Eur. Phys. J. C (2018) 78:329 https://doi.org/10.1140/epjc/s10052-018-5807-z Regular Article - Experimental Physics Measurements of the intrinsic quantum efficiency and absorption length of tetraphenyl butadiene thin films in the vacuum ultraviolet regime Christopher Benson1,2,a, Gabriel Orebi Gann1,2, Victor Gehman2 1 University of California, Berkeley, CA 94720, USA 2 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received: 18 September 2017 / Accepted: 13 April 2018 © The Author(s) 2018 Abstract A key enabling technology for many liquid noble and collecting the ionization signal. The specifics of the scin- gas (LNG) detectors is the use of the common wavelength tillation process in noble gasses means they are almost trans- shifting medium tetraphenyl butadiene (TPB). TPB thin films parent to their own scintillation light. Additionally, the time are used to shift ultraviolet scintillation light into the visible structure of that light is dependent on the incident particle spectrum for detection and event reconstruction. Understand- type, allowing for pulse shape analysis to distinguish nuclear ing the wavelength shifting efficiency and optical properties from electron recoils. of these films are critical aspects in detector performance and Detection of the vacuum ultraviolet (VUV) scintillation modeling and hence in the ultimate physics sensitivity of such light produced by noble gas targets is a critical challenge experiments. This article presents the first measurements of common to this class of detectors. As shown in Fig. 1,the the room-temperature microphysical quantum efficiency for scintillation wavelengths can range from 175 nm for Xenon vacuum-deposited TPB thin films – a result that is indepen- down to near 80 nm for Helium and Neon. Light of these dent of the optics of the TPB or substrate. Also presented wavelengths is strongly absorbed by most materials, includ- are measurements of the absorption length in the vacuum ing those commonly used for optical windows. Many exper- ultraviolet regime, the secondary re-emission efficiency, and iments sidestep the issue of directly detecting VUV light more precise results for the “black-box” efficiency across a though the use of wavelength shifting (WLS) films which broader spectrum of wavelengths than previous results. The absorb the VUV light and re-emit photons, typically in the low-wavelength sensitivity, in particular, would allow con- visible spectrum. The visible photons can then easily be struction of LNG scintillator detectors with lighter elements detected using photomultiplier tubes (PMTs). (Ne, He) to target light mass WIMPs. As experimental programs look toward lower energies, the use of helium and neon as a detecting medium is attractive. The scintillation wavelengths of these noble gases are around 1 Introduction and motivation 80 nm, deep into the VUV.This provides motivation to extend previous measurements of wavelength shifting materials into Noble-gas detectors are becoming important to numerous these extreme VUV regimes. experimental efforts involving: dark matter searches; neu- One commonly used WLS is tetraphenyl butadiene (TPB). trino and other particle detectors; searches for the neutron’s TPB thin films have been widely studied [16–26] and are electric dipole moment; and measurements of the neutron regularly used for many experimental programs [2–9] due lifetime [1–15]. The properties of these elements, particularly to their relatively high efficiency. TPB may also be easily in the liquid phase, are very attractive. They have exception- applied to surfaces using standard techniques, such as vac- ally high scintillation yield (20,000–40,000 photons/MeV), uum deposition in a thermal evaporator. The reemission spec- which results in excellent energy resolution and low thresh- trum of TPB has been measured and its shape well understood old. Their high density makes them more self-shielding than at cryogenic temperatures [22] and room temperature [16]. water or organic liquid scintillators. Tracking detectors can The wavelength shifting efficiency (WLSE) of thin films of be constructed by applying an electric field across the bulk TPB vacuum deposited on an acrylic substrate has been mea- sured, without reference to other materials, in the range of a e-mail: [email protected] 123 329 Page 2 of 19 Eur. Phys. J. C (2018) 78:329 Fig. 1 Scintillation wavelengths for various noble gases, along with the transmission of some commonly used optical windows [16] Fig. 2 A cartoon schematic of the experimental setup for the long wavelength configuration (Sect. 3.1) 120–250 nm [16]. The WLSE as defined in [16] is a “black- 2 Experimental apparatus box” definition of the efficiency that includes both the intrin- sic QE of the TPB as well as certain optical properties of The primary objective of this work is to measure the intrinsic the TPB film and substrate. The resulting quantity is the effi- QE of TPB and the characteristic VUV photon absorption ciency that a photon absorbed by the sample is reemitted length in TPB, and its dependence on incident wavelength. from the sample. This measurement is thus sample depen- Also measured are the room-temperature reemission spec- dent, including effects of scattering and absorption, and can- trum and the black-box WLSE, with its dependence on sam- not be directly applied to other apparatus. ple thickness and incident wavelength. These measurements This article presents the first ever measurement of the are performed by exposing TPB thin film samples to UV intrinsic quantum efficiency (QE) of TPB evaporated thin light of a known wavelength and intensity and measuring the films, defined as the probability that a photon absorbed by a amount and spectrum of light reemitted from fluorescence. TPB molecule is reemitted, and its dependence on incident A cartoon representation of the experimental apparatus wavelength. Also presented are measurements of the absorp- is shown in Fig. 2. The setup consists of a broad spectrum tion length of TPB in the VUV regime and a measurement of UV light source, filters, optical elements, a monochromator, the average efficiency of the secondary reemssion process, a sample mounting assembly and photon detectors. All of referred to as SQE, in the region where the TPB absorp- the apparatus components are installed within a windowless tion and reemission spectra overlap. As part of this work, vacuum chamber and held at vacuum. The setup is designed room temperature measurements of the reemission spectrum to produce and project monochromatic UV light onto thin of TPB as a function of incident wavelength are presented. film WLS samples in a repeatable fashion. A photodiode is The WLSE as defined in [16] is also studied, for the purpose used to separately measure the flux of photons incident on and of a direct comparison to this work. These results cover the reemitted by the samples. A spectrometer is used to measure spectral range studied by [16] with improved precision, while the reemission spectrum of the samples. extending the measurements down to 50 nm. Individual elements used in the experimental apparatus are Section 2 provides an overview of the hardware compo- described in this section. Because this work covers a wide nents of the experimental apparatus. Section 3 describes the range of wavelengths (50–250 nm), several distinct hardware various apparatus configurations used to cover the full wave- configurations are used. Each of these configurations are con- length region of interest (ROI). Section 4 describes the sam- structed from elements described in this section. The distinct ple fabrication. Section 5 describes the data acquisition meth- apparatus configurations are described in Sect. 3. ods. Section 6 contains descriptions of the analysis methods used to calculate the absolute WLS efficiency, the detailed Monte Carlo model used to simulate the setup, and the tech- 2.1 UV light source niques to extract the VUV QE, VUV absorption lengths and SQE. Section 7 presents results. Section 8 provides a compar- This work uses two distinct light sources. Each light source ison of this work to previous studies and discusses the impact is specialized to a subset of the entire 50–250 nm wavelength of various improvements to the measurements. Conclusions ROI. Only one light source may be attached to the setup at a are presented in Sect. 9. time. 123 Eur. Phys. J. C (2018) 78:329 Page 3 of 19 329 2.1.1 Deuterium light source pass optical filters which allow measurements of background levels (Sect. 5.1.4). The McPherson Model 623 light source, referred to as the Deuterium Light Source (DLS), produces light for measure- 2.2.2 Focusing mirror ments in the range of 125–250 nm. The DLS contains a sealed deuterium gas volume which is ionized to produce A McPherson Model 615 focusing elbow sits between the broad spectrum light with a bright UV component. The out- filter wheel and monochromator entrance slit. The focus- + put light passes through a circular 1” diameter MgF2 exit ing elbow contains a curved Al MgF2 focusing mirror window. The MgF2 window has a transmission cutoff wave- which focuses the light passing through the filter wheel onto length of 115 nm which drives the lower limit of emission the monochromator entrance slit. The focus of the mirror is for this source [27]. The DLS is powered using a McPherson adjusted using three set screws and is configured to maximize Model 732 power supply. the light output at the monochromator’s exit for wavelengths in the ROI. 2.1.2 Windowless light source 2.2.3 Monochromator The McPherson Model 629 Hollow Cathode Gas Discharge Source, referred to as the windowless light source, produces A McPherson Model 234/302 VacuumUltraviolet Monochro- light for measurements in the range of 45–150 nm. Unlike the mator is used to output monochromatic light of a selected DLS, the windowless light source does not have a window wavelength from a broad spectrum input.