Characterization of Germanium Detectors for the First Underground Laboratory in Mexico
A. Aguilar-Arevalo, S. Alvarado-Mijangos, X. Bertou, C. Canet, M. A. Cruz-Pérez, A. Deisting, A. Dias, J. C. D'Olivo, F. Favela-Pérez, E. A. Garcés, A. González Muñoz, J. O. Guerra-Pulido, J. Mancera-Alejandrez, D. J. Marín-Lámbarri, M. Martínez Montero, J. Monroe, C. Iván Ortega-Hernández, S. Paling, S. Peeters, D. Ruíz Esparza Rodríguez, P. R. Scovell, C. Türkoğlu, E. Vázquez-Jáuregui, J. Walding
PPrepared for submission to JINST
Characterization of Germanium Detectors for the FirstUnderground Laboratory in Mexico.
A. Aguilar-Arevalo a S. Alvarado-Mijangos b X. Bertou c C. Canet d M. A. Cruz-Pérez e A.Deisting f A. Dias f J. C. D’Olivo a F. Favela-Pérez a , c E. A.Garcés b , A. González Muñoz b , J.O. Guerra-Pulido a J. Mancera-Alejandrez g D. J. Marín-Lámbarri b M. Martinez Montero a J.Monroe f C. Iván Ortega-Hernández a S. Paling h S. Peeters i D. Ruíz Esparza Rodríguez b P. R.Scovell h C. Türkoğlu i , E. Vázquez-Jáuregui b J. Walding f a Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, CDMX, México b Instituto de Física, Universidad Nacional Autónoma de México, A. P. 20-364, México D. F. 01000, Mexico c Centro Atómico Bariloche, CNEA/CONICET/IB, Bariloche, Argentina d Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, CDMX, 04110 Mexico e Programa de Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, CiudadUniversitaria, Coyoacán 04510, Ciudad de México, Mexico f Royal Holloway, University of London, Egham Hill, United Kingdom g Facultad de Ingeniería, Universidad Nacional Autónoma de México, Mexico h Boulby Underground Laboratory, Boulby Mine, Saltburn-by-the-Sea, United Kingdom i Department of Physics and Astronomy, University of Sussex, Brighton, United Kingdom
E-mail: [email protected]
Abstract: This article reports the characterization of two High Purity Germanium detectors per-formed by extracting and comparing their efficiencies using experimental data and Monte Carlosimulations. The efficiencies were calculated for pointlike γ -ray sources as well as for extendedcalibration sources. Characteristics of the detectors such as energy linearity, energy resolution andfull energy peak efficiencies are reported from measurements performed on surface laboratories.The detectors will be deployed in a γ -ray assay facility that will be located in the first undergroundlaboratory in Mexico, Laboratorio Subterráneo de Mineral del Chico (LABChico), in the ComarcaMinera UNESCO Global Geopark [1].Keywords: Gamma detectors, Radiation monitoring Corresponding Author. Now at :AstroCeNT - Particle Astrophysics Science And Technology Centre, ul. Rektorska 4, 00-614, Warsaw,Poland a r X i v : . [ phy s i c s . i n s - d e t ] S e p ontents Pb calibrated samples 14
LABChico (Laboratorio Subterráneo de Mineral del Chico) will be an underground laboratoryin a decommissioned silver mine from colonial times, at the Comarca Minera UNESCO GlobalGeopark [1], in the Mexican state of Hidalgo. The LABChico research program will focus on ap-plications for environmental radiation monitoring, γ -ray assay and screening, as well as prototypedesign for detectors in underground astroparticle physics experiments, see [2–7] for some exam-ples. This program requires a variety of state-of-the-art detector technologies which operate in alow radioactivity background environment, such as High Purity Germanium detectors. LABChicowill be located at a depth of approximately 100 m of rock, 300 m.w.e. (meter water equivalent) over-burden. The background cosmic ray flux is attenuated, from 1 muon/cm /minute to approximately1 muon/cm /hour.LABChico will consist of a 125 m cavern with a usable surface area of 37 m (including aplatform), as well as a user/visitor center and storage facility outside the mine. The infrastructure willinclude ventilation and air conditioning (HVAC), electrical power, compressed air and networking.This work reports the characterization of two High Purity Germanium detectors that will be part ofthe LABChico underground facility.This report is organized as follows. In section 2 the detectors characterization with radioactivesources is presented; energy calibration and resolution are reported in sections 2.1 and 2.2 respec-tively. The efficiency measurement is described in section 2.3. The simulation of the germanium– 1 – igure 1 . The High Purity Germanium detector (ICN-HPGe) with its liquid nitrogen dewar at the Institutode Ciencias Nucleares, UNAM. detectors used in the efficiency measurement is described in section 3. The validation with extendedsources is described in section 4. The LABChico assay facility will initially consist of one High Purity Germanium detector manu-factured by ORTEC, enclosed by a 5 inch thick lead shield. The detector has been characterized, forthis work, at the Instituto de Ciencias Nucleares (ICN), Universidad Nacional Autónoma de México(UNAM) in Mexico City. This detector will be hereafter referred to as ICN-HPGe, see figure 1.The physics research and assay program will be complemented with a Broad Energy Germaniumdetector, located at the Instituto de Física (IF), also at UNAM in Mexico City, manufactured byCanberra. This detector will be hereafter referred to as IF-BEGe, see figure 2. Both laboratoriesare on surface (Mexico City altitude 2,250 meters above mean sea level).Both detectors are cooled down using liquid nitrogen, in a 30 l cryogenic storage dewar. TheICN-HPGe has an operational bias of -3200 V, with a p-type structure and a coaxial closed verticalgeometry. The data acquisition system (DAQ) consists of a PX5-HPGe multichannel analyzer(MCA) and the digital pulse processor (DPP) software analyzer provided by Amptek [8].The dimensions of the internal components of the ICN-HPGe detector were estimated fromX-ray images and by measuring the external parts of the device (see figure 1). The identifiedcomponents included in the simulations (see figure 7, section 3) are:1. Cryostat: an aluminium cylinder of 34.0 mm radius, 142.0 mm height and 500 µ m thickness.It contains the germanium crystal, contacts and endcap.2. Outer contact: lithium cylinder of 700 µ m thickness, surrounding the germanium crystal.– 2 – igure 2 . The Broad Energy Germanium detector (IF-BEGe) at the Instituto de Física, UNAM.
3. Dead layer: this is the part of the germanium crystal that is not sensitive to the incoming γ -rays. Its thickness was determined using Monte Carlo simulations as described in section3.4. Germanium crystal: a cylinder of 25.5 mm radius and 54.0 mm height with a cylindrical holein the middle for the inner contact.5. Endcap: made of carbon fiber with a 1.0 mm width aluminium window to allow low energyphotons to interact with the sensitive crystal.6. Thermal strap: an aluminum disc of 120 mm radius.7. Electronics chamber: an aluminum cylinder of 39.0 mm radius, 70.0 mm height and 500 µ mwidth that contains the preamplifier electronics.8. Inner contact: a 0.3 µ m inner contact of boron implanted ions.9. Cold finger: an aluminum cylinder of 7.50 mm radius.The IF-BEGe is a Canberra model BE2820 [9] with planar horizontal crystal configuration,30 mm radius and 20 mm height, an operational bias of +3000 V, a DAQ consisting of a PocketMCA 8000 A and the ADMCA software also provided by Amptek [8], see figure 2.For the IF-BEGe detector the dimensions were obtained from the technical sheet provided bythe manufacturer and also measuring the external parts of the device (see figure 2). The componentsidentified and later included in the simulations (see figure 8, section 3) are:1. Cryostat window: a disk with a 32.0 mm radius and 0.6 mm thickness, made of carboncomposite.2. Infrared (IR) window: two thin disks of 30.0 mm radius. One made of polyethylene and7.4 µ m thickness and the other made of aluminum and 0.1 µ m thickness.3. Side electrode dead layer: region of the germanium crystal, in contact with the electrode,unable to detect γ -rays. – 3 –. Germanium crystal: a germanium cylinder of 28.55 mm of radius and 17.15 mm height.5. Teflon cup: this volume is located between the detector holder and the germanium crystal. Itis a cup with inner radius 30.1 mm, outer radius 30.6 mm and 25.5 mm height.6. Vacuum space: inside the Cryostat a vacuum environment protects the germanium crystalsurfaces from moisture and condensible contaminants.7. Endcap: an aluminum cylinder with 41.3 mm radius, 237.0 mm long and 1.6 mm thickness.8. Front electrode dead layer: region of the germanium crystal unable to detect γ -rays.9. Detector holder: This volume is made of copper and has the shape of a cup, with inner radiusof 30.6 mm and outer radius of 31.4 mm. In the top side of this cup there is a ring that widensthe outer radius up to 33.6 mm. The height of the holder is 27.1 mm.The main difference between the ICN-HPGe and the IF-BEGe detectors is the geometry ofthe germanium crystals. The ICN-HPGe has a bulletized coaxial crystal, that is, a crystal in whichthe corners facing the front of the detector have been rounded to avoid charge collection in regionswhere the electric field is weak [10]. The IF-BEGe detector has a planar crystal in which the appliedelectric field is more uniform than in a coaxial crystal. These differences have an effect in the chargecollection produced by γ -ray interactions which become visible in the detector response. A set of pointlike radioactive sources, provided by
Spectrum Techniques [11], described in table 1,have been used to calibrate the energy response of the detectors. A 20 % uncertainty on the sourceactivities is reported by the manufacturer.Source γ peaks [keV] Half-life [yr] Am 59.54 432.2
Ba 81.0, 276.0, 303.0, 356.0 10.5
Cd 88 1.27 Co 122.0, 136.0 0.745 Na 511.0, 1275.0 2.6
Cs 662.0 30.1 Mn 835.0 0.855 Zn 1115.0 0.668 Co 1173.2, 1332.5 5.27
Table 1 . Energy and half-life of the γ -ray sources. All having an initial activity (January 2019) of 1 µ Ciand a 20 % uncertainty, except for
Cs (0.1 µ Ci) [11].
Each source was deployed individually, at a distance of 25 cm from the detector endcap alongits symmetry axis. The measurements with the ICN-HPGe detector were performed with the sourceand the detector enclosed by the lead shield, whereas the measurements with the IF-BEGe detectorwere performed without shield. A spectrum was acquired for each source; an example of the
Cs– 4 –
500 1000 1500 2000 2500 3000 3500 4000Channel number110 C oun t s Cs K Bi Tl
800 820 840 860 880 900 920
Channel number C oun t s Figure 3 . Raw data spectrum of the IF-BEGe detector placing the
Cs radioactive source in front of thedetector, in the vertical axis are shown the number of counts per channel and in the horizontal axis is thechannel number to which an energy is associated. Left panel shows the full spectrum range, 4096 channelsequivalent to up to 3 MeV,the
Cs photopeak is shown in red. The other peaks observed in the energyspectrum,
Bi and
Tl, are decay products of the primordial radionuclides
U and
Th, respectively. K is also a primordial radionuclide. These naturally occurring isotopes are ubiquitously present in nature.Right panel is a zoom around the peak corresponding to the
Cs photopeak, the red line is the fitted Gaussianfunction, the black line is the order one polynomial and the green line shows the fitted function composed bythe sum of both, the fit is performed in a six sigma range about the photopeak mean. spectrum is shown in figure 3, left panel. For each spectrum, the photopeak was fitted to a Gaussianplus a polynomial order one function using ROOT analysis tools [12]. The Gaussian mean fitparameter is the channel value associated to the γ -ray energy and its sigma is related to the detectorenergy resolution presented in section 2.2, see figure 3, right panel. The integral of the fitted linearfunction background model is subtracted from the total number of counts in a six sigma range aboutthe mean. From this exercise with each spectrum the linear distribution of γ -ray energy vs. channelis obtained. The fit parameters of the energy response distributions shown in figure 4, are listed intable 2 .For the IF-BEGe detector, an extra two natural background radiation points ( K 1.46 MeVand
Bi 1.76 MeV) are present in the spectrum and were included for the energy calibration, also
Tl peak is present at 2.6 MeV; these can be seen in the left panel of figure 3.Parameter ICN-HPGe IF-BEGem 0.1800 ± ± ± ± Table 2 . Fit parameters for the detector calibrations in figure 4, energy E as a function of the associatedchannel N is E ( N ) = m × N + b . After the energy scale calibration, the spectrum is fitted to a Gaussian function plus an order onepolynomial with negative slope. The full width at half maximum (FWHM) is given as
FW H M = For the IF-BEGe detector, if the b parameter is fixed to zero there is no significant change in the slope parameter m. – 5 – hannel0 1000 2000 3000 4000 5000 6000 7000 8000 E ne r g y ( k e V ) ICN-HPGe Am Ba Cd Co Co Ba Na Cs Mn Zn Co Na Co E ne r g y ( k e V ) IF-BEGe Ba Cd Co Ba Na Cs Zn Co Na Co K Bi Mn IF-BEGe
Figure 4 . Linear energy response of the detectors for different γ -ray sources. Deposited energy (keV) vs.channel number. In the top panel is presented the ICN-HPGe detector calibration, in the bottom panel thecalibration curve for the IF-BeGe detector. Fitting parameters of the linear functions can be found in table 2. √ × σ (cid:39) . σ , where σ [keV] is the Gaussian fit parameter to each photopeak. Thedetector energy resolution is defined as the ratio of the FWHM to the true gamma peak energy, R = FW H M / E , the uncertainty in R is obtained from the uncertainty in σ and E, which areobtained from the fit . This distribution is shown in figure 5 for both detectors, fitted to an empiricalthree parameter inverse square root function [13]: R = [ P ] (cid:112) [ P ] + E + [ P ] , (2.1)the best-fit values to the fitted parameters P , P and P , in [keV], are listed in table 3. Absolute efficiency, also known as full energy peak efficiency, is defined as the ratio of the numberof counts detected in a peak to the total number emitted by the source, (cid:15) = N FE P P γ N TOT , (2.2) The errors in σ and E were augmented by a factor χ /n.d.f. given by the fit to account for non-Gaussianities in themeasured photopeaks when this number was greater than one, for χ /n.d.f. smaller than one, this correction was notapplied. – 6 –arameter ICN-HPGe IF-BEGe P (keV) 1.38 × − ± × − × − ± × − P (keV) -36.73 ± ± P -1.63 × − ± × − -8.45 × − ± × − Table 3 . Fit parameters for the detectors resolution function, Equation 2.1.
Energy (keV)0 200 400 600 800 1000 1200 1400 E F W H M R = Ba Am Ba Cd Co Co Ba Ba Ba Cs Mn Zn Co Na Co ICN-HPGe E F W H M R = IF-BEGe Ba Cd Co Ba Na Cs Mn Zn Co Na Co IF-BEGe
Figure 5 . Resolution as a function of gamma energy, R ( E ) = [ P ]/ (cid:112) [ P ] + E + [ P ] . The top panel showsthe resolution curve vs. energy for the ICN-HPGe detector, and bottom panel the resolution curve for theIF-BEGe detector. The ICN-HPGe shows a better resolution performance compared to that of the IF-BEGe,which does not meet the expected resolution performance from factory settings [9], having almost twice theexpected FWHM. The ICN-HPGe shows an energy resolution at 1.3 MeV compatible with values reportedby ORTEC [14] for similar detectors. Fit parameters can be found in table 3 for both detectors. where N FE P is the full energy peak count rate in counts per second, P γ is the emission probabilityof the γ -ray being measured, and N TOT is the total number of γ -rays emitted at the specific energy,which was corrected for decay from the date of preparation, see table 1.The total number of counts in each full energy peak has been computed by integrating thefitted function in a standard interval of six times the standard deviation (see right panel of figure 3),which is symmetric about the mean of each photopeak. Counts below the straight line, used to fit P γ is the fraction of γ emissions of a given energy to the total number of isotope disintegrations, this values andtheir corresponding uncertainties are taken from [15]. – 7 – nergy (keV) F u ll E ne r g y P ea k E ff i c i en cy % - - ICN-HPGe Ba Am Ba Cd Co Ba Na Cs Mn Zn Co Na MC simulationData 25cm - - F u ll E ne r g y P ea k E ff i c i en cy % IF-BEGe Ba Cd Co Ba Na Cs Mn Zn Co Na Co IF-BEGe
MC simulationData 25cm
Figure 6 . Germanium detectors efficiencies measured with pointlike γ -ray sources, 25 cm away from theendcap of the detectors, compared with Monte Carlo simulations. Top panel shows the efficiency curve forthe ICN-HPGe detector and bottom panel shows the IF-BEGe detector efficiency curve. the background, have been subtracted.The number of emitted γ -rays for each source is determined using the radioactive decay lawand the age of the source. The half-life source values are presented in table 1 and the branchingratios for each γ -ray were taken from the National Laboratory Henri Becquerel decay tables [15].Figure 6 shows the efficiency measurements for all photopeak energies at a distance of 25 cm.For the ICN-HPGe detector the expected behavior was observed: a fast rise of the efficiency fromlow energies up to a maximum expected around 100 keV, between the Cd and Co γ -rays, andthen a slower steady decrease towards high energies, which is consistent with similar detectors byORTEC [16] . Uncertainties in figure 6 are assessed assuming uncorrelated errors from branchingratios, half-lives of the isotopes [15], number of counts in the photopeak (statistical) and sourceactivities (20 %).Broad Energy Germanium detectors have a typical relative efficiency that can range from 9 %to up to 50 % depending on crystal volume and front face area [9]. For the IF-BEGe detector, modelBE2820, a 13 % relative efficiency at 1332 keV from Co is reported by Canberra [9]. For historical Due to a lack of documentation for the ICN-HPGe detector, a comparison of relative efficiency values with manu-facturer specifications is not possible – 8 – igure 7 . Left panel, the layout of the ICN-HPGe detector taken from a X-ray scan and right panel,its implementation in GEANT4. The components are: 1) Cryostat, 2) Outer contact, 3) Dead layer, 4)Germanium crystal, 5) Carbon fiber endcap, 6) Thermal strap, 7) Electronics chamber, 8) Inner contact and9) Cold finger. reasons, relative detection efficiency of germanium detectors is defined at 1.33 MeV relative to theabsolute efficiency of a standard NaI(Tl) scintillator, this standard is a crystal of 3 in diameter and3 in long using a Co source placed 25 cm from the endcap face which value is 1.2 × − [10]. Thegermanium detector full energy peak efficiency measured in this conditions divided by 1.2 × − is the relative efficiency specification of germanium detectors. The 13 % relative efficiency valuereported for the IF-BEGe detector is equivalent to a full energy peak efficiency of 0.0156 %, andthe measured value, (0.0117 ± Monte Carlo simulations for both detectors were performed using the simulation toolkit GEANT4 [17],10.01.p03 version. The simulations included all the geometric elements that affect the propagationof γ -rays between the source and the germanium crystal. For the ICN-HPGe detector, the components listed in section 2 were included in the simulations(see figure 7). The preamplifier electronics (inside the electronics chamber) was not simulated.There was no information available about the dead layer thickness surrounding the activevolume of the germanium crystal. The X-ray images did not provide information about thisparameter either. Dead layer thicknesses of the order of 0.75 mm have been measured for a similarcoaxial vertical germanium detectors manufactured by ORTEC by bombarding its crystal witha collimated γ -ray source [18]. Having similar dimensions, the ICN-HPGe detector crystal wasexpected to have a dead layer of around the same order. Monte Carlo simulations with varyingdead layer thicknesses were performed to estimate this parameter by comparing the measured andsimulated pointlike source photopeak efficiencies, a best match was found at a dead layer thicknessof 0.65 mm. The IF-BEGe detector was simulated including the components and dimensions listed in section 2(see figure 8). The dead layer thickness of the germanium crystal was modeled as 0.05 mm on the– 9 – igure 8 . Geometry of the IF-BEGe detector. Left side, the detector cross-section provided by themanufacturer (Canberra) [9] and right side, the GEANT4 simulation of the detector. The componentsincluded in the simulations are: 1) Cryostat window, 2) IR window, 3) Side electrode dead layer, 4)Germanium crystal, 5) Teflon cup, 6) Vacuum space, 7) Endcap, 8) Front electrode dead layer and 9)Detector holder. front, 1.45 mm on the sides and 2.8 mm on the back, after the study described in the followingsection.A dead layer on the front side of the germanium crystal of 0.3 µ m and 500 µ m on the sides isreported by Canberra in the technical sheet of the IF-BEGe detector. These settings were used inthe simulations as a first approach, pointlike γ -ray sources located 25 cm from the detector cryostatwindow were simulated. The efficiencies for these simulated sources were calculated similarly tothose in section 2.3. A disagreement greater than 30 % was found when comparing the efficienciesobtained in the simulation with the experimental ones. Increasing the dead layer on the front sidereported by the Canberra technical sheet up to two orders of magnitude was not sufficient to find anagreement between experimental data and the Monte Carlo simulations.The discrepancy between data and simulated efficiencies could be explained if the crystal activevolume had a major alteration with respect to that reported in the technical sheet. In order to explorethis possibility, a crystal scan was performed following a similar method as in [19, 20].For the scan, a pointlike γ -ray source ( Cd) was fixed on top of a mechanical translationstage, in order to move it in a plane parallel and 4 cm away from the frontal face of the germaniumcrystal. The length of the mechanical translation stage was such that the scan was performed alongthe diameter of the germanium crystal as shown in figure 9.A lead collimator with an aperture of 4 mm diameter and 1 cm thickness was employed toobtain an homogeneous and focused γ -ray beam. A set of 33 measurements were taken along thehorizontal axis, from left to right in steps of 1 mm, see figure 10. Runs of approximately one hourwere taken for each position. A clear asymmetry can be seen in figure 10 between the left handside and right hand side of the crystal with respect to its center. This was a clear indication that thesensitive volume of the germanium crystal is indeed most likely smaller than the one indicated inthe technical sheet of the detector.In order to find a better match between the experimental data and the simulation, the followingprocedure was performed. First, a Co γ -ray source, placed 25 cm away from the cryostat windowwas simulated. Most of the γ -rays from such source reaching the germanium crystal volume willpass through most of this volume [21], if the crystal active volume is reduced in the Monte Carlo– 10 – igure 9 . Experimental setup for the IF-BEGe detector scan. The mechanical translation stage, γ -raysource (orange disk), lead block (collimator) and detector endcap are shown. - - - - C oun t s / s @ k e V Figure 10 . Active volume scan along the cylinder diameter using an 88 keV γ -ray from the Cd source.The size of the error bars in the X-axis are due to the collimator aperture. The vertical blue-dashed linesindicates the crystal size of the IF-BEGe detector. simulation, it can match the experimental data, regardless of the dead layer thickness in the frontcrystal face. A sensitive volume reduction of ∼
15 % was found to produce good agreement betweenthe experimental and simulated data. The second step was to find the correct position of the sensitivevolume in the crystal, that yields the dead layer thickness in the front side that matches the efficiencyfor a source with lower energy γ -rays. Simulating a Ba source, also placed 25 cm away fromthe cryostat window, an agreement with the experimental data fixes a dead layer of 0.05 mm on thefront face, 2.8 mm on the back and 1.45 mm on the sides of the crystal.– 11 – keV] dep E · · · · E ve n t s SimulationExperiment
Figure 11 . Background subtracted data and Monte Carlo simulation spectra comparison for a pointlikesource with γ -rays at 835 keV (corresponding to a Mn emission) pointlike source, located 25 cm away fromthe ICN-HPGe detector frontal face. An arbitrary normalization is used for qualitative comparison purposes.
For both detectors, the sources Na, Mn, Co, Co, Zn,
Cd and
Ba were simulatedfor comparison with experimental data, placed 25 cm away from the cryostat window of thedetector. In the case of the IF-BEGe, the sources were simulated as ions with the GEANT4 GeneralParticle Source and their decays with the Radioactive Decay Module [22]. For the ICN-HPGe,the simulations were performed emitting mono-energetic γ -rays corresponding to the emissionenergy of each isotope, see table 1. The deposited energy in the sensitive volume was computedadding up the deposited energy, via ionization processes, by all the secondary particles producedby the primary γ -ray. All information is stored in ROOT histograms [12]. For each primary γ -raysimulated, a count is stored in the corresponding deposited-energy bin.Figure 11 shows a comparison between a calibrated spectrum of a Mn source and the MonteCarlo simulation for a pointlike source emitting γ -rays at 853 keV. The simulated spectrum repro-duces with good agreement the experimental energy spectrum features such as energy resolution,photopeak, Compton continuum and Compton edge: maximum energy which can be transferredto an electron [10], escape peak at 511 keV has been removed by background subtraction. Theenergy resolution was introduced in the simulation following the empirical function obtained fromexperimental data, presented in section 2.2.The full energy photopeak efficiencies are computed as in section 2.3, equation 2.2, comparingthe number of counts in the fitted Gaussian photopeak to the original number of simulated primary γ -rays. The comparison to the experimental data is shown in figure 6. For both detectors, theefficiencies from the simulations are in agreement with the experimental efficiencies within uncer-tainties. The shapes of the curves drawn by the experimental data and Monte Carlo simulationsof the detectors reflect the differences between these two. The curve for the ICN-HPGe is whatis expected from a p-type coaxial detector, having a maximum in efficiency around 100 keV. Onthe other hand, the IF-BEGe detector does not show a maximum, as expected from a planar typedetector [10]. The efficiency of both detectors decreases with the energy since the higher energy γ -rays need more sensitive volume for multiple interactions to be fully absorbed.– 12 – Validation with extended calibration sources.
In order to measure the activity in a given sample, the efficiency calibration with pointlike γ -raysources is not sufficient, since full energy peak efficiency is a geometrically dependent quantity.Instead, an extended source efficiency is required, for the geometry of a given extended source,e.g. a sample vial. This is described for each detector in the following subsections. For practicalreasons, each detector was validated with different extended sources; for the ICN-HPGe detector, apotassium chloride (KCl) solution was used while for the IF-BEGe detector, a Pb solution wasused.
The simulation of the ICN-HPGe detector was validated using water samples with a salt substitutecontaining KCl at different concentrations.
Diluted salt [g]0 2 4 6 8 10 P ho t opea k E ff i c i en cy a t k e V SimulationExperiment
Figure 12 . Top panel, geometry of the simulation of the ICN-HPGe detector with an extended source(container in pink) on top. Bottom panel, Simulated and experimental data extended source, K photopeakefficiency in the ICN-HPGe detector in a potassium salt solution, as a function of the diluted salt mass. A20 % systematic uncertainty is estimated in the simulation.
Five samples of Novoxal brand KCl based salt substitute diluted in injectable water (sterile waterused for medical applications) at different concentrations were prepared in cylindrical polypropylene– 13 –ontainers of 600 ml volume. Figure 12 (top) shows the sample container geometry together withthe ICN-HPGe cryostat in the Monte Carlo simulation.In order to determine the K isotope concentration per gram of salt, a background spectrumof the container with no diluted salt is recorded and subtracted from the spectra of the salt solutionsamples. Then, the remaining number of counts in the photopeak is determined following the samemethodology as in the pointlike source case.The geometry of the sample was implemented in the simulation, as shown in figure 12 (top).The γ -rays emission from active K nuclei within the sample was incorporated in the simulationby sampling random points inside the physical volume of the solution and shooting mono-energeticand isotropic γ -rays with energy 1460 keV, the most probable γ -rays channel of K. The efficiencyis calculated as the ratio of the number of γ -rays counted inside the K photopeak to those initiallyemitted. As the concentration approaches zero, the number of emitter centers from the solutionapproaches zero, but the efficiency approaches a finite value, corresponding to the probability ofdetecting 1460 keV γ -rays emitted from within the container inner volume when it is filled onlywith water.Higher concentrations of KCl salt lead to a self-absorption effect of the γ -rays in the samples.This effect is clearly seen in the computed 1460 keV photopeak efficiencies for different concentra-tions, as shown in figure 12 (bottom). With these simulated efficiencies the activity of the extendedsources were determined to obtain the masses of K in the samples.From the experimental data and Monte Carlo simulations, the concentration ratio of K permass of diluted potassium salt (PS) was obtained as η s = . ± . m g [ K ]/ g [PS]. Fromthe Novoxal label information, in which there is 309.33 mg of pure potassium per salt gram,and taking into account the natural abundance of the K (0.012 %), the ratio η l = ( . ± . ) m g [ K ]/ g [PS] was obtained. Both results are compatible within errors, validating themeasurement and the Monte Carlo simulation. This comparison assumes an uncertainty of 5 %of the potassium concentration in the salt and a 20 % systematic uncertainty is estimated in thesimulated efficiency. Pb calibrated samples
For the Monte Carlo simulation validation of the IF-BEGe detector, five calibrated liquid samplesof
Pb in a solution of deionized water were used. This samples were prepared at Royal Holloway,University of London (RHUL) and measured with a broad energy germanium detector in BoulbyUnderground Germanium Suite (BUGS) [23] in the UK. These samples were shipped to Mexicoand independently measured with the IF-BEGe detector.The response of the Boulby BEGe P-type detector, with a 60% relative efficiency and 0.9 kgcrystal weight used in this study is characterized using the method discussed in [23]. An extendedIAEA-385 [24] sample with known
Pb contamination is placed on the front face of the detectorand the response to this is used to tune a GEANT4 simulation of the experimental setup. Usingthe simulation, the detector efficiency was determined at the 46.5 keV
Pb peak. The extendedsources were assayed on the Boulby BEGe P-type detector and their activities were determined.The activities measured at BUGS are compared with those measured with the IF-BEGe detector,as a cross-check to evaluate the IF-BEGe detector performance and also to validate the Monte Carlosimulation of the extended source. – 14 – igure 13 . GEANT4 simulated geometry of the
Pb calibrated sample inside of a polyethylene Marinellibeaker, placed in the front face of the IF-BEGe detector.
The IF-BEGe efficiency at lower energies was evaluated using X-rays from a pointlike
Cdsource. The agreement between the experimental data and Monte Carlo simulations is 10 % at25 keV.For the measurement of the
Pb calibrated sample using the IF-BEGe detector, the containerswere held inside of a Marinelli beaker, which maximizes the solid-angle coverage of the detector bythe sample (both containers made out of polyethylene). The experimental setup was enclosed by a14 cm thick lead shield, purged with the nitrogen gas from the detector dewar. After ∼
24 hours ofexposure time, the 46.5 keV peak was clearly identified. The γ -rays were counted as in section 2.3.Figure 13 shows the Monte Carlo simulation of the experimental configuration of the IF-BEGe detector with the Pb calibrated sample. Since the position of the water container insidethe Marinelli beaker could be shifted during experiment, as the lid of the beaker was closed, asystematic uncertainties study was performed. The nominal value for the measurement was takenin the position in which the axes of symmetry of the beaker and water container are aligned amongthemselves and with the detector symmetry axis. Then, the position of the water container in thesimulation was shifted up + − γ -ray events from theexperimental data, the activities of each Pb sample were computed. The results are listed intable 4, showing a good agreement with the activity values measured at BUGS, in the UK. RowI in table 4 shows a control water sample where no radioactive material was incorporated, in thissample there was no identifiable signal at 46.5 keV when counting in the IF-BEGe detector.– 15 –ass Expected Boulby IF-BEGe(g) (Bq/kg) (Bq/kg)(stat) (Bq/kg)(stat+sys)I 170.8 blank N/A N/AII 165.6 40 26.9(6) 26(4)III 177.9 22 23.3(3) 17(2)IV 163.6 N/A 0.029(7) 2.5(4)V 124.6 70 77(1) 71(10)VI 120.45 744 754(3) 837(124)
Table 4 . Lead-210 (46.5 keV) calibration standard samples measured with IF-BEGe detector. The thirdcolumn lists the specific activities (Bq/kg) expected from the solution preparation at RHUL, in the fourthcolumn those measured at BUGS [23], and the last column shows the measurement reported using theIF-BEGe detector.
The High Purity Germanium detectors described in this work are part of the detector suite plannedfor the first underground laboratory in Mexico. The energy linearity, energy resolution and fullenergy peak efficiency of the detectors, in conjunction with the low radioactive background providedby the mine environment, will allow the study of a wide range of samples with low radioactivecontent. These detectors will be used for experiments in nuclear and astroparticle physics, biologyand geology. Other applications of the facility in mining heritage, among other areas of the physicaland social sciences are under development.The energy scale and resolution of the ICN-HPGe and IF-BEGe detectors were obtainedover the energy range relevant to low-background astroparticle experiments and environmentalradiation studies. The detection efficiency was quantified in this energy range using a suite ofradioactive sources. From a scan to the IF-BEGe crystal, an alteration in its active volume wasfound. Simulations considering this alteration were performed, to find the optimal configuration,matching the measured detector efficiencies within uncertainties. The detector characterization andsimulation results were validated through tests with externally-calibrated KCl and
Pb sources.This work presents the first step towards establishing the performance of the LABChico radio-assaycapabilities.
Acknowledgments
This work is supported by the STFC Global Challenges Research Fund (Foundation Awards,Grant ST/R002908/1), DGAPA UNAM grants PAPIIT-IT100420 and PAPIIT-IN108020, CONA-CyT grants CB-240666 and A1-S-8960. The authors thank Hesiquio Vargas Hernández, technicianat the machine shop at IFUNAM. They also thank Juan Estrada, Kevin Kuk and Andrew Sonnen-schein for their support in the Fermilab donation of germanium detectors and instrumentation.– 16 – eferences [1] Comarca Minera Hidalgo UNESCO Global Geopark, 2020. .[2] Ian Lawson. Low Background Measurement Capabilities at SNOLAB.
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