Lead perovskites as CEνNS detectors

Jesús-Valls, César; Sánchez, Federico

doi:10.3389/fphy.2023.1191954

ORIGINAL RESEARCH article

Front. Phys., 17 August 2023

Sec. High-Energy and Astroparticle Physics

Volume 11 - 2023 | https://doi.org/10.3389/fphy.2023.1191954

Lead perovskites as CEνNS detectors

César Jesús-Valls¹*

Federico Sánchez²*

¹Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo Institutes for Advanced Study, University of Tokyo, Kashiwa, Japan
²Section de Physique, University of Geneva, Geneva, Switzerland

Introduction: The recent discovery of coherent elastic neutrino-nucleus scattering (CEvNS) has created new opportunities to detect and study neutrinos. The interaction cross section in CEvNS scales quadratically with the number of neutrons, making heavy-nuclei targets such as active lead-based detectors ideal. Lead perovskites have emerged in the last decade as revolutionary materials for radiation detection due to their heavy and flexible element composition and their unique optoelectronic properties that result in an excellent energy resolution at an economic cost.

Methodology: In this study, we discuss, for the first time, the physics potential and feasibility of building neutrino detectors using semiconductor lead perovskite crystals as a target.

Results and Discussion: We indicate that existing data with x-rays suggest the suitability of existing lead perovskite sensors to study CEvNS using neutrinos from π decay at rest (π- DAR) sources. Although dedicated research and development will be necessary, we have found significant benefits and no inherent obstacles for the development of lead perovskites as CEvNS detectors.

1 Introduction

Neutrinos are the only known fermions carrying exclusively weak charges and, therefore, are clean probes of the weak interaction and unique messengers of dense matter environments, unaffected by strong and electromagnetic interactions. These appeals, however, result in notably suppressed interaction cross sections, hampering the study of neutrino physics and rendering most applications impractical.

In 1974, the existence of coherent elastic neutrino-nucleus scattering (CEνNS) was pointed out as a consequence of the standard model [1]. In CEνNS, a neutrino transfers momentum to a whole nucleus via the exchange of a virtual Z boson, forcing it to recoil. The interaction cross section for this process is

\begin{align} \frac{d σ^{CE ν NS}}{d E_{R}} = & \frac{G_{F}^{2}}{8 π \cdot {(ℏ c)}^{4}} {(N + (1 - 4 \sin^{2} θ_{W}) Z)}^{2} \\ \cdot m_{N} \cdot (2 - E_{R} m_{N} / E_{ν}^{2}) | f (q) |^{2} \end{align} (1)

where G_F is the Fermi constant, N (Z) is the number of neutrons (protons), θ_W is the Weinberg angle, and m_N and E_R are the nucleon mass and its recoil energy, respectively. The nuclear form factor f(q) characterizes the loss of coherence as a function of the transferred momentum q = $\sqrt{2 m_{N} E_{R}} / ℏ$ , and it is close to unity for small q, associated with typical neutrino energies E_ν ≲ 50 MeV. Notably, given that 4 sin²θ_W ∼ 1, $σ^{CE ν NS} \overset{\infty}{\sim} N^{2}$ [2]. This remarkable interaction cross section enhancement, however, offers a very challenging detection signal as the nucleon recoil needs to be identified. The maximum recoil energy scales as $E_{R}^{max} \approx 2 E_{ν}^{2} / m_{N}$ so that detectors need to be able to measure recoil energies of, at most, several tens of keV. Thanks to recent advancements in detector technology, experimentally studying CEvNS has become possible recently, as demonstrated by the COHERENT collaboration using a CsI target [3] and an Ar target [4].

2 Motivations

The discovery of CEνNS and its enhanced cross section shows potential to mitigate the elusiveness of neutrinos and therefore revolutionize their study at energies on the order of a few tens of MeV, which include geoneutrinos [5], reactor neutrinos [6], accelerator neutrinos from meson decays at rest [7-10], solar neutrinos [11], and supernova neutrino bursts [12]. Characterizing the cross section of CEνNS is also essential for dark matter searches as CEνNS constitutes an irreducible background, the so-called neutrino floor [13]. Being mediated by flavor-insensitive neutral currents, the detection of CEνNS provides extended sensitivity to sterile neutrinos [14-16] and other new physics signatures [17-20], and allows the study of the neutrino magnetic moment [21, 22], its effective charge radius [23], and the nuclear neutron form factor [24, 25]. Applications, such as deploying neutrino detectors to increase nuclear security [26, 27], might also be possible. Moreover, CEνNS is relevant to theoretical astrophysics as a key actor during stellar collapse [28-30].

3 CEνNS experiments

Because of the aforementioned findings, an increasing number of CEνNS detector technologies have been proposed [31-42], and several experiments are ongoing or have been proposed: COHERENT [43], using CsI, NaI, high-purity Ge (HPGe), and liquid-Ar targets; CONUS [44], NCC-1701 [45, 46], and νGEN [47] using cryogenic HPGe; MINER [48], using cryogenic HPGe/Si; NUCLEUS [49], using cryogenic CaWO₄ and Al₂O₃; CONNIE [44], using Si charge-coupled devices (CCDs); TEXONO [50], using p-type point-contact Ge; RES-NOVA, using cryogenic PbWO₄ [51, 52]; RICOCHET [53], using cryogenic HPGe bolometers; and RED100 [54], using liquid-Xe.

To get the most from CEνNS, an ideal detector should be inexpensive to produce and operate, have excellent energy resolutions to identify nuclear recoils with an energy of a few keV, and be made of heavy nuclear targets to exploit the quadratic scaling of the cross section. In this study, we point out, for the first time, the excellent prospects of lead perovskites to build up future CEνNS detectors and discuss their experimental feasibility in light of existing measurements.

4 Lead perovskites

Lead halide perovskites (LHPs) are novel semiconductors with exceptional optoelectronic properties, a versatile chemical composition, and low-cost synthesis. They typically consist of crystals with structure APbX₃, as shown in Figure 1, where A is CH₃NH₃₊ (MA⁺), ${C H N H}_{3}^{+}$ (FA⁺) or Cs⁺; B is Pb²⁺; and X is Cl⁻, Br⁻, and I⁻ [55].

FIGURE 1

FIGURE 1. Schematic representation of the perovskite ABX₃ crystal structure.

The study of halide perovskites as photosensors was sparked about a decade ago in the context of solar cell development [56] and quickly emerged as an active field of research due to record energy conversion efficiencies [57-64]. Along the process, much has been learned about the basic properties of this material, which combines a low exciton binding energy on the order of few meV [65] with exceptionally long electron–hole diffusion lengths exceeding 1 μm [66], a tunable band gap in the range of 1.2–2.4 eV [67-68], and a high bulk resistivity of 10⁷⁻¹⁰Ω⋅cm at room temperature [69]. The aforementioned combination is unique as it pairs efficient charge carrier production and mobility at a low voltage bias with a high bulk resistivity and orders of magnitude higher than those of Si and Ge, suppressing dark current and noise. Moreover, LHPs naturally allow for the manufacture of crystals with very high atomic numbers, such as CsPbI₃, and the design of application-specific perovskite sensors by means of stoichiometry engineering [70, 71]. Furthermore, the synthesis of LHPs is easy and flexible through techniques such as solution processing and melt growth, and single crystals with sizes > 1cm³ can be routinely built [72]. The production cost is also low, with an estimated price of <0.3$/cm³ [55], namely, at a density of 4 g/cm³ and an inexpensive cost of 75 $/kg. Finally, LHPs can be operated inexpensively at room temperature.

5 Perovskites as radiation detectors

Lead perovskites' striking performance as solar cells and their high atomic numbers¹ quickly attracted the interest of the medical imaging community toward this material as x/γ-ray detectors [73-81]. In 2015, MAPbI₃ was proven to detect γ-rays from ¹³⁷Cs [82], and the first x-ray images were obtained [83]. Since then, a steady improvement in x/γ-ray performance metrics and achievements has been reported over time [72, 74, 83, 84], including the best x-ray sensitivities yet achieved in any material [85, 86]. Other radiation types have also been studied with perovskites, specifically neutrons [87] and α [88, 89] and β [90] particles. For a recent review, see [91]. Moreover, perovskite nanoparticles show enormous potential as wavelength shifters, for a review see [92], making them interesting doping materials (≈1 g/L) to build nanocrystal-doped liquid scintillators, with applications in neutrino detection [93]. In this study, nevertheless, we focus on solid lead perovskite crystals as a target, i.e., a detector where 100% of the active volume is made of lead perovskite, enhancing the cross section of CEνNS enhanced due to the presence of lead. As crystals, despite the many achievements that are previously listed, all reported precision measurements involving low-energy O (10–100) keV particles have been based on the detection of recoiling electrons induced by x-ray interactions. Possibly because of this finding, no mentions exist in the literature about the possibility of measuring CEνNS using perovskites. In CEνNS, a nuclear recoil, instead of an electron recoil, needs to be measured. For Ge, it has been measured that nuclear recoils generate about a third of the ionization signal of their electronic counterparts [94]. For lead perovskites, this fraction, the so-called quenching Q, is still unknown. Quenching acts by reducing the signal, therefore, reported sensor metrics in x/γ-ray measurements that are expected to degrade when used to study CEνNS. If Q^perovskite ≈ Q^Ge, then the energy resolution E_res for nuclear and electron recoils can be related by

E_{res}^{nuclear} \approx E_{res}^{electron} \times Q^{perovskite} \approx E_{res}^{electron} / 3 . (2)

Existing $E_{res}^{electron}$ measurements are presented in Figure 2. These data were reported in 2021 using CsPbBr₃ lead perovskite crystals at room temperature [95]. A stable operation was achieved with them for over 18 months. A fit to the data smoothly reproduces the trend. Using the fit, we calculate that the energy resolution would get worse than 100% for photon energies below 4.3 keV. Taking this value as a reference to define an approximate detection threshold, Eq. 2 suggests that, if lead perovskite quenching is similar to that of Ge, existing lead perovskite sensors could have a detection threshold similar to 15 keV for nuclear recoils. Certainly, a definitive answer requires an experimental determination of Q^perovskite, a measurement that we encourage for the first time in this study. It must be emphasized that existing lead perovskite sensors are still far from their ultimate energy resolution [95], and therefore, future sensors should lead to even lower detection thresholds. Room for improvement ranges from an increase in the detected signal, e.g., reducing crystal defects [96] and improving the electrode contacts [97, 98], to a decrease in noise, e.g., passivating the sensor surfaces [99], using dopant compensation [100, 101], or operating at cryogenic temperatures. In this way, even if future measurements show that Q^perovskite > Q^Ge, current data and sensor improvement trends suggest that reaching O (10) keV nuclear recoil detection thresholds will likely be possible in the near future.

FIGURE 2

FIGURE 2. Fit to the energy resolution measured for single photons using a CsPbBr₃ perovskite at room temperature. Data from [95]. The best parameters are A, B, and C = {4.95, −1098.24, and 189690.18}.

6 Prospects as CEνNS detectors

Producing low-activity lead perovskites should be readily possible, e.g., CsPbI₃ consists of Cs and I, both used in the first historical detection of CEνNS [3], and archaeological Pb has recently been demonstrated to be adequate for CEνNS detection [102]. Moreover, CsPbI₃ and other lead perovskites are made up of strikingly heavy elements, significantly advantaging the CEνNS interaction cross section of mainstream alternative materials and, in particular, that of Ge. However, the maximum recoil energy decreases linearly with m_N, and therefore, the ability of the detector to identify the recoiling nucleus needs to be considered. To account for it, we define the effective cross section, σ_eff, as a figure of merit, defined as

σ_{eff} \equiv \int_{E_{threshold}^{recoil}}^{E_{R}^{max}} \frac{d σ}{d E_{R}} ϵ d E_{R} (3)

which can be calculated from Eq. 1 if the detector efficiency, ϵ, is specified. Using it, in Figure 3 CsPbI₃ and Ge targets² are directly compared for some neutrino energies, assuming a detector with perfect (null) efficiency above (below) a certain energy recoil threshold, $E_{threshold}^{recoil}$ .

FIGURE 3

FIGURE 3. CEνNS interaction cross section per nucleus, σ, multiplied by the detector efficiency, ϵ, as a function of the recoil energy threshold, $E_{threshold}^{recoil}$ . Solid (dashed) lines correspond to CsPbI₃ (Ge).

If, as suggested in the previous section, $E_{threshold}^{recoil} \approx$ 15 keV in existing lead perovskite sensors, studying 30–50 MeV neutrinos could be readily possible. Interestingly, this neutrino energy range overlaps with the energy spectrum of neutrinos produced in pion decay at rest (π-DAR) neutrino sources [103, 104]. π-DAR neutrinos have been used in the only two CEνNS measurements so far, using 14.6 kg of CsI [3] and 24 kg of argon [4]. Building and operating similar masses of lead perovskite poses no apparent impediment, with the driving cost being the number of electronic channels. If sensor masses similar to 1 g are deployed, a reasonable and potentially scalable O (10⁴) number of electronic channels would be needed to set up the experiment. CEνNS experiments at π-DAR sources are primarily counting experiments that observe the event rate variation induced by switching on and off the neutrino beam, allowing to characterize the background levels and cancel out effects related to the detector efficiency. Neutrino energy is not reconstructed. Instead, the measurement observable is directly the reconstructed signal distribution above the detection threshold (see, for instance, [3]). Signal interactions are contained in individual sensors, and therefore, no spatial resolution is needed. If the detector is deployed as a dense array of lead perovskite sensors, identifying nearby sensor coincidences could be used to assist auxiliary veto modules to reject the background.

The comparison between CsPbI₃ and Ge in Figure 3 reflects that for a given fixed neutrino energy, lead perovskites require a smaller $E_{threshold}^{recoil}$ to observe CEνNS. However, if the detection threshold is achieved and mildly lowered, it results in a large enhancement of the interaction cross section. This trade-off is characterized by the ratio $σ_{eff}^{C s P b I_{3}} / σ_{eff}^{G e}$ presented in Figure 4. For $E_{threshold}^{recoil} \approx$ 15 keV, as previously suggested, Ge and CsPbI₃ would lead to similar event rates for π-DAR neutrinos. However, although the fabrication and operation of Ge sensors are nearly optimal, perovskite R&D shows potential to lower its $E_{threshold}^{recoil}$ in the next few years, resulting in an up to six-fold event rate increase compared to Ge. Moreover, such a detection improvement would also open the door to investigating the use of lead perovskites to measure neutrinos from other sources, e.g., supernova and reactor neutrinos. Lastly, perovskites are orders of magnitude cheaper to manufacture and potentially operate³ than existing alternatives, including HPGe.

FIGURE 4

FIGURE 4. The color map depicts the ratio of $σ_{eff}^{C s P b I_{3}} / σ_{eff}^{G e}$ as a function of the neutrino energy, E_ν, and the recoil energy threshold, $E_{threshold}^{recoil}$ . A ratio value of zero indicates that $E_{threshold}^{recoil}$ is above the necessary level to observe any recoil in CsPbI₃. To help the visualization, values of some particular integer ratios are highlighted by color lines.

7 Discussion and outlook

In just one decade, lead perovskites have been established as novel materials with transformative potential as radiation detectors due to their unique optoelectronic properties.

In this study, we highlight their potential as neutrino detector targets and discussed, for the first time, their suitability for the study of CEνNS. In particular, we note that existing x-ray data indicate that current lead perovskites sensors might already be suitable to study π-DAR neutrinos and discuss their implications. In general, with the available data, no impediments are apparent that prevent further development of the concept of lead perovskites for neutrino detection. Nonetheless, we highlight the necessity of determining the quenching fraction for recoiling the nucleus in lead perovskites to evaluate its exact effect. In any case, to bring perovskites to their ultimate detection potential and enable their full range of applications, active R&D is required. In particular, efforts to optimize lead perovskite sensors for the detection of single low-energy particles would be significantly beneficial for the development of this technology within the field of experimental neutrino physics.

Lastly, we note that CEνNS and some dark-matter models share the same signal mechanism, i.e., the detection of nuclear recoils. Therefore, any progress in this direction might benefit both the neutrino and dark-matter research communities.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

Author contributions

CJ-V and FS contributed to the conception and design of the study. CJ-V performed the cross section calculations. CJ-V wrote the first draft of the manuscript. CJ-V and FS wrote sections of the manuscript. All authors contributed to the article and approved the submitted version.

Funding

This project was partially inspired by the ZPro project funded by the Barcelona Institute of Technology (BIST). Open access funding was provided by the University of Geneva.

Acknowledgments

The authors acknowledge fruitful discussions with E. Palomares and valuable feedback from J. I. Collar and L. Pattavina.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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Footnotes

¹Photon attenuation increases ∝ Z⁴, where Z is the atomic number.

²For CsPbI₃, the weighted average (Cs + Pb+3I)/5 is used in the result of Eq. 3.

³Perovskites might be able to operate without the need for cryogenic systems, as supported by existing x-ray data.

References

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