors for lower resolution gamma ray spectroscopy Gas detectors, principally GM tubes and neutron detectors Si(Li) for X-ray detection CdTe/CdZnTe new technologies for room temperature gamma ray detection

Documents

Dr Paul Sellin Page 1 Atkins Nuclear Radiation Physics Course University of Surrey, 3-6 October 2006 Radiation Dosimetry and Detectors: Part 1 Dr Paul