gamma spectroscopy for environmental radiation measurements

Gamma spectroscopy Gamma spectroscopy for environmental for environmental

radiation radiation measurementsmeasurements

L.Visca L.Visca 1,21,2, R. Cirio, R. Cirio1,21,2, A. , A. SolanoSolano1,21,2

1. Universita’ degli Studi di Torino, Dipartimento di Fisica Sperimentale2. INFN, Sezione di Torino

XI ICFA School San Carlos de Bariloche 11-22 January 2010

SummarySummary

Overview about natural background radiationOverview about natural background radiation

Description of a portable scintillation detector Description of a portable scintillation detector

system NaI(Tl)system NaI(Tl)

Calibration of a scintillation detection system Calibration of a scintillation detection system

Analysis of gamma spectra from different samplesAnalysis of gamma spectra from different samples

Activity assessment from measured spectraActivity assessment from measured spectra



Natural radiation Natural radiation exposuresexposuresAll living organisms are continually exposed to ionizing radiation, which

has always existed naturally. The sources of that exposure are cosmic rays that come from outer space and from the surface of the Sun, terrestrial radionuclides that occur in the Earth’s crust, in building materials and in air, water and foods and in the human body itself. Some of the exposures are fairly constant and uniform for all individuals everywhere, for example, the dose from ingestion of potassium-40 in foods. Other exposures vary widely depending on location. Exposures can also vary as a result of human activities for example building materials of houses and the design and ventilation systems strongly influence indoor levels of the radioactive gas radon and its decay products, which contribute significantly to doses through inhalation.


Gamma rays from Gamma rays from terrestrial sourcesterrestrial sourcesNaturally occurring radionuclides of terrestrial origin are present in all

media in the environment, including the human body itself. Irradiation of the human body from external sources is mainly by gamma radiation from radionuclides in the 238U and 232Th series and from 40K. Indoor exposure to gamma rays, mainly determined by the materials of construction, is inherently greater than outdoor exposure in earth materials have been used; the source geometry changes from half-space to a more surrounding configuration indoors. When the duration of occupancy is taken into account, indoor exposure becomes even more significant.

ScintiblocScintibloc is a scintillator is a scintillator cristal NaI[Tl] directly coupled cristal NaI[Tl] directly coupled to PhotoMultiplierTube (PMT). to PhotoMultiplierTube (PMT).

NanoSPEC NanoSPEC is a NaI(Tl) is a NaI(Tl) detector base. It includes HV detector base. It includes HV module, spectroscopic module, spectroscopic amplifier and amplifier and MultiChannelAnalyser MultiChannelAnalyser (MCA). It can be connect to a (MCA). It can be connect to a PC using a serial interfacePC using a serial interface

Instruments and methodsInstruments and methods

winTMCA32 winTMCA32 is the software is the software required to set-up the detectorrequired to set-up the detector, , acquire the gamma ray acquire the gamma ray spectrum from the sample and spectrum from the sample and analyse it.analyse it.

San Carlos de Bariloche 11-22 January 2010

WinTMCA32: InitializationWinTMCA32: InitializationBefore starting the set-up, set the switch of the NanoSpec to: Int1. Select the acquisition mode (Acquisition_Mode) on: PHA (pulse height analysis). In this way every value is assigned to one channel and the channel content is incremented by one if a corresponding signal is counted.

2. Define the Spectrum length (Spectrum_Length): this command changes the number of channels and sub spectra of the actual spectrum. You can select values from the list in the LENGTH field of the input (i.e. 1024, 2048, 4096…) or enter an optimal value (in our case, 2048).3. Setup the hardware (Hardware_Setup) :

a) High Voltage: type the desired voltage value (in our case 450 V; WARNING: do not exceed 500 V) in the text field, then confirm the input by pressing the ENTER button.

b) ULD…:

i. ULD (upper level discriminator): defines the upper level of the signal acquisition. The settings 0 to 255 don’t belong to a channel directly, but 0 relates to the lowest channel and 255 to the topchannel. (set ULD to 255)


WinTMCA32: InitializationWinTMCA32: Initialization

ii. LLD (lower level discriminator): discriminates the lowest level of signal acquisition. The settings 0 to 255 are not directly related to to the channels. In order not to discriminate a channel, Set LLD to 0.

iii.Conversion Gain: defines the number of channels for spectra acquisition, thus the relation of channels and voltage. The value varies from 0 to 255. Set Conv. Gain to 0 (this corresponds to a spectrum with 2048 channels).

iv. Noise: defines which of the signals registered are events and which are noise. Only events which give a higher voltage value than the noise value will be registered. In principle Noise works like LLD but events beyond the Noise-level are not considered for the dead time and base line calculations. Set Noise to 0.


WinTMCA32: InitializationWinTMCA32: Initialization

c) Gain: Sets up the coarse gain for the acquisition hardware. Set the coarse gain to 20


WinTMCA32: WinTMCA32: Spectrum Spectrum acquisitionacquisition

Starts spectrum acquisition

Stops spectrum acquisition

Erases the actual spectrum

Defines spectrum length (2048)

Opens the hardware setup menu

Integrates the marked area within the actual spectrum

Opens a dialog to select spectra files

Save the spectrum

Show the preset menu

Real time Status: STP =acquisition is stopped ACQ=acquisition is active

Select window


Energy calibrationEnergy calibrationThe energy calibration of a spectrum has to be realized with calibration sources.

As example we could used a Marinelli calibration source, containing a mixture of nuclides.Nuclide Gamma-ray

energy (keV)Activity @ 1 february

2000 (Bq)

Emission rate (s-1)

Caesium-137 662 3.18E+03 2.70E+03

Cobalt-60 1173 3.49E+03 3.48E+03

Cobalt-60 1333 3.49E+03 3.49E+03


Energy calibration procedureEnergy calibration procedure1. Select Spectrum, ROIs, New.

2. Define Left Margin and Right Margin using the red marker.

3. Press Set button and close the window.

4. With the red marker inside the ROI, use the command Compute, Integrate to determine the Centroid of the peak.

5. Repeat the procedure for all the available peaks.

6. Select Spectrum, Energy calibration and insert the data as showed below:

Nuclide Gamma-ray energy (keV)

Centroid

Caesium-137 661.8 738.4

Cobalt-60 1173 1252

Cobalt-60 1333 1412


Energy calibration resultEnergy calibration result

Remember to write down in another file the Remember to write down in another file the energy calibration table, the system cannot save energy calibration table, the system cannot save

it separately. it separately.


Sample analysisSample analysis

Acquire the spectrum from your sample using the same set up used for the calibration.

- Load the energy calibration.

- Select Spectrum, ROIs, New.

- Define Left Margin and Right Margin using the red marker.

- Press Set button and close the window.

- With the red marker inside the ROI, use the command Calculus, Integrate to determine:Peak energy, Net area, CPS


Sample analysisSample analysis

Nuclide search:

- Load the nuclide database

- DBEdit: Database_Open: Nuclide.db (Nota: the nuclide database is in in the Folder target/winTMCA32 english/NUCDATA

- Use the search command to search by photopeak energy the nuclide (half life and secondary peak emission can help you !)


Sample analysisSample analysisEdit your nuclide database:

- Create your DB (Dbedit ->new)

Import the desired nuclide (Dbedit ->import)

Activate the Check field of your DB


Sample analysisSample analysisEdit your nuclide database:

- Load the nuclide database (C:\Programmi\target\winTMCA32 english\Dbedit.exe ->new)

If the peak centroid lies within the range of a peak energy of one of the nuclides in the selected nuclide data base, the name of the nuclide will be displayed. Therefore it is required that the CHECK field of this line in nuclide database is activated.

The Bq-field shows the activity calculated from the net count rate; the efficiency factor of the nuclide database line was taken into consideration.


Sample analysisSample analysis-From the program WinTMCA32, load your nuclide DB in the spectrum (Program->select nuclide database)

- Define ROIs.

-With the red marker inside the ROI, use the command Compute, Integrate. The program will recognize the matching DB nuclide and will give you the Activity.

PAY ATTENTION THE ACTIVITY HAS TO BE NORMALIZED USING THE EFFICIENCY OF THE DETECTOR.Activity (Bq) =

where: Br = Branching ratio

= efficiency

T = live time

TεBr

AreaNet

Since the efficiency reported in nuclide database is 100%, the activity value provided by software needs to be corrected using the efficiency calibration curve of your detector:

Ln(’ )= -4.4364 + 1.436452 Ln(Energy) – 0.21634 ((Ln(Energy))2

Corrected activity = Activity/ ’


Samples analysisSamples analysisActivity (Bq) =

where: Br = Branching ratio

= efficiency

T = live time

TεBr

AreaNet

Since the efficiency reported in nuclide database is 1, the activity value provided by software needs to be corrected using an efficiency calibration curve:


Corrected activity = Activity/ ’


Efficiency calibrationEfficiency calibrationThe efficiency calibration of NaI detector has to be realized with calibration sources.

As example we could used Marinelli calibration source constituted by radionuclidic mixture.Nuclid

eGamma-

ray energy (keV)

Activity @

1/02/2000 (Bq)

Activity @ present

(Bq)

Net Area ’

Caesium-137

662 3.18E+03

2.5331E+03

2250151 36.7671732

1.4515E-02

Cobalt-60

1173 3.49E+03

9.534E+02

421944 5.869137039

6.1560E-03

Cobalt-60

1333 3.49E+03

9.534E+02

342489 4.757619492

4.9902E-03

TBr

AreaNet

Activity (Bq) = T=72000 s

efficiency = ‘=

efficiency calibration curve:

Tε'Br

AreaNet

TActivityBr

AreaNet



ConclusionsConclusions

Give a brief report of your experiment comparing the activity form your sample with the ones of a background spectrum.

In which way you can assess the gamma absorbed dose rate?

The mass of the detector is 0.378 kg

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