RadiationRadiation

DetectionDetection

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• Now that we know the Now that we know the characteristics of atomic structure characteristics of atomic structure and radioactivity, we now need to and radioactivity, we now need to discuss methods of detecting discuss methods of detecting radioactivityradioactivity

• Since radiations are Since radiations are submicroscopic and cannot be submicroscopic and cannot be seen, they must be detected seen, they must be detected indirectly indirectly

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• This is accomplished through the This is accomplished through the fact that radiations ionize matterfact that radiations ionize matter

• Since we cannot detect the Since we cannot detect the radiations directly we detect the radiations directly we detect the ions that it produces ions that it produces

• Consequently, most radiation Consequently, most radiation detectors work on the process of detectors work on the process of ionizationionization

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• There are two types of detectorsThere are two types of detectors– PassivePassive– ActiveActive

• Passive detectors do not require a Passive detectors do not require a constant source of energy, i.e. constant source of energy, i.e. “plugged in”, batteries, etc.“plugged in”, batteries, etc.

• Active detectors require a constant Active detectors require a constant energy sourceenergy source

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• Passive detectors – The first Passive detectors – The first passive detector we need to passive detector we need to discuss is the first ever radiation discuss is the first ever radiation detector… Photographic Filmdetector… Photographic Film

• In 1895 Wilhelm Roentgen used In 1895 Wilhelm Roentgen used photographic film to record the first photographic film to record the first X-RaysX-Rays

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• In 1896 Henri Becquerel used In 1896 Henri Becquerel used photographic film to discover photographic film to discover natural radioactivitynatural radioactivity

• Photographic film is a mixture of Photographic film is a mixture of silver nitrate, when silver nitrate is silver nitrate, when silver nitrate is ionized, they change colorionized, they change color

• Photographic film is still the most Photographic film is still the most commonly used detectorcommonly used detector

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• Both x-rays used the medical field Both x-rays used the medical field and PMD (personnel monitoring and PMD (personnel monitoring device) i.e. Film Badgedevice) i.e. Film Badge

• The quantity of radiation The quantity of radiation determines the darkness of the film determines the darkness of the film exposure exposure

• The type of radiation can be The type of radiation can be determined by the use of filters determined by the use of filters placed in front of the filmplaced in front of the film

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• The advantages of photographic The advantages of photographic film are…film are…– Cost effectiveCost effective

• CheapCheap• Length of useLength of use

– Requires no user interactionRequires no user interaction

• Disadvantages of filmDisadvantages of film– Not immediate resultsNot immediate results– Not reusable Not reusable

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• Another passive detector not used Another passive detector not used anymore was called an anymore was called an electroscopeelectroscope

• Electroscopes used the charging Electroscopes used the charging and uncharging of ions to show the and uncharging of ions to show the presence of radiationpresence of radiation

DetectionDetection• The rate at which the electroscope The rate at which the electroscope

discharges, tells how strong the discharges, tells how strong the radiation source isradiation source is

• Advantages of electroscopesAdvantages of electroscopes– ReusableReusable

• DisadvantageDisadvantage– Not qualitative (what kind of radiation)Not qualitative (what kind of radiation)– Not accurately quantitative (how Not accurately quantitative (how

much)much)

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• Dosimeters are yet another type of Dosimeters are yet another type of passive detectorspassive detectors

• Dosimeters work like a miniature Dosimeters work like a miniature electroscope, inside there is a electroscope, inside there is a small ion chambersmall ion chamber

• This chamber contains a Mylar fiber This chamber contains a Mylar fiber that works like the gold leaf of the that works like the gold leaf of the electroscopeelectroscope

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• The dosimeter is charged by The dosimeter is charged by placing the contact point on a placing the contact point on a corresponding charging contact on corresponding charging contact on a dosimeter chargera dosimeter charger

• They are used for personnel They are used for personnel monitoring monitoring

• They are used where the potential They are used where the potential for higher doses of radiation are for higher doses of radiation are possible i.e. Nuclear Power Plantspossible i.e. Nuclear Power Plants

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• Advantages of dosimeters Advantages of dosimeters – ReusableReusable– Accurately Quantitative (How Much)Accurately Quantitative (How Much)– Immediate resultsImmediate results

• Disadvantages of dosimetersDisadvantages of dosimeters– Expensive (initial cost)Expensive (initial cost)

DetectionDetection

• The next passive detector we need The next passive detector we need to look at are TLD’sto look at are TLD’s– (Thermo Luminescent Dosimetery)(Thermo Luminescent Dosimetery)

• Early on in the study of radiation it Early on in the study of radiation it was learned that radiation has very was learned that radiation has very unique affects on crystalline unique affects on crystalline structuresstructures

• Radiation energy causes electrons Radiation energy causes electrons to move in these structures to move in these structures

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• When electrons are moved to When electrons are moved to higher energy levels, they store higher energy levels, they store energyenergy

• In many cases this causes a color In many cases this causes a color change in crystal i.e. brown glasschange in crystal i.e. brown glass

• Gemologists learn of this effect and Gemologists learn of this effect and used it to enhance the color of gem used it to enhance the color of gem stonesstones

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• Consequently approximately 6 Consequently approximately 6 thousand metric tons of gemstones thousand metric tons of gemstones are irradiated each yearare irradiated each year

• Approximately 85% of all the dark Approximately 85% of all the dark blue topaz sold in the US has been blue topaz sold in the US has been irradiated irradiated

• Other effects to gemstones include Other effects to gemstones include turning yellow into green diamonds turning yellow into green diamonds

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• Some crystals when heated and Some crystals when heated and the electrons move back to their the electrons move back to their original positions, will give off original positions, will give off visible lightvisible light

• The amount of light given off is The amount of light given off is proportional to the amount of proportional to the amount of radiation received i.e. lithium radiation received i.e. lithium fluoride fluoride

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• Consequently, crystals can be used Consequently, crystals can be used as personnel monitoring devices as personnel monitoring devices

• The advantages of TLD’sThe advantages of TLD’s– Readers are inexpensiveReaders are inexpensive– They are reusable They are reusable

• Disadvantages of TLD’sDisadvantages of TLD’s– More quantitative than qualitative More quantitative than qualitative

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• Cloud Chambers are another Cloud Chambers are another passive detectorpassive detector

• The work on the process of a gas The work on the process of a gas changing to a liquidchanging to a liquid

• They are similar to that of a jet They are similar to that of a jet airplane giving off a vapor trail at airplane giving off a vapor trail at high altitudehigh altitude

• In a cloud chamber the cold area is In a cloud chamber the cold area is at the bottomat the bottom

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• The atmosphere is made up of The atmosphere is made up of ethyl alcohol which is slightly ethyl alcohol which is slightly heavier than airheavier than air

• As the ethyl alcohol sinks towards As the ethyl alcohol sinks towards the bottom, it gets cold and wants the bottom, it gets cold and wants to condenseto condense

• It condenses on the trails of ions It condenses on the trails of ions created by the radiation, therefore created by the radiation, therefore you see the effects of ionization you see the effects of ionization

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• Advantages of cloud chambersAdvantages of cloud chambers– You can physically see the effects of You can physically see the effects of

the radiationthe radiation• Disadvantages Disadvantages

– Clouds don’t stay longClouds don’t stay long

• Bubble Chambers work on the Bubble Chambers work on the process of liquid changing to gas… process of liquid changing to gas… opposite of a cloud chambersopposite of a cloud chambers

DetectionDetection

• They use a chamber of liquid They use a chamber of liquid hydrogen, as the radiation passes hydrogen, as the radiation passes through the liquid, it causes the through the liquid, it causes the atoms to move and the friction atoms to move and the friction changes the liquid to a gaschanges the liquid to a gas

• We do not do bubble chambers We do not do bubble chambers because hydrogen is explosive and because hydrogen is explosive and tough to keep in a liquid statetough to keep in a liquid state

DetectionDetection

• AdvantagesAdvantages– Once the bubbles are created, they Once the bubbles are created, they

are frozen in position, therefore you are frozen in position, therefore you can study bubble tracks…can study bubble tracks…

• Disadvantages we discussed Disadvantages we discussed already…already…

• The final passive detector is the The final passive detector is the “missing link” between passive and “missing link” between passive and active detectorsactive detectors

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• A spark chamber contains many A spark chamber contains many metal plates with a gap between metal plates with a gap between themthem

• The plates all have alternating The plates all have alternating charges (+ - + - +)charges (+ - + - +)

• Inside the chamber and around the Inside the chamber and around the plates is an easily ionized gasplates is an easily ionized gas

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• As the radiation passes through the As the radiation passes through the chamber, it creates ions that chamber, it creates ions that bridged the gap and creates trails bridged the gap and creates trails of sparksof sparks

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GM TubeGM Tube

• Detects Alpha, Beta, and Gamma Detects Alpha, Beta, and Gamma • Low efficiency Low efficiency • But won’t tell you what kind of But won’t tell you what kind of

radiationradiation

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ScintillationScintillation

• Only Gamma Only Gamma • And will differentiate between low And will differentiate between low

and high energy gammaand high energy gamma