LANTERN: Characterization technology for low threshold cryogenic detectors

Abstract

The use of low-temperature detectors, such as cryogenic calorimeters, has pioneered the recent advancements in low-energy rare event searches. These detectors provide a low-noise environment essential for the direct detection of dark matter and neutrinos. Characterizing these detectors within the region of interest (ROI), typically spanning from O(10~eV) to O(1~keV), has proven to be a challenging task. Conventional radioactive sources produce signals above this range, leading to nonlinearities and saturation effects. Moreover, these detectors are usually deployed in low background environments, meaning that having a radioactive source during physics runs can spoil the measurement making the use of this type of solution unfeasible. As a solution to these issues, we introduce LANTERN, an optical calibration system designed for highly segmented cryogenic calorimeters. LANTERN utilizes the photostatistics resulting from the absorption of monochromatic UV-Vis photons emitted by LEDs to analyze the detector response curve, without needing prior knowledge of the total energy deposited. The system employs a fast-switching LED matrix that operates at excitation times faster than the typical response of cryogenic detectors and can currently characterize up to 64 calorimeters independently. In this work, the validation of the final electronics designed for the project is shown. The first test was carried out by calibrating one of the cryogenic detectors of the BULLKID-DM experiment and checking the energy-reconstruction error of the spectral features produced by the surrounding lead casing. An error of $\approx 2\%$ has been observed in the energy reconstruction. The second validation was carried out by cross-calibrating one of the CALDER thin detectors with a commercial LED driver, and compatible results between the two setups were achieved.

Figures (9)

• Figure 1: In blue a simulated pulse generated during an optical calibration is show along side the triggering signal sent to the LED driver plotted in orange. In the inset a zoom showing the fine structure of the trigger is presented: the 10 periods of a 5 MHz square wave is used as the trigger and each period corresponds to a single LED burst.
• Figure 2: Example of the optical calibration procedure. In the top panel the different energy distributions of the light depositions are shown and fitted with a Gaussian. In the bottom panel, the mean and variance of the distributions are plotted and fitted with a first degree polynomial to extract the calibration parameters of Eq. \ref{['eq:abscal_func']}. The figure is taken from lantern1.
• Figure 3: Optical spectrum of the photons emitted by different LED sources. In orange the LLS-UV400 LED used in the LANTERN electronics, while in blue the SP5601 CAEN commercial LED driver are shown. Both spectra have been measured using a PyLoN:100BR_eXelon CCD pylon and renormalized for the quantum efficiency of the device.
• Figure 4: Example of the estimation of the nonlinearity of the detector using the optical calibration setup. The mean of the energy depositions of Fig. \ref{['fig:calib_example']} is plotted against the number of bursts used to produce the distribution. The data points are then fitted with Eq. \ref{['eq:non_linear']} to extract the energy nonlinearity correction parameters.
• Figure 5: Development of the LANTERN optical calibration electronics. Panel A: design of the single LED driver developed around the use of the BSS123 MOSFET (M) to bias the LED with an externally generated triggering signal T. The R$_1$ resistor is used to speed up the discharge of the LED and the capacity C is used to protect the circuit from possible high frequency fluctuations of the bias voltage V$_\text{CC}$. Panel B: conceptual design of the multiplexing scheme used to up-scale the number of optical channels. Four multiplexers are employed to redirect the triggering signal to one of the available LED drivers, which are all connected in parallel to the bias voltage. A digital potentiometer is used to accurately regulate the voltage reaching the LED driver, making the luminosity easily adjustable. Panel C: Picture of the final printed circuit board of LANTERN with 64 channels. Only one LED is mounted to perform the initial validation of the electronics.