tl gamma spectroscopy

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Gamma spectroscopy

KHVASTUNOV Illia & RASPOPOV Sergii

October 27, 2014

Abstract

The aim of this paper is to present the result ofstudent laboratory work on gamma spectroscopy ofunknown samples. The -spectra of natural back-ground radiation, uranium glass collectable, radioac-tive polluted soil and monazite sand were charac-

terized. For this purpose high purity germanium(HPGe) detector was employed. In order to reducethe data contamination in the signal region by Comp-ton suppression, anti-coincidence arrangement usingbismuth germanate (BGO) scintillator was devised.

1 Introduction

Gamma spectroscopy is very powerful method to de-termine nuclei that is widely used in nuclear industry,geochemical investigation and astrophysics. Most ra-dioactive nuclei produce -rays, which has different

energies and intensities. Collecting this -rays withdifferent detectors (scintillators, semiconductors, etc.)and analyzing the -rays energy spectra, radioactivenuclei could be certainly identified.

Radioactive nuclei in general emit-rays in the en-ergy range from a few keV to few MeV, correspondingto the typical energy levels in nuclei with reasonablelong lifetimes. The three main effects of photons in-teraction with matter are photoelectric effect, Comp-ton scattering and pair production. Just in case offull -ray thermalization inside detector volume itstotal energy could be identified. When-ray inter-act via Compton effect and escape from detector vol-ume or in case ofe+e pair creation and escaping oneor two-rays, according to possible electron-positronannihilation, one could not determine total energy ofincident -ray. Compton scattering inside detectorbulk causes noise in energy spectrum and disturbpeak energy measurements. To reduce signal fromCompton scattering-ray in the detector bulk an anti-coincidence technique can be used. The cross-sectionof photoelectric effect depends on charge number ofnucleus asZ5, Compton effect doesnt depend on it,and pair production depends linear (if one consideratom field then it depends as Z2). For a good-ray

detectionZnumber of the detector material shouldbe high enough.

For gamma spectroscopy one of the main criteria ishigh resolution of the detector. It means that neigh-bor lines of nucleus should be distinguishable inenergy spectrum. Different characteristics of the de-tectors, that was proposed for-ray spectroscopy, aredescribed in chapter 2. Experimental setup of themeasurements and Compton suppression technique

are described in chapter 3. It was obtained differ-ent spectra from background, Uranium glass andtwo samples (radioactive soil and sand). Detailed de-scription, explanation and discussion of the obtainedresults are negotiated in chapter 5.

2 Semiconductor and scintillator

detectors

The detectors that were proposed for spectroscopywere big and small NaI(Tl) detectors, BaF2, Oil scintil-lator (with Gadolinium), planar and 2 coaxial HPGedetectors and BGO (Bi4Ge3O12) scintillator.

The main comparable characteristics of the detec-tors are shown in Tab.1.

Rise time, ns Decay time,s Zeff , g cm3

BaF2 5 0.04 54 4.89HPGe 350 100 -Oil 10 0.03 -NaI(Tl) 250 2 50 3.67BGO 2000 - 74 7.1

Table 1: Main characteristics of the proposed detec-tors.

The activity of proposed sources dont exceed300 kBq. This means that all proposed detectorswere too fast to collect data from all from proposedsources.

When a gamma ray passes through matter, theprobability for absorption is proportional to the thick-ness of the layer, the density of the material, and the

absorption cross section of the material. The total ab-sorption shows an exponential decrease of intensity

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with distance from the incident surface:

I(x) =I0ex , (1)

where x is the distance from the incident surface, =nis the absorption coefficient, n - number of atoms

per cm3

, - absorption cross section. So for effectivedetection the material of the detector should havehigh density and Z number.

Due to different band gap between valence andconductive level, different structure of the material(organic and inorganic scintillators), methods of theproduction scintillation light the energy for creationinformation carriers (electron-h pairs and visible pho-tons) are different. For HPGe detector it needs ap-proximately 3 eV to create one electron-hole pair, thelight yield of BaF2 is about 12 photons per keV, forNaI scintillator - 38 photons per 1 keV, BGO - 32 pho-

tons per keV. Thus, the less energy it needs to create1 information carrier, less energy distribution of thedetector will be gained.

3 Experimental set-up

In the present work the Canberra High Purity Germa-nium (HPGe) Detector has been employed. It consistsof the planar and two coaxial germanium detectors.These detectors provide a very good energy resolu-tion as well as can be calibrated up to very high en-ergy using appropriate-ray sources.

Figure 1: Scheme of the set-up: connection of HPGe

detector and BGO scintillator.

When photons interact with the material within thedetector, charge carriers (holes and electrons) are pro-duced; total collected charge, which is in proportionto the energy, deposited in the detector by the incom-ing photon, is converted into a voltage pulse by an in-tegral charge-sensitive preamplifier. The signal frompreamplifier goes to the amplifier, which shape thesignal to a convenient form for further processing.Analog-to-Digital Converter (ADC) converts the ana-log signal from amplifier to equivalent digital form:

takes the incoming continuous signal and assigns adigital number that represents the amplitude of the

signal. From ADC the signal goes to MultichannelAnalyzer (MCA) (see Fig. 1), which sorts out incom-ing pulse and keeps count of the number at eachheight in a multichannel memory. The contents ofeach channel can then be displayed on a screen to givethe pulse height spectrum. The MCA is connectedto a computer which shows the energy spectrum ofthe -rays. MCA converts the signal into channels.So the obtained spectrum shows counts versus chan-nels. The spectrum is then calibrated by linear cal-ibration (e.g. using 60Co source) or multi-point cal-ibration (e.g. using 152Eu) to get the counts versusenergy spectrum. This energy spectrum is then an-alyzed to find the resolution and efficiency for eachenergy peak in the spectrum.

The main source of background for the gammaspectroscopy is the radioactivity of the surroundingmaterials. When photons hit the HPGe detector they

undergo Compton scattering and with high probabil-ity deposit only part of their energy before escapingthe detector; the escaped s deposit their energy inthe nearby environment. If the surrounding space isfilled with scintillating material, the escaped s canbe detected and the events with incomplete energydeposition in the HPGe detector can be vetoed. Suchan arrangement is called aCompton suppression[1] be-cause it strongly rejects the Compton spectrum be-tween thelines in the energy spectrum (see Fig. 2).

Figure 2: Qualitative comparison of Compton sup-

pressed with respect to the intact 152Eu-spectrum.

For this purpose in our laboratory work the Bis-muth Germanate (BGO) scintillator was chosen be-cause of its high detection efficiency and high -rayabsorption due to the high atomic number of bismuth(Z= 83) and its high density (= 7.13g cm3).

A veto time gate is defined to accept only eventwith a certain hit pattern via available NIM signal (seeFig. 1). Varying delay and duration of the NIM signal,the necessary values were found in order to suppressall simultaneous events in HPGe and BGO, which areresponsible for Compton scattering.

The optimal position of radioactive source wasfound on axis of the detector and on certain distancefrom the inlet as soon as the efficiency of registrationstrongly depends on geometry of the source and itsposition towards the detector.

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4 Analysis description

All the data that was got from MCA board is numberevents per channel. As 60Co and 152Eu were very in-tensive (intensity much bigger than background) andhave 2 and 14 lines respectively it was used for energyand efficiency calibration. Arbitrary peak in channelspectrum was fitted with Gaussian function for a sig-nal and linear polynomial for a background. Fit wasdone with Minuit software that minimize2 function.As errors inx and y coordinates it was taken half ofthe bin size and square root of the events number inbin respectively.

As it was expected the energy calibration has lineardependence from channel number. The obtained2

was enormous big that could be explained very littleerrors in channel number. That means that channelnumber error was overestimated. To decrease nor-

malized2

the number of bins in spectrum were de-creased by a factor 9 or 11. In that case 1 bin approx-imately correspond to 1 keV. This value is compara-ble with energy resolution of the HPGe detector. Incase of bins decreasing the2 was decreased but stillleft big. To know that errors dont have systemati-cal component the subtraction of experimental pointsfrom obtained fit function was built. As it also wasexpected, the efficiency calibration has decreasing lin-ear dependency in twice logarithmic scale. The ob-tained 2 4 provides the information that the errorsfor channel number and events number are overesti-

mated even in case of decreasing number of bins.

5 Results

The background spectrum (with and without sur-rounding lead shielding) was measured with andwithout Compton suppression technique. Besides, itwas proposed to measure following unknown sam-ples: Uranium glass, soil from Chernobyl region andmonazite sand. These spectra were obtained with us-age of Compton suppression. Thus it was achieved60% reduction of events, that are not belong to a sig-

nal, while the signal was not suppressed.To reduce errors in the samples spectra with sub-

tracted background spectrum, the following methodwas proposed. Both spectra were scaled to the sametime of the measurement. Then spectra were scaled tothe same potassium line (1460 keV) total events num-ber. After all background spectrum was subtractedfrom the samples and obtained spectrum was con-sidered as a final result.

It was agreed that in order to improve the Comptonsuppression it could be more effectively to use LiquidArgon as external detector for HPGe detector in per-

spective instead of the BGO scintillator, which size islimited.

5.1 Background

One of the most heavy nobel gas that is occuring asan indirect decay product of uranium or thorium isRadon. The most stable isotope is 222Rn. 222Rn is anelement of 238U decay chain. So thats why in back-

ground it could be possible to seelines of214Bi - 609keV, 727 keV, 934 keV, 1119 keV, 1238 keV, 1764 keV,2118 keV and 2204 keV.

When HPGe was operated additional lead shield-ing was used. The main component of cosmic raysis cosmic proton. When protons interact with leadshielding the(p, n)reaction occurs. That means, thatnuclei of 207Bi and 208Bi are producing. These nu-clei arent stable, so they decay with ray emission.Thats why it could be possible to see lines - 569 keV,1770 keV, 2614 keV. Unfortunately it wasnt measuredthe spectrum without additional lead shielding (ring,

which is put on detector) to approve or refute linesfrom 207Bi and 208Bi.

In human body there are a lot of potassium K, andone of its isotopes 40K (0.0117%) has big lifetime -1.25109 years and decays with 1460 keV line emis-sion.

The last component of the background is the impu-rities of Uranium and Thorium in lead blocks. 238U,235U and 232Th have big lifetimes, so their decayproducts accumulate inside lead blocks. Thats whyits possible to see lines - 63 keV, 92 keV, 186 keV,911 keV, 1630 keV, etc.

All these lines could be seen in Fig.3. It was foundand explained every peak, and using efficiency cal-ibration it was confirmed that the lines, that wasemitted from the same nucleus, belong to the samenucleus indeed.

Figure 3: Background spectrum.

5.2 Uranium glass

First proposed unknown sample was a uranium glasscollectable. The characteristic green-yellow hue givenby Uranium glass was observed.

There are two main isotopes of the Uranium - 235Uand 238U. The fraction of the 235U and 238U in naturalUranium is 0.7% and 99.2%. It well-known that 235Uis used for nuclear bombs and reactors, so such kindof toys were done from depleted uranium - uranium,which 235U fraction is less than 0.3%. The task wasto find out the ration of 235U and 238U in collectableglass.

This analysis was done with234

Th (which was pro-duced from 238U)lines - 63 and 92 keV and 235U

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Figure 4: Subtraction of background from the U glassspectrum.

line - 186 keV (See.Fig.4). The ratio of 235U and 238Ucould be found via:

N0(235U)

N0(238U) =

T1/2(235U)

T1/2(238U)

NUev/IU

NThev /ITh

, (2)

whereI- intensity of the line. The derived fractionof 235U and 238U - 0.0236 0.0016% and 99.9764 0.0016%respectively.

The activities of 63 keV line of 238U is 86 2 Bqand 92 keV - 214 4Bq. Thus it was found that foruranium glass light emission the fraction of the glass

should be0.35 2%. That means that for 0.35% frac-tion the activity of 238U should be 2 kBq, that is 10times more than expected. Maybe the color of the dollcould give some pigment.

5.3 Soil from the Chernobyl region

In the spectrum of soil from the Chernobyl region wasfound only line from 137Cs - 662 keV. The obtainedactivity of the soil is 38 2 Bq, that corresponds to6.80.3 Ci km2, that is supposed correspond to Kyivregion (50-100 km from Chernobyl).

The main uncertainty that gives the problem formeasuring activity of137Cs is methodology of soil ac-tivity measurement [2]. In article the Geiger counterwas taken to measure the activity of the soil. I thatcase its not clear how much the soil was taken. Itwas better to take some amount of soil, weigh it andthen burn. The residue put on some substrate andthen measure the activity with Geiger counter.

5.4 Monazite sand

The last unknown sample was monazite sand that has

red-brown hue. Cerium monazite contains Thorium,which has long decay chain. In spectrum it was seenlines from 228Ac, 212Bi, 212Pb and 208Tl - 89 keV, 92keV, 238 keV, 338 keV, 582 keV, etc. (See.Fig.5)

Figure 5: Subtraction of background from the mon-azite sand spectrum.

For monazite sand spectrum as it was expected themost activelines were from the nuclei that has the

smallest T12 , because activity in general is inverselyproportional to half-life of nucleus.

References

[1] Canberra Industries Inc., Germanium Detectors.Users Manual(2003)

[2] Pilot investigation of food products contamina-

tion by 137

Cs in selected areas of Ukraine affectedby the Chernobyl catastrophe in 1986, April 2011

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