A Background Study with the CdTe Detector for the 14-4 kev Solar Axion Search

Transcription

1 A Background Study with the CdTe Detector for the 14-4 kev Solar Axion Search Shelvia Wongso Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University Aihara Laboratory, Department of Physics, The University of Tokyo (Dated: August 19, 2016) Axion is one of dark matter candidates and its existence would be able to explain the strong CP problem in Physics. The objective of the overall study is to seek the 14.4-keV axions emitted from the M1 transitions in the 57 Fe nuclei. The resulting photons can be detected subsequently using the 57 Fe nuclei target and detector in the laboratory. Since the signal is very rare and there are different background components that contaminate the signal, a detailed study of the detector background is essential with the aim to understand and further reduce the background level of the detector. This background study employs the CdTe detector to evaluate the background rate near the signal region between 13.6 kev to 15.2 kev (14.4 ± 0.8 kev) and to assess the effectiveness of different shield materials in suppressing the background near the signal region. I. INTRODUCTION A. Strong CP Problem The Lagrangian for QCD includes a CP-violating term: L Θ = Θ(α s /8π)G µνa Ga µν (1) where π Θ +π is the effective Θ parameter after diagonalizing quark masses, G a µν is the color field strength tensor, and G a,µν ɛ µνλρ G a λρ /2. Experiment on the neutron electric dipole moment imposed a limit Θ even though Θ = O(1) is otherwise acceptable [1]. The most attractive explanation to the strong CP problem is currently based on the global chiral symmetry U(1) P Q proposed by Peccei and Quinn [2]. As noted by Weinberg [3] and Wilczek [4], the U(1) P Q symmetry is spontaneously broken at energy scale f a, yielding a new neutral spin-zero pseudoscalar particle called axion.the symmetry is broken due to the axion s anomalous triangle coupling to gluons, and the CP-violating term in the lagrangian for QCD becomes: L = ( φ A f A Θ) α s 8π Gµνa Ga µν (2) where φ A is the axion field and f A is the axion decay constant. Non-perturbative topological fluctuations of the gluon fields in QCD induce a potential for φ A whose minimum is at φ A = Θf A, thus cancelling the Θ term and restoring the CP symmetry in strong interactions. B. Axions Earlier standard axion (PQWW-axion) model assumes that f A is fixed at the electroweak scale f A = (2G F ) GeV [2 4], but this model has been experimentally shel0009@e.ntu.edu.sg excluded [5]. In this model, the tree-level flavour conservation fixes the axion properties in terms of a single parameter: the ratio of the vacuum expectation values of two Higgs fields that appear as a minimal ingredient. The axion mass for this model would be of the order of 100 kev to 1 MeV. Variant axion models keep f A 250 GeV while relaxing the constraint of tree-level flavour conservation [6], but these models are also ruled out by experiment [7]. The mass of axion for this model is predicted to be about 1.7 MeV. New axion models propose that f A is much greater than 250 GeV and can be extended up to the Planck mass GeV. Since the coupling constants of axions with matter and radiation (e.g. axion couplings to photons (g Aγ ), leptons (g Ae ), and hadrons (g AN )) and the axion mass (m A ) are inversely proportional to f A, these axion models are often referred to as invisible axions. There are two classes of invisible axion models that have been proposed and developed: the KSVZ or hadronic axion model [8, 9] and the DFSZ or GUT axion models [10, 11]. In both models, the axion mass is expressed in terms of π 0 properties: m A [ev ] = f πm π z ( f A (1 + z + w)(1 + z) )1/2 (3) where f π = 93 MeV is the pion decay constant, z = m u /m d 0.56 and w = m u /m s are quark-mass ratios. For the given values of z and w (existing experimental data allow for rather broad ranges for possible values of z and w), the mass of axion can be expressed as m A = /f A where m A and f A are in ev and GeV respectively. The astrophysical considerations based on axionphoton and axion-electron couplings constrain the mass of the invisible axions to m A 10 2 ev [12]. In addition, the axion-nucleon coupling is constrained by the data on neutrino signal from the supernova SN1987A. However, it should be noted that the data also leave open a small window known as the hadronic axion window of 10 ev m A 20 ev provided that the axion-photon coupling constant is sufficiently small [13]. Furthermore, the latest cosmological limits on hot dark matter consisting of

2 2 hadronic axions suggest that m A < 1 ev [14, 15]. The objective of the overall study is to seek the kev monochromatic axions emitted from the M1 transitions in the 57 Fe nuclei through the process of resonant absorption. The resulting photons and conversion electrons can be detected subsequently using the 57 Fe nuclei target and detector in the laboratory. Searches for kev axions have been performed in [16 23]. Since the signal is very rare and there are different background components (from naturally-occurring radioactive isotopes, muons, intrinsic radioactive impurities in the set-up, etc.) that contaminate the signal, a detailed study of the detector background is essential with the aim to understand and further reduce the background level of the detector. This background study employs the CdTe detector to evaluate the background rate near the signal region between 13.6 kev to 15.2 kev (14.4 ± 0.8 kev) and to assess the effectiveness of different shield materials in suppressing the background near the signal region. II. SOURCE AND DETECTION TECHNIQUE OF 14.4-KEV SOLAR AXIONS The mechanism for the detection of 14.4-keV solar axions using 57 Fe is proposed by Moriyama [24]. Due to the high temperature in the centre of the Sun ( 1.3 kev), some nuclei having low-lying nuclear levels such as 57 Fe can be excited thermally. 57 Fe is one of the stable isotopes of iron (with natural abundance of 2.2%) and is exceptionally abundant among heavy elements in the Sun (solar abundance by mass fraction ). The transition between the first excited state and the ground state of 57 Fe is an M1 transition. Its first excitation energy is 14.4 kev. Hence, the de-excitation of these nuclides yields the solar axions of energy 14.4 kev. These monochromatic axions are Doppler broadened owing to the thermal motion of the axion emitter in the Sun. Consequently, they can excite the same nuclide in the laboratory. The possibility of decay of the axion into two photons during their journey from the Sun to the Earth is insignificant. The resonant absorption of these axions by M1 transition in the 57 Fe target in the laboratory will be followed by the decays of the excited nuclei, emitting either a gamma ray of energy 14.4 kev or an internal conversion electron of energy 7.3 kev. The detection technique of these solar axions serves as an experimental test of the hadronic axion window, independent of the axion-photon coupling. The prevailing experiment for the 14.4-keV axion search using 57 Fe led by Derbin utilizes the Si(Li) detector with sensitive area of 66 mm in diameter and 5 mm in thickness [23]. The detector is cooled to liquid-nitrogen temperature and is arranged in a vacuum cryostat. The surface of the detector is positioned at a distance of 1.5 mm from the iron target enriched in the isotope 57 Fe to 91%. The target has a mass of 1.26 g and is 70 mm in diameter, the corresponding thickness being x 0 = 30 mg/cm 2.The passive shield employed in this experiment consists of a copper envelope 10 mm thick, which is adjacent to the cryostat; an iron layer 35 mm thick; and a lead layer 50 mm thick. It suppresses the external γ activity by a factor of about 500. Within the cryostat, the Si(Li) detector is mounted on a copper plate 50 mm thick, which shields it from the radioactivity of zeolite. Furthermore, active shields consisting of six plastic scintillators cm 3 in dimension are used to suppress the cosmic-ray and fast-neutron backgrounds. The total detection efficiency for 14.4-keV gamma rays in this experiment is ε = (8.91 ± 0.3)%. III. EXPERIMENTAL SETUP The CdTe detector Model 579/CdTe was used in this background study. The detector effective area is thick mm. The maximum biasing voltage for this detector is 200 V. One of the advantages of using this detector is that it can be operated at room temperature. The energy resolution is 1.5 kev fwhm at 60 kev (measured using 241 Am). From this information, it is found that the energy resolution is 5.3% at 14.4 kev. However, it must be noted that the energy resolution may get worse over time due to polarization effect [25]. The CdTe detector has the characteristics of a Schottky diode. The band gap energy and the electron-hole pair creation energy for this detector are 1.52 ev and 4.43 ev respectively at 300 K. Figure 1. The wired connection in experimental set-up. Figure 1 shows how the different apparatus used in this experiment were connected in this experiment. A voltage of 180 V was applied to the amplifier. The final output would be the ADC count. Energy calibration was performed using the 137 Cs and the 133 Ba. The decay diagram and the energy spectrum are shown in Figure 2 and 3 respectively. The photoelectric peaks were then matched to the respective energies based on the decay diagram. Then, a graph of photoelectric peak energy against ADC count was plotted to measure the calibra-

3 3 tion constant, as shown in Figure 4. fitting, we obtain: Applying linear E [kev ] = ADC Count (4) The small chi square is due to insufficient data used for the calibration. Employing other isotopes for the energy calibration would improve the fitting. 2. With Pb Shields (5 cm) 66 hours 3. With Pb Shields (5 cm) and Cu Shield (3 mm) 39 hours 4. With Pb Shields (5 cm) and Cu Shield (6 mm) 48 hours Although the duration for each of the experiment is different, the number of events will be normalized against time in order to be able to make comparison of the results. The Cu shield is oxygen free and has a purity of 99.99%. It is used to block the gamma rays that originate from the Pb shields. Figure 5 and 6 show the set-up of the shields. Figure 2. The decay diagram of 137 Cs and the 133 Ba. Figure 5. (i) Pb Shields (ii) CdTe detector inside Pb Shields (iii) CdTe detector inside Pb Shields and Cu Shield. Figure 3. The energy spectrum of 137 Cs and the 133 Ba measured using CdTe detector. Figure 6. (i) Cu Shield of 3 mm thickness (ii) Cu Shield of 6 mm thickness. IV. RESULTS Figure 4. Graph of photoelectric peak energy against ADC count for 137 Cs and the 133 Ba measured using CdTe detector. Four sets of experiments were conducted to measure the environmental radiation under different conditions using the CdTe detector: 1. Without shield 40 hours Figure 7 shows the plot of the environmental radiation under the four different conditions using the ROOT software. The continuous background is likely to be due to the naturally occurring radioisotopes in our environment. At the very low energy region, two peaks can be observed. The first peak could be possibly due to the thermal noise of the detector and the second peak at around 5 kev could be possibly due to the combined characteristic X- rays emitted by the Cd and Te from the detector. The rationale behind this explanation is due to the fact that these peaks exist in both shielded and unshielded cases. Hence, it is suspected that these peaks originate from the

4 4 internal of the set-up rather than external factors. Overall, it can be observed that the use of shields is effective in reducing the environmental background. region for the four different measurements, the integrated event rate (area under the histogram) was computed using the ROOT software. The results are shown in Table I. From the table, it can be seen that the background suppression rate (94.9%) is the highest with the use of both Pb Shields and thicker Cu Shield. Condition of Experiment Integrated Event Rate [Counts/keV/day] Without Shield 43.1 With Pb Shields (5 cm) 7.24 (-83.2%) With Pb Shields (5 cm) 3.80 (-91.2%) and Cu Shield (3 mm) With Pb Shields (5 cm) 2.19 (-94.9%) and Cu Shield (6 mm) Table I. Table to show the integrated event rate in Counts/keV/day for the four different measurements in the signal region. The background suppression rate for the conditions with shields are also shown in terms of percentage. Figure 7. Plot of environmental radiation with four different conditions: (i) Blue line: without shield, (ii) Red line: with Pb Shields (5cm), (iii) Green line: with Pb Shields (5 cm) and Cu Shield (3 mm), and (iv) Pink line: with Pb Shields (5 cm) and Cu Shield (6 mm). The red dots encircle the region of low energy (between 0 to 30 kev) which is our region of interest. Figure 8. The zoomed in plot of the low energy region of Figure 6. The black line encircles the signal region between 13.6 kev to 15.2 kev. Since the signal region lies between 13.6 kev to 15.2 kev (14.4 ± 0.8 kev), it is more useful to focus on the low energy region as depicted in Figure 8. In this plot, it can be seen clearly that the condition with use of thicker (6 mm) of Cu shield (pink line) is able to suppress the background the most compared to the other conditions. To compare the background suppression rate in the signal V. DISCUSSION AND FUTURE WORK Based on the results from the prevailing experiment conducted by Derbin [23], the plot of the spectrum is obtained as shown in Figure 9. In the signal region of 13.6 kev to 15.2 kev, the integrated event rate obtained by Derbin is 89.8 counts/kev/day. Normalizing the rate by the detector s surface area, we obtained a count rate of counts/kev/day/mm 2 for Derbin s experiment with Si(Li) detector and a count rate of counts/kev/day/mm 2 for our experiment with CdTe detector. Setting Derbin s result as our benchmark, we would be required to improve our condition by a factor of 5. We considered a possibility of muons as background in our experiment. In order to calculate the amount of energy deposited by muons in the CdTe detector, we considered the factors: the intensity of vertical muons for horizontal detectors to be 1 cm 2 min 1, the surface area of CdTe detector to be cm 2, incoming muon rate to be s 1 and the density of CdTe detector to be 6.2 g cm 3. Since there is no data on the energy loss of muons in 48 Cd and 52 Te, we took the data of 50 Sn which has the average atomic number of the two. The energy deposited by muons of momentum 1.0 GeV/c in 50 Sn is 1.4 MeV g 1 cm 2 according to Figure 10. Assuming the -de/dx is constant, the energy deposited by muons in the CdTe detector is approximately 4 MeV. Although this energy scale is not within our energy region of interest, this figure is obtained with the assumption that the muons hit the detector vertically. In the oblique case, the muons will travel longer distance, and thus deposit more energy in the detector. In order to reduce the possible background due to muons bremsstrahlung induced signals, veto counter could be employed and anticoincidence technique could be applied.

5 5 Figure 10. The energy loss of muons of different momentum. VI. CONCLUSIONS Figure 9. The result of approximating spectrum measured by Derbin [23] in anticoincidence with an active-shield signal in the range kev. Possible improvements could be made to the set-up of the experiment. Firstly, hermeticity of the lead shields could be improved further by modifying the geometry of the shields. Secondly, thicker copper could be used to test whether further background suppression could be achieved. Thirdly, the polarization effect of the CdTe detector due to long hours of operation should be checked carefully as it will result in poor energy resolution. One method is to analyse the shift of the photoelectric peak of a known isotope such as that used in the energy calibration. In the main experiment of axion search, XRPIX5 detector will be used. XRPIX5 is one of the SOI (Siliconon-Insulator) pixel detector developed for X-ray detection. Each pixel has readout circuit that enables the process of extracting the trigger signal and reading pixel by pixel. One advantage of using XRPIX5 lies in its high sensitivity due to its small readout noise and great energy resolution. Its effective area is 13.8 mm 21.9 mm 0.5 mm thickness and its energy resolution is less than 4% at 14.4 kev. The iron target is enriched in the isotope 57 Fe to 91%. The experimental technique that we are going to employ in the search for the resonance absorption of kev solar axions by 57 Fe nuclei is to detect the resulting photons using the XRPIX5 which a type of SOI pixel detector. In order to detect the weak signal, we conducted background study with the aim to understand and further reduce the background level of the CdTe detector. It was found that the use of Pb Shields (5 cm) and Cu Shield (6 mm) is the best condition among the four different conditions we tested, yielding a background suppression rate of 94.9%. The integrated event rate for this condition is counts/kev/day/mm 2. By comparing this result to that of the prevailing experiment led by Derbin, it was concluded that we still need to improve the background suppression by a factor of 5. Improvement in the set-up of the experiment and analysis of the polarization effect of the CdTe detector would need to be made in order to refine this background study. ACKNOWLEDGEMENT I would like to thank Professor Hiroaki Aihara for providing me with the valuable opportunity to participate in his research group, Professor Yoshiyuki Onuki for his kind and patient guidance in my overall research experience, my fellow labmates for their kind assistance and heartwarming welcome, the staff members of the University of Tokyo Research Internship Program (UTRIP) for their administrative help and care, and lastly the Graduate School of Science (GSS) for the generous scholarship.

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