Radiation Detection & Measurements - 1 The topic of now discussion is “ ………………………..” Day 3 – Lecture 3

Objective To learn about different types of radiation detectors used in radiation protection

Contents Detector Material Detector Principles Detector Types

Detectors The detector is a fundamental base in all practice with ionizing radiation Knowledge of the instruments potential as well as their limitation is essential for proper interpretation of the measurements

Detector Material Any material that exhibits measurable radiation related changes can be used as detector for ionizing radiation. Change of colors Chemical changes Emission of visible light Electric charge Active detectors: immediate measurement of the change. Passive detectors: processing before reading Any material which shows measureable change when radiation fall/pass through it can be used as a radiation detector material. E.g. when radiation pass/on fall on the material may be its colour change, may some chemical changes take place, may be visible light emitts from the mateiral and may be electric charge produced in the material. So by using these behaviors of the materials against ionization radiations detectors may be fabricated to detect & measure the ionization radiations. Radiation detectors are of two type…………..

Detector Material Any material that exhibits measurable radiation related changes can be used as detector for ionizing radiation. Change of colors Chemical changes Emission of visible light Electric charge Active detectors: immediate measurement of the change. Passive detectors: processing before reading Any material which shows measureable change when radiation fall/pass through it can be used as a radiation detector material. E.g. when radiation pass/on fall on the material may be its colour change, may some chemical changes take place, may be visible light emitts from the mateiral and may be electric charge produced in the material. So by using these behaviors of the materials against ionization radiations detectors may be fabricated to detect & measure the ionization radiations. Radiation detectors are of two type…………..

Detector Principles Gas filled detectors ionisation chambers proportional counters Geiger Müller (GM) - tubes Scintillation detectors solid liquid Other detectors Semi conductor detectors Film Thermoluminescense detectors (TLD) In some detectors Gas is used a detecting medium e.g. ……………….. And in some detectors scintillation is used as a detecting principle e.g ……….. scintillating. Some used semi conductor , film or TLD as detecting material. These are the some basic detector principles

Detector Types 1) Counters Gas filled detectors Scintillation detectors 2) Spectrometers Solid state detectors 3) Dosimeters Thermoluminiscent detectors Film There are three types of detectors called counters, spectrometers & dosimeters, Gas filled detectors & scintillation detectors may be used as a counter. And scintillation detectors & solid state detectors may be used as spectrometers and similarly, gas filled detectors, solid state detectors, scintillation detectors, thermoluminiscent detectors, films may used as a dosimeters. We see that some detectors may be used for multipurpose as a counter or spectrometers or dosimeters .

Detector Types Effect Type of Instrument Detector Electrical Ionizing Chamber Proportional Counter GM Tube Solid State Detector Gas Semiconductor Chemical Film Chemical Dosimeter Photographic Emulsion Solid or Liquid Light Scintillation counter Crystal or Liquid Thermo- luminescense Thermo - luminescense dosimeter Crystal Heat Calorimeter First column shows the behavior of detectors against ionization radiations. Either electric charge produced, chemical changes produced, visible light produced either thermoluminescense or heat produced when radiation pass/fall on the detecting material. 2nd column shows the types of instruments………….. And third column shows the detector medium used.

Gas Filled Radiation Detectors These detectors consist of: a gas filled tube a positive electrode (anode) and negative electrode (cathode)

Regions Of Operation For Gas-filled Detectors Gas filled detectors are operated in the regions mentioned here. We plot a graph b/w the applied voltage V and the current produced due to the collection of ion pairs at the respective electrodes. when the incident radiation pass through the gas tube, it ionize the gas molecules and produces ion pairs. Initially when the applied current is zero or minimum, the produced ions will not move towards the respective electrodes rather they recombine to form again neutral molecule. When we further increase the voltage to the electordes the ion pairs start to collect at their respective electrodes and current will pass through the circuit and show the indication of radiation. When further increase the applied voltage to the electrodes the increase in the current will be very small about negligible and from the curve you see that it is about flat region and this region is called saturation region or Ionization Chamber Region. Ionization chambers are operated in this region at normal mode. When we further increase the applied voltage, the electron when are coming towards the anode get so much k.E that they produced further ionization on their way towards the anode and as a result current in the circuit sufficiently increased. As the voltage is increased still further, the threshold field at which gas multiplication begins is reached. The collected charge then begins to multiply, and the observed pulse amplitude will increase. Over some region of the electric field, the gas multiplication will be linear, and the collect-ed charge will be proportional to the number of original ion pairs created by the incident radiation. This is the region of true proportionality and represents the mode of operation of conventional proportional counters,

Ionization Chamber Simplest of all gas filled radiation detectors An electric field (104 V/m) is used to collect all the ionizations produced by the incident radiation in the gas volume In most ionization chambers, the gas between the electrodes is air. The chamber may or may not be sealed from the atmosphere. Many different designs for the electrodes in an ionization chamber, but usually they consist of a wire inside of a cylinder, or a pair of concentric cylinders. Here are some important points about ionization chambers..

Ionization Chamber Electrometer Negative ion Positive ion + HV - 1234 HV - The response is proportional to ionization rate (activity, exposure rate) Some genral properties of ionization chamber are....................... General Properties Of Ionisation Chambers High accuracy Stable Relatively low sensitivity

Examples Of Ion Chamber here are the snaps of different kinds of ion chambers.

Applications of Ion Chambers Current Mode Radiation Survey Radiation Source Calibrator Radioactive Gases Measurement Pulse Mode Counting Alpha Spectroscopy Ion chambers may be operated into two modes…………………

General Properties of Ionisation Chambers High accuracy Stable Relatively low sensitivity

Problems With Ion-chambers A basic problem with ionization chambers is that they are quite inefficient as detectors for x and gamma-rays. Only a very small percentage (less than 1percent) of X- or gamma rays passing through the chamber actually interact with and cause ionization of air molecules. for x and gamma- rays, their response changes with photon energy because photon absorption in the gas volume detection efficiency and relative penetration of photons through the chamber walls both are energy-dependent processes There are some problems with the ion chambers especially for X-ray and gamma measurements …………………..

Proportional Counter Proportional counter are operated at an electric field strength 106 V/m for Gases at STP causing Avalanches Applications Low Energy X-Radiations Neutron Detection Spectroscopy

Gas Multiplication and Avalanche in Proportional Detector The avalanche will stop after the electric field reduced to a threshold caused by the space charge of accumulated positive ions in the gas. anode an electron cathode

Properties of Proportional Counter Can be applied to situations in which the number of ion pairs generated by the radiation is too small to permit satisfactory operation in pulse-type ion chambers. A little higher sensitivity than the ionisation chamber Used for particles and low energy photons

GM Counters When the electric field strength across a proportional counter is increased (> 106 V/m), the device enters a GM region of operation. GM counter is gas-ionization device in which, the ionization effect creates a response which can be converted to an electrical output. It is a gas-filled detector designed for maximum gas amplification effect.

GM Tube Structure The center wire (anode) is maintained at high positive voltage relative to the outer cylindrical electrode (cathode). The outer electrode may be a metal cylinder or a metallic film layer on the inside of a glass or plastic tube. Some GM counters have a thin radiation entrance window at one end of the tube. The cylinder or tube is sealed and filled with a special gas mixture, typically argon plus a quenching gas.

Fill Gases Gases used in a Geiger tube must meet some of the same requirements as for proportional counters. noble gases are widely used for the principal component of the fill gas in G-M tubes, with helium and argon the most popular choices. A second component is normally added to most Geiger gases for purposes of quenching, the electron avalanches.

Uses of GM Tubes Simple, low cost, easy to operate Pulse type counter that records number of radiation events All energy information is lost-no ability to do spectroscopy Dead time greatly exceeds any other commonly used radiation detector It has a high sensitivity but has a lower accuracy.

Types of Geiger-Mueller (GM) Tubes

Scintillation Detectors Scintillation is a means of detecting the presence of ionizing radiation Ionizing radiation interacts with a scintillator which produces a pulse of light This light interacts with a photocathode which results in the production of an electron The electron is multiplied in a photomultiplier tube that has a series of focused dynodes with increasing potential voltage which results in an electrical signal

Scintillation Detectors The number of counts is dependent on the activity that is present The energy of the electron, and consequently the associated current is proportional to the incident energy of the ionizing radiation By analyzing the energy and corresponding number of counts, the nuclide and activity may be determined

Scintillation Detectors There are several types of Scintillator Detectors: scintillator NaI (sodium iodide): restricted to the detection of the gamma; plastic scintillator: solution of fluorescent compounds included in a transparent plastic material (gantry); scintillator ZnS (Zinc Sulfide): used for the detection of alpha radiation

Scintillation Detector (alpha) The alpha scintillator is typically zinc sulfide.

Alpha Scintillation Detector The photomultiplier tube is located in the handle.

Scintillation Detection (photon) This is a review of an earlier slide showing the photocathode and photomultiplier tube. This slide shows the use of the scintillation detector (typically this would be sodium-iodide, NaI) in a shielded counting system – this configuration is used in counting low levels of activity, such as environmental monitoring. In addition, it shows how lead X-rays end up creating a signal that is detected (although the graphic shows the lead X-ray heading for the PM tube). Lead x-ray interference may be reduced using a graded shield-the inside of the lead walls are shielded with cadmium (Cd), and next with a layer of copper (Cu). The cadmium absorbs the lead x-ray, while the copper absorbs any cadmium X-rays produced. The figure also shows how Compton backscattered photons are produced.

Spectral Analysis Scintillation detectors, when used with a multichannel analyzer (MCA) provide information on the energy of a photon that has interacted with the detector as well as the activity present The spectra can be analyzed to determine which isotopes are present

Thermolumniscent Dosimeter (TLD) Thermoluminescence Mechanism: Thermoluminescence is the emission of light from a crystal on heating, after removal of excitation (i.e. ionizing radiation). Radiation dose causes the electrons in the crystal to move from low energy states to higher energy states. Some of these excited electrons are trapped in metastable states These photons can be collected with a photomultiplier tube. By proper calibration, the dose delivered to the crystal can be measured.

Simplified scheme of the TLD process Thermoluminescence dosimeters (TLDs) are crystals that can store some of the energy deposited by ionising radiation in a retrievable form. The figure illustrates the principle of Thermo , (one applies heat) Luminescence (and the crystal emits light) and Dosimetry (of which the intensity is related to the dose of ionizing radiation absorbed by the crystal prior to heating",). While the emitted light is proportional to the absorbed radiation the proportionality constant varies with radiation energy, total dose, TLD material and - most difficult to account for - thermal history of the crystals. As such, TLD is mostly used as a relative dosimetric technique in which the dose to be determined is compared to a similar known dose given to the same or a similar TL detector. TLDs have the advantages of small physical size and that no cables are required during irradiation. As such they are particularly well suited for measurements within solid phantoms and in vivo dosimetry. The chief disadvantages are the delay between irradiation and the readout process and the complexity of the whole TLD set-up.

Thermoluminescence TLD principle heating filament emitted light photomultiplier thermoluminescent material

TLD glow curves Shown here is the fact that different trap depths lead to different temperatures required for freeing the electrons from the traps, leading to differnet glow peaks.

TL Dosimeters Following are few TL materials used as TL dosimeters. LiF Feldspars CaF2 Quartz CaSO4 Topaz Li2B4O7 Diamond KBr

TLD Advantages: Small size High sensitivity Integrating Tissue equivalent Disadvantages: Time consuming No permanent record

BF3 Neutron Detectors BF3 Tube Construction Tube dimensions and geometry Large size tubes at higher pressure of fill gas Constructed of cylindrical geometry Cathode Al : low neutron absorption cross-section SS : preferred over Al because Al show alpha activity

BF3 Neutron Detectors Ageing effect Degradation in performance after operation of 1010 - 1011 registered counts Detection Efficiency Efficiency decreases abruptly with increase of neutron energies Dead spaces for charge collection reduce detection efficiency Li have energy 0.84 MEV and Alpha have 1.47 MeV

Lithium Containing Slow Neutron Detectors Neutron induced reaction is detected by lithium based scintillators LiI(Eu) scintillator function like NaI(Tl) detector Crystal size is greater than the range of reaction products, pulse height response is free of wall effect and a single is formed Scintillation efficiency is almost same for heavy charged particles and secondary electrons H-3 has energy 2.73 MeV whereas Alpha has energy 2.05 MeV

The 3He Proportional Counter Design of 3He Tube Diameter as large as possible Pressure of 3He is increased to reduce range of charged particles Add a small amount of a heavier gas to increase stopping power H-3 has energy 0.191 MeV and Proton has energy 0.573 MeV.

Solid State Detectors Solid State detectors are also called Semiconductor detectors In these radiation detector, a semiconductor material such as a silicon (Si) or germanium (Ge) crystal constitutes the detecting medium. In the detecting medium electron-hole pairs are produced when a particle of ionizing radiation pass through it As a result a pulse of current generated is measured Operation of HPGe detectors require Liquid Nitrogen

Solid State Detectors

Using Solid as Detection Medium In many radiation detection applications, the use of solid medium is of great advantage For high energy electrons and gammas, solid state detectors are much smaller than gas filled detectors Energy resolution can be improved by increasing number of charge carriers – possible in semiconductors

Semiconductor Detectors Desirable features of – (semiconductor diode detectors) or solid state detectors Superior Energy Resolution Compact Size Fast Timing Characteristics Effective Thickness – Can be varied according to the requirement Semiconductor Materials Silicon – Used for charged particle spectroscopy Germanium - Used for gamma ray spectroscopy

Semiconductor Detectors When a positive voltage is applied to the n-type material and negative voltage to the p-type material, the electrons are pulled further away from this region creating a much thicker depletion region The depletion region acts as the sensitive volume of the detector Ionizing radiation entering this region will create holes and excess electrons which migrate and cause an electrical pulse

Semiconductor Detectors Reverse Bias Intrinsic/Depletion Region Cathode (-) Anode (+) + + - -

Semiconductor Detectors Gamma rays transfer energy to electrons (principally by compton scattering) and these electrons traverse the intrinsic region of the detector e (+) (-)

Kodak Type 2 Radiographic Film Film Badge Dosimeter Open Window 0.8 mm Pb filter Cu filters (0.05, 0.3 and 1.2 mm) Kodak Type 2 Radiographic Film

Film Dosimeter Film dosimeters (commonly known as film badges) consist of a piece of photographic film in a holder The holder is fitted with a range of filters which allows us to distinguish between beta, x-ray, gamma and thermal neutron radiations and also allows determination of the personal dose equivalent for Hp(10), Hp(0.07) and Hp(3)

Film Dosimeter By determining the degree of blackening (optical density) on the developed film and comparing it with calibrated films that have been exposed to known doses, it is possible to ascertain both the total dose received by the wearer and also the contribution to total dose by each type of radiation The various filters used in film badges to ascertain whole body Hp(10), skin Hp(0.07) and eye Hp(3) doses are shown in the following Figure and Table

Film Dosimeter Filter Material Application Open Window beta and very soft x-rays Plastic (50 mg cm-2)  and x-ray dose and energy* Plastic (300 mg cm-2)  and x-ray dose and energy* Dural (0.040”)  and x-ray dose and energy* Sn + Pb (0.028” 0.012”)  and x-ray dose and energy* Cd + Pb (0.028” 0.012”) slow neutrons** Lead (0.012”) edge shielding+ Indium (0.4 g) neutron accident monitoring *quantitative determination of ** by gamma emitted after capture by cadmium +to prevent overlap of film blackening due to angled incident radiation

Film Badge Dosimeter A - Plastic filter B to E - Metallic filters Package A - Plastic filter B to E - Metallic filters O - Open window A B C D E O Film Package A - Plastic filter B to E - Metallic filters O - Open window

Film Badge Dosimeter The density on the film results from three basic sources: Base+Fog Exposure Black = exposed White = not exposed Al Filter Pb Filter

Where to Get More Information Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009) International Atomic Energy Agency, Postgraduate Educational Course in Radiation Protection and the Safety of Radiation Sources (PGEC), Training Course Series 18, IAEA, Vienna (2002)