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Journal of Environmental Radioactivity 113 (2012) 77e82

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate/ jenvrad

Recent gamma background measurements at high mountain altitude

A.L. Mishev*, E. HristovaInstitute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria

a r t i c l e i n f o

Article history:Received 9 May 2011Received in revised form6 April 2012Accepted 18 April 2012Available online 30 May 2012

Keywords:Gamma backgroundRadiation measurementsCosmic rayMonte Carlo

* Corresponding author. Tel.: þ359 2 9746310; fax:E-mail addresses: alex_mishev@yahoo.com, mishe

0265-931X/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.jenvrad.2012.04.017

a b s t r a c t

Results from recent measurements of radiation gamma background at high mountain altitude, namely atBasic Environmental Observatory Moussala (42.11 N, 23.35 E, 2925 m a.s.l.) are reported. Themeasurements are fulfilled with several devices, namely IGS-421 gamma probe and MDU Liulin. Acomparative analysis with previous measurements performed with SBN-90 SAPHYMO NaI(Tl) gammaprobe is carried out. A temperature effect during winter period of SAPHYMO probe is observed. Inaddition the measurements are compared with CaSO4:Dy TLD. The obtained results are widely discussed.A numerical model for galactic cosmic ray contribution to the dose rate in air is presented. The model isbased on a full Monte Carlo simulation of cosmic ray induced cascade in the atmosphere. The simulationis carried out with CORSIKA 6.52 code using FLUKA 2006b and QGSJET II hadron interaction models.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The radioactive elements and their radiation are integral part ofthe environment. Their influence on live organisms is imminentand very important to study. The natural radioactivity is duemainlyto cosmic radiation and the availability of natural radio-nuclides(terrestrial sources) in the environmental components (Eisenbudand Gesell, 1997; United Nations Scientific Committee on theEffects of Atomic Radiation, 2008). The cosmic ray intensity,respectively their contribution to the natural radioactivity dependon geographic position (geomagnetic and rigidity cut-off effect),altitude above sea level and solar activity. The main contribution toexternal exposure is due to gamma emitting radio-nuclides, whichare present in the soil such as 40K, 238U and 232Th families.

A significant contribution to the exposure comes from the radongas and its radioactive progeny (United Nations ScientificCommittee on the Effects of Atomic Radiation, 2008). It isreleased from the Earth’s crust and subsequently decays intoradioactive atoms that become attached to airborne dust, aerosolsand other particulates. Another contribution arises from theradioactive atoms produced in the upper atmosphere by high-energy cosmic rays. About 3% of the background radiation comesfrom other man-made sources. The level of natural backgroundradiation depends on location over the world. In some areas thelevel is significantly higher than the average (Hendry et al., 2009).

þ359 2 9753619.v@inrne.bas.bg (A.L. Mishev).

All rights reserved.

The high mountain stations are privileged places for environ-mental studies and monitoring. Their advantage is the smallanthropogenic influence. The Basic Environmental Observatory(BEO)Moussala is located on the top of the highestmountain on theBalkan Peninsula, namely at 2925 m above sea level (Fig. 1). It isa clean region with small anthropogenic influence. It gives goodpossibility for environmental monitoring (Angelov et al., 2007;Stamenov, 2007) and spaceweather studies (Mishev and Stamenov,2008; Mishev et al., 2009; Mishev, 2010). The existing devices atBEO Moussala permit to study changes and processes in theatmosphere with good precision. The gamma background moni-toring is a significant part of BEOMoussala radiationmeasurements(Gelev et al., 2007; Masson et al., 2011).

2. Instrumentation and methods

After the Chernobyl nuclear reactor accident in 1986, themajority of the countries of the European Union establishednetworks for outdoor dose rate monitoring. The general aimwas toprovide a system for early warning. Basically the measured data arecomposite values: gamma-dose rate due to terrestrial, cosmic andartificial radiation sources. In most cases the data include someinstrument background. The large data sets potentially containvaluable information about spatio-temporal variations, termwhichcould be used for tracer applications and validation of atmospherictransport models. Several devices for radiation gamma background(RGB) measurements are operational at BEO Moussala. Theyprovide continuous recordings of RGB in a specific climate condi-tions at high mountain, specifically during the winter period.

Fig. 1. Basic Environmental Observatory (BEO) Moussala.

A.L. Mishev, E. Hristova / Journal of Environmental Radioactivity 113 (2012) 77e8278

2.1. SBN-90 SAPHYMO gamma probe based on NaI scintillator

Awell knownmethod for gamma-ray detection involves the useof crystal scintillators such as NaI(Tl). When it is used as a gamma-ray detector, the scintillator does not directly detect the gamma-rays. They produce charged particles in the scintillator crystals,which interact with the crystal and emit photons. These low energyphotons are subsequently collected by a photomultiplier. Histori-cally the first device installed at BEO Moussala was SBN-90SAPHYMO gamma probe based on NaI(Tl) scintillator (Fig. 2). Itwas granted by the French electrical company - Electricite de France(EDF). The energy range of the probe is between 50 keV and 3 MeV.It gives the possibility to measure dose rates in the range of40e100,000 nGy h�1. The device was upgraded withmicrocontroller PIC16C77 and new data acquisition system (Mitevand Lenev, 2000). A network of four devices, distributed in Southof Bulgaria was established.

The transformation of ionizing energy into an electrical signal isvery complicated process and causes a complex temperaturedependence of the scintillator light output (Birks, 1964; Knoll,1999). The redistribution of the light intensity is temperaturedependent. It is significant at temperatures around �20 �C

Fig. 2. SBN-90 gamma probe based on NaI(Tl) scintillator at BEO Moussala.

(Ianakiev et al., 2007, 2009). The temperature dependence of lightoutput of SBN-90 gamma probe at Moussala is very importantduring the winter period (low temperature, high humidity, strongwinds) (Mihnevski, 1993). The effect was observed during the 5years exploitation period of the device, till the end of 2005 (see thesection below).

2.2. IGS-421 gamma probe

The IGS-421 gamma probe is developed by German companyTECHNIDATA. The recently established network for permanentgamma background monitoring at BEO Moussala - INRNE-BAS isbased on IGS-421. The probe consists of two low-energy and onehigh-energy GeigereMuller tubes. The sensitivity range of thedevice is 10 nGy/h-10 Gy/h, with accuracy of 15% respecting to137Cs. The technical characteristics are as follows: The operatingtemperature is between þ60 �C and �40 �C. The latter is veryimportant taking into account the negative annual temperature atthe top of Moussala and low temperatures during the winterperiod. The probe interface is RS-232, which permits up to 15 mdirect connection with a PC. The probe is mounted outside on themain bridge of the station (Fig. 3). Several important technicalcharacteristics of the probe are presented in Table 1. The probe isoperational since September 2006 (Mishev et al., 2007).

2.3. Liulin spectrometer

The mobile dosimetry unit (MDU) Liulin is originally developedfor onboard spacecraft studies. It has been used as high precisiondevice for near Earth radiation measurements (Spurny and Dachev,2003; Dachev, 2009). The equipment has been acquired for Alomarobservatory in the North of Norway and Jungfraujoch high moun-tain observatory in Switzerland. It has been used for radiationmeasurements at high mountain altitude (Spurny and Langer,2006). The MDU Liulin demonstrated high capabilities fordifferent problems, specifically measurements at flight altitudesand registration of solar energetic particles (Spurny and Dachev,2001; Spurny et al., 2002, 2005). Quite recently it was installed atBEO Moussala (Gelev et al., 2007; Spurny et al., 2007).

TheMDU-Liulin is based on a Si-semiconductor diode. The diodeis situated at the head of the MDU unit. The dimensions of the unitare 10� 20 mm2 with thickness of 0.3 mm. It is covered by 1 mm ofAl and about 0.5 mm of copper. There is an air-gap between Si-

Fig. 3. IGS-421 probe mounted at the bridge of BEO Moussala.

Table 1Main characteristics of low energy GM tube and high energy GM tube of IGS-421.

Probe Range Sensitivity Detector background

Low-energy 10 nGy/he10 mGy/h 1976 counts/min 38 counts/minHigh-energy 0.1 mGy/he10 Gy/h 1.24 counts/min No

Fig. 4. Monthly average of gamma background and air temperature at BEO Moussalameasured with SAPHYMO for 2000 and 2001.

A.L. Mishev, E. Hristova / Journal of Environmental Radioactivity 113 (2012) 77e82 79

diode and Al-cover of about 5 mm thick. The equipment monitorssimultaneously the doses and numbers of energy deposition eventsin Si-diode. The amplitude of the pulses is proportional by a factorof 240mV/MeV of the energy loss (deposited) in Si. The adjustmentof the energy scale is carried out through 60 keV photons of 241Am.The amplitudes are digitized and organized in a 256-channelspectrum. The dose D, in Gy in Si diode is calculated from thespectrum as:

D ¼ K*X

ðEi�AiÞ=Md (1)

whereMd is the mass of the detector, in kg; Ei is the energy loss in J,in the channel i; Ai is the number of events in this channel and K isa coefficient.

The MDU unit at BEO Moussala operates using an externalpower supply and direct PC connection.

3. Measurements of gamma background at BEO Moussala

At present the gamma background at BEOMoussala is measuredby IGS-421 gamma probe and Liulin MDU as well as with TLDdetectors. In this section are presented the data obtained during thefirst year of exploitation of the devices.

3.1. Previous measurements

The first measurements of gamma background at BEO Moussalaare carried out with SBN-90 SAPHYMO gamma probe. The lowestmeasured dose rate is 87.50 nSv/h (May 2003). The maximalmeasured dose rate is 128.10 nSv/h (July 2004). The average is118.0 nSv/h with corresponding standard deviation of 7.5 nSv/h(Table 2). In Fig. 4 are presented the monthly average gammabackground for 2000 and 2001 year (solid squares). A floating of themeasured values is observed during rain flows. The dose rateincreases in months with intensive rains. The rains collect aerosolsfrom atmosphere and transport them to the ground-level.

As was mentioned above, the total measured light yield ofNaI(Tl)detectors is a nonlinear function of the ambient tempera-ture. The effect is significant around - 20 �C (Tsankov and Mitev,2006). Significant part of the monthly average decrease duringthe winter period is due to low temperatures (Fig. 4 open circles).The detailed study of the temperature effect is beyond the scope ofthis paper and it is a topic of further investigations. However thepreliminary data analysis of the measurements confirmed thetemperature effect.

Table 2Mean, median, minimal, andmaximal values of themeasured gamma background atBEO Moussala with SBN-90 SAPHYMO, IGS-421 gamma probe, Liulin spectrometerand CaSO4:Dy TLD.a

Device Mean[nGy/h]

Standard deviation Median[nGy/h]

Min[nGy/h]

Max[nGy/h]

SBN-90 118 7.5 119 97.5 128.1IGS-421 144 5.19 145 128 169MDULiulin 189 33.6 186 98 383TLD 150 20

a The dose rate measured with CaSO4:Dy TLD is in [Sv].

The observed temperature effect is one of the reasons of the notproper and stable work of the device, which lead to his replacementby IGS-421 gamma probe.

3.2. Recent results

Presently the gamma background measurements at BEOMoussala are carried out with several devices. These devices arebased on different type of detectors and have different accuracy andsensitivity. Since September 2006, the described above IGS-421 andMDU Liulin are both operational at BEO Moussala.

The average of gamma background with the correspondingstandard deviations for December 2006 obtained with IGS-421 ispresented in Fig. 5. The monthly mean value is 144 nGy/h withstandard deviation of 5.19 nGy/h. The maximal measured value is169 nGy/h, respectively the minimal is 128 nGy/h. Additionalanalysis of the symmetry of the distribution is carried out. Themedian value is 145 nGy/h. The data are obtained on the basis of10 min integration of the gamma background. The data aresummarized in Table 2.

Fig. 5. Average gamma background at BEO Moussala measured with IGS-421 forDecember 2006.

Fig. 6. Comparison of average gamma background at BEO Moussala measured withLiulin and IGS-421 for December 2006.

A.L. Mishev, E. Hristova / Journal of Environmental Radioactivity 113 (2012) 77e8280

An improvement of the gamma probe software is carried out.The improvement permits the automatic setting-up and remotecontrol of the probe. As a result in December 2006 (see Fig. 5) thenon operational cycle of the probe is reduced to less then 5% (ina previous months it was in the order of 20%). The non operation isdue essentially to power supply problems on the top duringintensive thunderstorms. The recently developed telecommunica-tion system (Ivanov et al., 2002) permit to present the locally storeddata in a real time on BEO Moussala webpage (www.beo-db.inrne.bas.bg).

The gamma background at BEO Moussala is estimated also withMDU Liulin. A comparison between the measured daily average ofgamma backgroundwith IGS-421 gamma probe andMDU Liulin forDecember 2006 is carried out. The results are presented in Fig. 6.The monthly average gamma background at BEO Moussalameasured with Liulin and IGS-421 gamma probe is shown in Fig. 7.It is necessary to point out that the MDU Liulin is located inside thestation, while the IGS-421 probe is outside. Additionally similaranalysis of the measured data is made. The results are described inTable 2. The measurements carried out with Liulin spectrometersystematically have larger standard deviations. This is due to

Fig. 7. Monthly average gamma background at BEO Moussala measured with Liulinand IGS-421 gamma probe.

a smaller detector area of MDU Liulin. Moreover the measured withLiulin dose rates are around 20% larger then measured with IGS-421.

An additional comparison with CaSO4:Dy TLD measurements iscarried out. The design of TLD, with corresponding Monte Carlosimulations is presented in Isabey et al. (1997). The measured doserate with TLDs is 150 nSv/h with standard deviation of 20 nSv/h.This is in agreement with other measurements. All data aresummarized in Table 2.

Generally the described active devices, in addition with TLDdemonstrate proper work and can be used as a reference point ina Bulgarian national system for permanent gamma monitoring.The IGS-421 measurements are transmitted permanently toEURODEP.

4. The contribution of galactic cosmic ray to dose rate

An important contribution to radiation environment at highmountain altitudes represents cosmic radiation component(Spurny et al., 2008). The Earth is constantly bombarded by flux ofsubatomic particles, radiation from outside of solar system-thecosmic ray. Primary cosmic ray particles impinge the Earth atmo-sphere and release energy via nuclear interaction and ionizationlosses. Low-energy particles from cosmic ray are absorbed in theatmosphere, while those with energies above GeV generate newparticles through interactions with air nuclei. They initiate nuclear-electromagnetic-muon cascades in the atmosphere. The dose fromcosmic ray is mainly due to muons, neutrons, and electrons. Thedose rate varies with geomagnetic field, respectively geographicposition, altitude, and solar activity. It is possible to estimate thespectra of secondary particles resulting from interactions ofprimary galactic cosmic rays with atmospheric nuclei and subse-quently obtain the effective dose rate as a function of geomagneticcut-off and altitude using full Monte Carlo simulation of theatmospheric cascade (Ferrari et al., 2001; Roesler et al., 2002).

The evolution of atmospheric cascade is carried out with COR-SIKA 6.52 (Heck et al., 1998) code with corresponding hadroninteractionmodels FLUKA 2006b (Fasso et al., 2005; Battistoni et al.,2006) and QGSJET II (Ostapchenko, 2006). COsmic Ray SImulationsfor KASKADE (CORSIKA) code is one of the most widely used in thelast years atmospheric cascade simulation tool. It is a Monte Carlosimulation tool for detailed study of cascade evolution in theatmosphere. The code simulates the interactions and decays ofnuclei, hadrons, muons, electrons and photons in the atmosphereup to extreme energies. The result of the simulations is detailedinformation about the type, energy, direction, location and arrivaltime of the produced secondary particles at given selected obser-vation level. Moreover it is possible to obtain the energy deposit bydifferent shower components and particles at given observationlevels.

We simulate 50 000 events up to 85� of zenith angle, distributedisotropically, following steep spectrumwith slope of 2.7 (Nakamuraet al., 2010).

The dose rate in Gy, produced in 1 g of the ambient air at a givenatmospheric depth by one particle of the primary cosmic ray withgiven kinetic energy per nucleon is determined according expres-sion (2).

Dðh; lmÞ ¼Z N

E

Z p=2

0

Z 2p

0SðEÞOEðh; EÞ

OhsinðqÞdEdqd4 (2)

where DE is the deposited energy in an atmospheric layer Dh, S(E) isthe differential cosmic ray spectrum, lm is a geomagnetic latitude, Eis the initial energy of the incoming primary nuclei at the top of theatmosphere. The geomagnetic latitude lm governs the rigidity,

Fig. 8. Dose rate due to galactic cosmic rays as a function of the altitude above sealevel.

A.L. Mishev, E. Hristova / Journal of Environmental Radioactivity 113 (2012) 77e82 81

which is related to integration (integration above E) and is con-nected with the geographic position.

The estimated dose rate distribution due to cosmic ray asa function of the altitude above sea level is presented in Fig. 8.

The contribution of cosmic ray to dose rate in air at highmountain altitude is estimated to be of the order of 10e12 % (asexample at Moussala is roughly 11%, see Tables 2and 3). The modeland measured radiation background at Moussala are in a fullagreement with other simulations (Friedberg et al., 2005),measurements (Regulla and David, 1993; Schrewe, 1999; O’Sullivanet al., 2002) as well as bibliographic data (United Nations ScientificCommittee on the Effects of Atomic Radiation, 2008).

In this numeric model the detector response of different probesto secondary cosmic rays is not considered. It is a topic of furtherstudy beyond the scope of this paper.

5. Summary and discussion

The radiation gamma background at BEO Moussala is measuredwith several different devices. At the same time on the top theautomatic meteo-station gives permanent information about windvelocity and direction, temperature, pressure and amount ofprecipitation. This is very important, because an increase of RGBwas observed in rainy months. As example in Fig. 7 one observesthis fact for spring - summer period for 2007, specifically for IGS-421 measurements. This fact is not observed with MDU Liulin,because it is located inside the station. Similar fluctuations areobserved for daily averages.

The detected temperature effect, specifically during the winterperiod of a NaI(Tl) SBN-90 SAPHYMO gamma probe led to a non

Table 3Dose rate due to GCR as a function of altitude above sea level estimated withnumerical model.

Depth g cm�2 Altitude [m a.s.l.] Dose rate [nGy/h]

813 1980 12.59925 2920 16.27552 5000 39.2365 8000 128272 10,000 249195 12,100 388125 15,000 50975 18,200 52955 20,100 495

stable work of the device. This motivated the replacement of theprobe with a new IGS-421 gamma probe.

The IGS-421 gamma probe demonstrates good stability. Hencethis is the reason to use such type of device for a local network forgamma background monitoring, specifically at high mountainaltitude. The data for gamma background measured with IGS-421are presented on-line on the webpage of BEO Moussala. In addi-tion they are transmitted to EURODEP and national system forpermanent monitoring of gamma background in Bulgaria.

A big challenge is the investigations of the contribution ofcosmic radiation on aircrew exposure (Reitz, 1993). The variety ofprimary and secondary ionising particles, their wide energy rangereflects on aviation routes conditions, respectively exposure vari-ation (Spurny et al., 1996). On the basis of a full Monte Carlosimulation of atmospheric cascade by cosmic rays is estimated thecontribution of galactic cosmic rays to average of dose rate at highmountain latitude. The proposed formalism could be applied forvarious altitudes and geographical regions. This 3-D model isa contribution to the recentMonte Carlo studies related to radiationmeasurements (Makovicka et al., 2009) and could be applied atflight altitudes during large solar particle events (O’Brien et al.,1997).

At present the contribution of secondary cosmic rays to themeasured RGB at BEO Moussala is not subtracted, nor the detectorresponse to secondary cosmic rays. The experimental estimation ofcosmic ray contribution to the dose rate in air at high mountainaltitude is a topic of further works (Mishev, 2006) as well as theprecise estimation of various secondary components.

Acknowledgements

The authors are grateful to the colleagues from BEO Moussala,specifically electronics engineer Tc. Lenev, software engineer K.Davidkov and Assoc. Prof. S. Ushev. The authors acknowledge Assoc.Prof. L. Tsankov from physics department of Sofia University “St. Kl.Ohridski” for discussions related to temperature effect of NaIdetectors. This work is dedicated to the memory of Prof. F. Spurny,having significant contribution to improvement of radiationmeasurements at high-mountain stations BEOMoussala in Bulgariaand Lomnicki Stit in Slovak republic.

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