Light yield measurements of scintillator cocktails for use in detectors of neutrinoless double beta decay

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1 Light yield measurements of scintillator cocktails for use in detectors of neutrinoless double beta decay Y. Ungar August 29, 2014 Abstract The ongoing search for neutrinoless double beta decay is one of the most active fields in particle physics today. Neutrinoless double beta decay, if observed, will provide insight into physics beyond the standard model. Improving scintillator composition in order to optimize light yield and eliminate background is one way to increase detector sensitivity. This paper presents studies on the light yield of various scintillator cocktails, including quantum dot-doped scintillators. 1 Introduction The standard model has been called the theory of almost everything because it accurately describes most particle interactions. In the standard model, all known matter is composed of quarks and leptons that interact with each other by exchanging force carrier particles. One of the six leptons is the neutrino. Neutrinos are fermions, particles with half integer spin, that are produced in decay processes and nuclear reactions. Neutrinos have no electric or strong charge and therefore feel only the weak and gravitational forces. Thus, although very abundant, neutrinos hardly ever interact with matter. According to the standard model, neutrinos are massless. The discovery of neutrino oscillations in 2001 established that neutrinos do have mass: In order to oscillate between different flavor states, neutrinos must have different mass states [1]. The standard model is also symmetrical with respect to CP transformations and therefore does not explain why the universe is composed of mostly matter and hardly any antimatter. Neutrino mass and matter antimatter asymmetry, among other problems, leads to physics beyond the standard model. Currently, the most feasible way to experimentally address these issues is the search for neutrinoless double beta decay. 1

2 1.1 Neutrinoless Double Beta Decay Beta decay is a radioactive process in which a neutron decays into a proton, releasing an electron and an electron antineutrino. Double beta decay is a process that occurs in certain isotopes that, due to energy considerations, can not decay by emitting a single beta particle. Instead, two neutrons in the same nucleus simultaneously undergo beta decay, releasing two electrons and two electron antineutrinos. (Z, A) (Z + 2, A) + 2e + 2 ν. (1) Neutrinoless double beta decay (0νββ) is an even rarer process than the two neutrino double beta decay, and was first proposed in 1939 by Wolfgang Furry [2]. In neutrinoless double beta decay, two neutrinos simultaneously decay into two protons, releasing only two electrons. (Z, A) (Z + 2, A) + 2e. (2) An electron neutrino and antineutrino are produced, but only as virtual particles because they immediately annihilate each other. Due to conservation of energy, the two electrons emitted in the neutrinoless double beta decay process must carry off all the energy of the decay, and due to conservation of momentum, must travel in opposite directions. Detectors search for these two electrons with characteristic energy at the Q value of the decay. This energy spectrum is illustrated in Figure 1 on the right. This process is only possible if the neutrino has mass (which the 2001 discovery of neutrino oscillations has confirmed) and is a Majorana particle (i.e. its own antiparticle), and if lepton conservation may be violated. 2 Liquid Scintillator Detectors While liquid scintillator detectors have successfully used in the past to observe neutrino properties such as oscillations, they are not sensitive enough to observe 0νββ decay. Lower limits on the half life of 00νββ have been obtained. From these half life limits, the upper limits of the activity of candidate isotopes can be calculated and are listed in Table 1. Half life sensitivity of detectors must be improved to be able to observe such a rare decay. The sensitivity of a detector of 0νββ decay can be calculated from the equation T 0νββ 1 2 (n σ ) = yr ( ea Mt n σ W ) B E where n σ is the number of standard deviation related to a particular confidence level, e is the energy efficiency of the detector, a is the isotopic abundance of decay candidate, W is the molecular weight of isotope, M is the mass of isotope in the detector, t is the time the detector runs, B is the background and E is the detector resolution[7]. Half life sensitivity capabilities can be improved by optimizing the various components of the detector. (3) 2

3 Figure 1: A cartoon of the continuum of energy of the electrons produced in double beta decay in black and the spike of energy of the electrons produced in neutrinoless double beta decay in red. Note that the vertical scale of neutrinoless double beta decay is greatly exaggerated. From: Winslow, et al. Characterizing Quantum-Dot-Doped Liquid Scintillator for Applications to Neutrino Detectors[3] 2.1 Scintillator A scintillator is a material that emits light when a charged particle travels through it. Ionizing radiation excites the π bonds in the aromatic rings of the molecule. These excited electrons then de-excite to ground state, releasing optical photons. The amount of light emitted is proportional to the energy of the charged particle[8]. The absorption and emission spectra of a single-molecule based scintillator overlaps. In order to prevent degradation of efficiency due to the reabsorption by scintillator of scintillation light, scintillator is mixed with a wavelength shifter that absorbs the energy from the excited electron and subsequently releases light of a longer wavelength to which the scintillator is more transparent. Scintillator candidates are listed in Table 2. This work studies the characteristics of these scintillators. 2,5-Diphenyloxazole (PPO) was dissolved in all scintillator samples and was used as a wavelength shifter. The light yield of a scintillator cocktail can be calculated by the equation I(c P P O ) = p (4) 1 + p 2 /c P P O 1 + p 3 c P P O 3

4 Table 1: The lower limits of the half life and upper limits of activity of candidate isotopes for 0νββ decay. Name of isotope Lower limit on half life Upper limit on activity (yr) (events/yr/100 kg) 130 Te [4] Ge [5] Xe [6] 9 Table 2: Solvent and solute names and abbreviations used in this paper, the corresponding IUPAC names, CAS numbers and information on the source of the chemicals. Name IUPAC Name CAS Number Source Information Toluene Methylbenzene Sigma Aldrich [9] (Chromasolv Plus) Pseudocumene 1,2,4-Trimethylbenzene Aldrich (98 %) [9] (PC) Linear Alkyl Benzene various chain lengths Cepsa/Petresa [10] (LAB) PETRELAB 550-Q Phenyl-o-Xylylethane 1,2-Dimethyl Dixie Chemical (PXE) (1-Phenyl-Ethyl)-Benzene Company [11] Di-Isopropylnapthalene Isomer mixture courtesy of (DIN) PerkinElmer [12] Di-Isopropylnapthalene, Isomer mixture courtesy of high purity (DIN HP) PerkinElmer [12] Phenylcyclohexane Cyclohexylbenzene Aldrich ( 97 %) [9] (PCH) Diphenyloxazole 2,5-Diphenyloxazole Aldrich (99 %) [9] (PPO) suitable for scint. 4

5 where p 1, p 2 and p 3 are solvent-specific parameters [13]. Liquid scintillators are viable for increasing sensitivity because they scale easily to large volumes and thus inherently self shield and eliminate background. Also, increased quantities of scintillation light yield improves the efficiency and resolution of the detector. A particle passing through scintillator may also produce Cherenkov light. Cherenkov radiation is emitted whenever a particle travels through a medium at a speed faster than the speed of light in that medium. Unlike scintillation light that is emitted isotropically, Cherenkov light contains directional information of the particle. Background signals can be minimized if the trajectory of the particles can be tracked in order to eliminate any signatures other than that of neutrinoless double beta decay. Separating the Cherenkov from the scintillation light is a challenge: some of the Cherenkov light is absorbed by the scintillator and is reemitted isotropically by the scintillator. For a 1 MeV beta ray, only 60 Cherenkov photons are produced. Also, it is difficult to separate the two forms of light due to their often overlapping spectra and the similar time at which they arrive at the photomultiplier tube (PMT). Future detectors will manipulate materials such as quantum dots to be able to separate the two forms of radiation and glean more information about the trajectory of the particles encountered by the detector. 2.2 Quantum Dots Quantum dots (QDs) are superconducting nano-crystals. The size of a QD is inversely promotional to its band gap and therefore proportional to the wavelength of light it absorbs and emits. Doping a scintillator with quantum dots allows fine-tuning the wavelength of emitted light to match the wavelength at which the PMT is most efficient. Furthermore, QDs can be composed of candidate isotopes for neutrino less double beta decay, enabling the presence of large amounts of isotope in the detector[3]. A powerful application of QDs is utilizing them to separate scintillation from Cherenkov light. Doping the scintillator with QDs that absorb and emit only high energy photons enables the longer wavelengths of Cherenkov light to travel toward the detector unimpeded by scintillation process. Because long wavelengths travel faster than short wavelengths of light in a medium due to dispersion, and because the scintillation absorption and emission process takes time, the long wavelengths of of Cherenkov light will reach the detector 1 ns before the scintillation light, enabling measurement of Cherenkov light and ultimately directional reconstruction of particle trajectory. [3]. QDs can be used to manipulate response time of the scintillator, but must not do so at the cost of light yield of the scintillator. Quantum dots of various emission and absorption spectra that were studied in this paper are listed in Table 3. Future detectors will use other solutes such as quantum dots to manipulate the response time of the scintillator and help separate the Cherenkov from the scintillation light. In this paper, different scintillator cocktails were studied for suitability. 5

6 Table 3: Quantum dot composition, emission peaks, and source information. Composition Emission Peak Source Information (core/shell) (nm) CdS 360 NN Labs 360 nm[14] CdS 360 mk Nano CdS-T-360[15] CdS 380 mk Nano CdS-T-360[15] CdS 400 mk Nano CdS-T-400[15] CdS 400 NN Labs CdS 400 nm [14] CdS/ZnS 400 Ocean Nanotech QZP [16] CdS/ZnS 425 Ocean Nanotech QZP [16] CdSeS 450 Crystalplex NC-450-A [17] 3 Experiment The light yield measurements of various scintillator cocktails that had been prepared a year earlier were measured and compared to light yield of fresh samples. Also, the light yield of scintillator (toluene) doped with quantum dots of varying emission spectra was measured. 3.1 Setup and Procedure In order to ensure accuracy and reproducibility of measurements, standard operating procedures were defined. An ultraviolet-transparent quartz cell was filled with approximately five ml of scintillator solution. The scintillator sample was purged of oxygen by bubbling it with nitrogen for ten minutes. The cell was then sealed, cleaned with ethanol and coupled to the PMT with silicon grease (which had a similar index of refraction as the quartz cell). A microcurie source of 137 Cs was placed on an anti-reflecting teflon block and placed on the scintillator sample. A schematic of the experimental setup is shown in Figure 2. The 662 kev gamma rays from radioactive decay of Cs Compton scattered with the electrons in the scintillator sample. These electrons acted as the stimulant for the scintillator. A Hamamatsu R PMT [18] with a Hamamatsu E base[19] and high voltage of 1675V collected the scintillation light. Individual pulses were recorded by an AlazarTech ATS9870 PCI waveform digitizer with 8 bit resolution and a sample rate of 1 Gs/s [20]. To clean it in between different samples, the quartz cell was rinsed with isopropyl alcohol and dried with nitrogen twice, and then rinsed with cyclohexane and dried with nitrogen. In between runs, the outside of the bottle cleaned with ethyl alcohol, and the PMT with both ethyl alcohol and cyclohexane. 6

7 Figure 2: Schematic view of the light yield measurement setup. 3.2 Data Analysis A sample waveform of voltage collected by the PMT is shown in Figure 3. Each waveform consists of 192 samples taken over 192 nanoseconds. Each sample run lasted approximately 30 minutes and therefore on the order of waveforms were collected each run. Figure 3: A sample waveform collected by the PMT The light yield is proportional to the total charge of photoelectrons collected by the anode of the PMT. This charge was obtained by calculating the area of each pulse, in arbitrary charge units. The charge from each of the pulses of the run was placed on a histogram. A sample histogram is shown in Figure 4. The histogram is a Compton spectrum because Compton effects dominate. In order to compare the light yield different scintillator solutions, a characteristic point at which the number of events dropped to half the Compton edge maximum was chosen. 7

8 th4_floats Entries Mean RMS Events / 2 Charge units maximum half of maximum charge value Charge [a.u.] th4_floats Figure 4: The histogram of number of events collected by PMT versus charge value. Note the histogram is a Compton spectrum. To analyze these results and to compare different scintillators solutions, a characteristic charge that corresponds to the 662 kev gamma rays, at half the Compton edge was chosen. 4 Results Figure 5 displays the graphs of charge (in arbitrary charge units) versus PPO concentration of fresh and old samples. Solvent specific parameters for the year-old samples, p 1, p 2 and p 3 were obtained and are listed in Table 4. The parameters p 1, p 2 and p 3 correspond to the light yield normalization, the energy transfer from solvent to solute and the self-quenching effect, respectively[13]. From these parameters, the PPO concentration that yields maximum quantity of light can be calculated. Year-old samples yielded smaller quantities of light than new samples. This is due to spurious oxygen penetrating into the sample. In a large scale detector, this effect will not be significant as the ratio of volume to surface area will increase. Figure 6 displays the histograms of 8 samples of QD-doped scintillator and one sample of undoped scintillator, in red. The QDs significantly degrade the light yield of the scintillator mixture. Scintillator doped with QDs with shorter emission wavelengths produce more light than those with longer emission wavelengths. Toluene cocktails containing QDs produced a smaller quantity of light than those without QDs. Scintillators doped with QDs with shorter peak emission wavelength produced a larger quantity of light that those with QDs of longer peak emission wavelengths. 8

9 Table 4: Listed below are the fit results for each scintillator solution s data set. Solvent p 1 [a.u.] p 2 [g/l] p 3 [l/g] Toluene and 757 ± ± ± Toluene and 757 ± ± ± PPO (year-old) PC and PC and PPO (year-old) LAB and LAB PPO (year-old) PXE and PXE and DIN and DIN PPO (year-old) DIN HP and DIN HP PPO (year-old) PCH and PCH PPO (year-old) 9

10 5 Conclusion Next generation of liquid scintillator detectors searching for neutrinoless doublebeta decay will need to reduce backgrounds. The ability to extract a direction signal would be a powerful way to reduce background. Doping scintillator with QD allows fine tuning of emission wavelength of scintillator to shorter wavelengths thus enabling the Cherenkov light that contains directional information to reach the PMT before scintillation light. In this work, the light yields fifteen scintillator solutions, some doped with quantum dots, were measured and studied for suitability. Year-old samples of scintillator cocktail mixed with varying amounts of PPO yield quantities of less light than newly mixed samples. This effect was due to oxygen penetration, and therefore should not be significant for large scale detectors with larger volume to surface area ratio. QDs with shorter emission wavelengths are more viable for scintillators as they produce more light that QDs with longer emission wavelengths. QDs substantially degrade the light yield of scintillator. With improved technology in quantum dot production, QDs with better characteristics, such as shorter emission wavelengths, are expected to become available in the near future. These QDs should be able to improve the light yield of the the quantum dot-doped scintillator. 6 Acknowledgments The author would like to thank Dr. Lindley Winslow for her expertise and approachability. The author would like to thank Robert Schofield. Thanks to the National Science foundation for enabling this research project, and thanks to Dr. Francoise Queval for her dedication to making the summer REU experience both educational and enjoyable. 10

11 (a) Toluene (b) PC (c) LAB (d) PXE (e) DIN (f) DIN HP 11 (g) PCH Figure 5: Figures (a)-(g) show the charge value at half the Compton edge versus the PPO concentration of seven scintillators.

12 Figure 6: The histograms of charge (which is proportional to light yield) of various samples of.5g/l of QD in toluene and PPO solutions. 12

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