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Gamma radiation

José - Luis Gutiérrez Villanueva

Radonova Laboratories AB, Uppsala - Sweden

Overview

J.L. Gutiérrez–Villanueva IAEA regional workshop Vilnius 2019

Overview

Presentation

José Luis Gutiérrez–Villanueva

j PhD in Physics (U. Valladolid, Spain)j Currently working as specialist radon measurement advisor at

Radonova Laboratories AB (Sweden)j Secretary of the Executive Committee and founding member

of ERA (European Radon Association)

For any questions or comments please contact me at:[email protected]


Overview

Gamma spectrometry

I Instrumentation (HPGe) available in most of labsI No radiochemistry is neededI Pretreatment of the sample is very simpleI A large number of radionuclides can be measuredI Detection limits acceptable for environmental

determinations

I Each geometry needs different efficiency calibrationI Very dependent on density of sampleI Time consuming (several days in some cases)I Maintenance of detector is critical (refrigeration)I Monitoring of background levels is necessary


Overview

Gamma spectrometry


Overview

Short intro to gamma spectrometry

The gamma-ray spectrum of natural uranium, showing about a dozendiscrete lines superimposed on a smooth continuum, allows theidentification the nuclides 226Ra, 214Pb, and214Bi of the uranium decaychain. (Credit: wikipedia)


Overview


Gammna decay: Photon’s emission by a nucleus when reachingsteady state of energy

I Gamma line is the fingerprint of a radionuclideI One radionuclide can have several gamma lines with

different probabilities and different energiesI X Rays and Gamma Rays (with different energies)I Gamma rays = NucleusI X Rays = Atomic crust

Interaction of radiation (photon) with matter (detection’smaterial)


Overview


Gammna decay: Photon’s emission by a nucleus when reachingsteady state of energy

I Gamma line is the fingerprint of a radionuclideI One radionuclide can have several gamma lines with

different probabilities and different energiesI X Rays and Gamma Rays (with different energies)I Gamma rays = NucleusI X Rays = Atomic crust

Interaction of radiation (photon) with matter (detection’smaterial)


Overview


INTERACTION OF RADIATION WITH MATTER

I Charged particles produce a signal within a detector byionization and excitation of the detector material directly.

I Gamma photons are uncharged and consequently cannot do this

I Gamma-ray detection depends upon other types of interactionwhich transfer the gamma-ray energy to electrons within thedetector material

I Excited electrons charge and lose their energy by ionization andexcitation of the atoms of the detector medium, giving rise tomany electron–hole pairs

I The absorption coefficient for gamma radiation in gases is lowand all practical gamma ray detectors depend upon interactionwith a solid

I The electron–hole pairs can be collected and presented as anelectrical signal.


Overview



PHOTOELECTRIC EFFECT

COMPTON EFFECT

PAIR PRODUCTION


Overview




COMPTON EFFECT

PAIR PRODUCTION


Overview




COMPTON EFFECT

PAIR PRODUCTION


Overview




COMPTON EFFECT

PAIR PRODUCTION


Overview



The photon interacts with the atom and gives ALL its energy toone electron: one part of the energy is used as kinetic energy andthe rest is used to remove electron from the atom


Overview


COMPTON EFFECT

I “Elastic collision”: pool ballsI Main interaction of gamma raysI The photon collides with electron and hands over part of

its energy to it. The angle through which the photon isscattered, the energy handed on to the electron, andenergy lost by the photon are interconnected


Overview


PAIR PRODUCTION

I When the photon with energy in excess of 1.02 MeV passesclose to the nucleus of an atom, the photon disappears,and a positron (e+) and an electron (e−) appear

I Annihilation reaction: positron interacts with electrons andcreates 2 annihilation photons each of 0.51 MeV


Overview


I Photoelectric interactions are dominant at low energyI Pair production at high energyI Compton scattering being most important in the

mid-energy rangeI In practice, evidence of pair production is only seen within

a gamma-ray spectrum when the energy is rather morethan 1022 keV


Overview



Overview


I Counts to appear in the spectrum above the full energy peak(apart from the natural background): due to random summingor pile-up, determined by the statistical probability of twogamma-rays being detected at the same time and therefore onthe sample count rate

I The most troublesome photoelectric interactions will be thosewith the shielding, usually lead. There is a significant possibilitythat this fluorescent X-ray may escape the shielding and that itwill be detected by the detector: generation of a number ofX-ray peaks in the gamma spectrum in the region 70–85 keV(problems for low energy measurements that may be solved withCd and Cu)

I The normal geometric arrangement of source–detector shieldingmeans that most gamma-rays are scattered through a largeangle by the shielding: backscattering (difficult to solve)


Overview


I Pair production: surroundings of the detector give rise to theannihilation peak at 511 keV in the spectrum. This is caused bythe escape of one of the 511 keV photons from the shielding,following annihilation of the pair production positron. Theannihilation peak is clearly visible in the spectrum of 28Al (seeFigure) but not in that of 137Cs (Why?)


Overview

Practical points gamma detection

I Gamma spectrometry using germanium detectors is the besttechnique for identifying and quantifying radionuclides. This isdue to the very sharply defined and characteristic energies ofgamma-rays which are produced by the great majority ofradionuclides.

I There are a small number of ‘pure beta emitters’, which do notemit gamma radiation. These cannot be identified by gammaspectrometry (3H, 14C, 90Sr).

I X-ray energies will tell you the element present, but not whichisotope.

I Decay schemes give vital information on whether gammas are in‘cascade’. This has great significance in true coincidencesumming.


Overview

Practical points detection systems

Radiation INTERACTS with matter (detector’s material)

We must translate this interaction into an electrical signal thatwe are able to measure


Overview





Overview





Overview

Ideal properties of gamma spectrometry detector

I Output proportional to gamma-ray energy

I good efficiency, i.e. high absorption coefficient, high Z

I easy mechanism for collecting the detector signal

I good energy resolution

I good stability over time, temperature and operating parameters

I reasonable cost

I reasonable size


Overview

Band structure of solids


Overview

Charge carriers

I The interaction of a gamma-ray with the semiconductor materialwill produce primary electrons with energies considerably greaterthan thermal energies

I Electric field, carriers will migrate up (electrons) or down (holes)the field gradient.

I The number of electron–hole pairs produced, n, will be relateddirectly to the gamma-ray energy absorbed

I One important component of the detector resolution is afunction of n

I Avoid trapping centres which can make difficult mobility ofcarriers: the detector material must be available, at reasonablecost, with a high purity and as near perfect as possiblecrystalline state


Overview

Suitable materials?

I To have as large an absorption coefficient as possible (i.e. highatomic number)

I To provide as many electron–hole pairs as possible per unitenergy

I To allow good electron and hole mobility

I To be available in high purity as near perfect single crystals

I To be available in reasonable amounts at reasonable cost.


Overview

Type of HPGe detectors

Intrinsic semiconductor: A semiconductor material containing equalnumbers of electrons and holes

I acceptor impurities when distributed throughout thesemiconductor material give rise to extra energy states justabove the valence band, called acceptor states. Germanium withthis type of impurity would be called p-type germanium (‘p’ forpositive acceptor impurities)

I The impurity atom is a donor atom sitting in a donor site, it willintroduce donor states just below the conduction band.Germanium with such impurities is n-type germanium (‘n’ fornegative donor impurities)

I The p-type material has an excess of holes and the n-type anexcess of electrons.


Overview

Type of HPGe detectors


Overview

HPGe detectors

GERMANIUM DETECTORS ARE OPERATED AT LOW TEM-PERATURE IN ORDER TO REDUCE ELECTRONIC NOISEAND THEREBY ACHIEVE AS HIGH A RESOLUTION AS POS-SIBLE

T

he most common means of providing a suitably low temperatureis cooling with liquid nitrogen (boiling point 77 K).


Overview

HPGe detectors


Overview

HPGe detectors


Overview

HPGe detectors

HPGe detectors: Detection system


Overview

HPGe detectors

I The detector produces a signal proportional to the energyemitted by the source

I The measuring equipment accumulates on each channelthe number of emissions corresponding to a certain energy


Overview

HPGe detectors

I The detector produces a signal proportional to the energyemitted by the source

I The measuring equipment accumulates on each channelthe number of emissions corresponding to a certain energy


Overview

HPGe detectors: MCA (Multichannel analyzer)

I From the output from the amplifier, it rejects out-of-range pulses

I it measures the height of each of those accepted and adds acount into the memory location corresponding to the channelrepresenting the voltage range

I it displays the data as a spectrum and allows the data to beprinted or saved to a data storage device.

I In principle, the relationship between pulse height (and thereforeenergy) and channel number would be exactly linear, passingthrough zero


Overview

HPGe detectors: MCA (Multichannel analyzer)


Overview

HPGe detectors: MCA terms and definitions

Lower level discriminator (LLD) pulses below this level will not be analysed. Use thisto reject electronic noise and low-energy X-rays

Upper level discriminator (ULD) pulses above this level will not be analysed. Use thisto reject very high energy pulses. This will often beleft at its maximum, but still performs a usefulfunction in rejecting high- energycosmic gamma-rays

ADC zero level use this to adjust the energy calibration so that itpasses through 0 keV. Not ideal for eliminating theeffect of noise

Digital offset this is a means of shifting the spectrum to lowerchannel numbers by subtracting a fixed number (theoffset) from every channel number output by the ADC

Conversion range the maximum pulse height the MCA can accept,typically 10 V

ADC resolution is the total number of channels available within theADC. It varies from model to model, but MCAs forgermanium systems might incorporate a 16k (16384),8k (8192), or 4k (4096) channels ADC

ADC conversion gain is simply the number of channels actually used in aparticular application – in everyday parlance, thespectrum size


Overview

Standards, calibrations

Geometry the standard source shall have the same geometry as the sample tobe measured.

Geometry the standard radioactivy source shall be homogeneously distributed

Position centered position

Software the sorftware determines the presence of isotopes according to theirenergy. Thermal variations may cause spectrum shifts. Therefore itis necessary to carry out verifications frecuently


Overview

Calibration source

I Similar density ofstandard and analysissample

I Standard shall beuniformly distributedover the entirevolume (similarself-absorption asanalysis samples)

I Centre the sample


Overview

Examples of geometries


Overview

Calibration steps

Calibration energy versus channel

Calibration efficiency versus enery


Overview

Energy vs. channel

The object of energy calibration is to derive a relationship betweenpeak position in the spectrum and the corresponding gamma-rayenergy

Energy calibration is accomplished by measuring the spectrumof a source emitting gamma-rays of precisely known energy andcomparing the measured peak position with energy. It matters notwhether the source contains a single nuclide or several nuclides:152Eu source for routine energy calibration


Overview

Energy vs. channel


Overview

Energy vs. channel


Overview

Efficiency vs. energy

Relative efficiency general performance measure relating the efficiency ofdetection of the 60Co gamma ray at 1332 keV of thedetector to that of a standard sodium iodidescintillation detector

Absolute full energy peak relation between the peak area in our spectrum to theamount of radioactivity it represents. This relates thepeak area, at a particular energy, to the number ofgamma- rays emitted by the source and must dependupon the geometrical arrangement of source anddetector

Absolute total efficiency relates the number of gamma rays emitted by thesource to the number of counts detected anywhere inthe spectrum. This takes into account the full energypeak and all incomplete absorptions represented bythe Compton continuum

Intrinsic efficiency relates the counts in the spectrum to the number ofgamma rays incident on the detector. This efficiencyis a basic parameter of the detector and isindependent of the source/detector geometry. It isalso called full energy peak or total


Overview

Energy vs. channel


Overview

Energy vs. channel


Overview

Energy resolution

Resolution is a measure of the width of the peaks in a gamma-rayspectrum – the smaller the width, the better the detector, thehigher the resolutionFHWM: the Full Width of the peak at Half Maximum (keV)


Overview

Energy resolution


Overview

How to calculate activity

COUNTS

ACTIVITY (Bq kg−1; Bq l−1 . . . )


Overview

How to calculate activity

A =I

T · ε · P · X

I A: activity expressed in Bq kg−1; Bq l−1 . . .I I : number of counts correctedI T : time in secondsI ε: efficiencyI P : emission probabilityI X : mass (kg), volume (l), . . .

In some cases it is necessary to include corrections due to radioac-tivity decay, summing effects, etc.


Overview

Some statistics

Binomial distributionIn principle, the statistics of radioactive decay are binomial innature. If we were to toss a handful of coins onto a table and thenexamine the arrangement, we would find coins in one of twodispositions – heads up or tails up. Similarly, if we could prepare aradioactive source and, during a particular period of time, monitoreach individual atom we would see that each has only one of twopossible fates – to decay or not decay


Overview

Some statistics

Confidence limits. . . we must quote our limits in such a way that we have a stateddegree of confidence that the true value lies somewhere within them


Overview

Some statistics

Counting Decision Limits

Critical limit (LC) a decision level: ‘Is the net count significant?’

Upper limit (LU) ‘Given that this count is not statisticallysignificant, what is the maximum statisticallyreasonable count?’

Detection limit (LD) ‘What is the minimum number of counts Ican be confident of detecting?’

Determination limit (LQ) ‘How many counts would I have to have toachieve a particular statistical uncertainty?’

Minimum detectable activity (MDA) ‘What is the least amount of activity I can beconfident of detecting?’


Overview


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