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Characterization of Low-energy Ionization Signals in Silicon Detectors for the Nab Experiment

R. J. Taylor, August Mendelsohn, Arlee Shelby, W. C. McCray, Jin Ha Choi, Nicholas Macsai, Grant Riley, Erick Smith, Stefan Baeßler, Leah J. Broussard, Christopher B. Crawford, Michael Gericke, Francisco M. Gonzalez, David Harrison, Leendert Hayen, Mark Makela, R. R. Mammei, D. G. Mathews, D. D. Počanić, Glenn Randall, Americo Salas-Bacci, W. S. Wilburn, A. R. Young

TL;DR

The study provides a comprehensive characterization of large-area silicon drift detectors for the Nab experiment, focusing on low-energy proton response, energy calibration, and timing performance important for the beta-antineutrino angular correlation measurement. By combining 30 keV protons from a Manitoba source with calibration lines from 113Sn and 109Cd, the authors robustly determine the detector’s energy response, dead-layer thickness, and per-pixel calibration, including temperature and energy-loss corrections modeled with Geant4. They further employ NESSE pulse-shape simulations to connect impurity-density profiles to rise-time differences and timing biases, showing that proton–electron timing offsets can be predicted and constrained to well below the 0.3 ns requirement. The results indicate negligible cross-talk (<1%), stable proton peaks over extended periods, and operating conditions near -300 V and ~120 K that will enable Nab to reach its 0.1% precision in the beta-neutrino angular correlation measurement.

Abstract

The Nab (Neutron a b) experiment is designed to measure the beta-antineutrino angular correlation in free neutron $β$ decay with an ultimate precision goal of 0.1%, providing input for tests of Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity. This measurement is performed via detection of electrons and protons in delayed coincidence using custom large-area segmented silicon drift detectors. We present the characterization of one such detector system to establish the proton energy and timing response, using a dedicated proton accelerator. The detected proton peak was studied for 25 keV, 30 keV, and 35 keV incident protons on a set of detector segments and multiple cooling cycles over a one year period. Ionization losses were consistent with models of the detector dead layer with thicknesses less than 100nm. The detected proton peak was stable within the uncertainty from energy calibration (0.2 keV). The rise times of detector pulses from $^{109}$Cd and $^{113}$Sn conversion electron sources were used to extract the impurity density profile and establish a precise model for the detector timing response. The observed impurity density profile varied from $(2 \pm 2) \times 10^9$ cm$^{-3}$ at the center to $(26 \pm 2) \times 10^9$ cm$^{-3}$ at the edge. This impurity density profile was then used to characterize systematic effects in proton time-of-flight measurements due to detector pulse-shape effects; the resultant proton timing systematic uncertainties were below 0.3 ns, which is sufficient for the Nab experiment.

Characterization of Low-energy Ionization Signals in Silicon Detectors for the Nab Experiment

TL;DR

The study provides a comprehensive characterization of large-area silicon drift detectors for the Nab experiment, focusing on low-energy proton response, energy calibration, and timing performance important for the beta-antineutrino angular correlation measurement. By combining 30 keV protons from a Manitoba source with calibration lines from 113Sn and 109Cd, the authors robustly determine the detector’s energy response, dead-layer thickness, and per-pixel calibration, including temperature and energy-loss corrections modeled with Geant4. They further employ NESSE pulse-shape simulations to connect impurity-density profiles to rise-time differences and timing biases, showing that proton–electron timing offsets can be predicted and constrained to well below the 0.3 ns requirement. The results indicate negligible cross-talk (<1%), stable proton peaks over extended periods, and operating conditions near -300 V and ~120 K that will enable Nab to reach its 0.1% precision in the beta-neutrino angular correlation measurement.

Abstract

The Nab (Neutron a b) experiment is designed to measure the beta-antineutrino angular correlation in free neutron decay with an ultimate precision goal of 0.1%, providing input for tests of Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity. This measurement is performed via detection of electrons and protons in delayed coincidence using custom large-area segmented silicon drift detectors. We present the characterization of one such detector system to establish the proton energy and timing response, using a dedicated proton accelerator. The detected proton peak was studied for 25 keV, 30 keV, and 35 keV incident protons on a set of detector segments and multiple cooling cycles over a one year period. Ionization losses were consistent with models of the detector dead layer with thicknesses less than 100nm. The detected proton peak was stable within the uncertainty from energy calibration (0.2 keV). The rise times of detector pulses from Cd and Sn conversion electron sources were used to extract the impurity density profile and establish a precise model for the detector timing response. The observed impurity density profile varied from cm at the center to cm at the edge. This impurity density profile was then used to characterize systematic effects in proton time-of-flight measurements due to detector pulse-shape effects; the resultant proton timing systematic uncertainties were below 0.3 ns, which is sufficient for the Nab experiment.

Paper Structure

This paper contains 31 sections, 7 equations, 21 figures.

Table of Contents

  1. Introduction
  2. Nab Detector Requirements
  3. Apparatus and Measurement Overview
  4. The Nab Detection System
  5. Nab Silicon Drift Detectors
  6. Amplifier and Electronics
  7. Detector Cooling
  8. The Manitoba II Proton Source
  9. Radioactive Calibration Sources
  10. Data Summary
  11. Detector Energy Calibration
  12. Trapezoidal Filter Optimization
  13. Integral Cross Talk Assessment
  14. Calibration with Radioactive Sources
  15. Temperature Correction
  16. ...and 16 more sections

Figures (21)

  • Figure 1: Top-down view of the vacuum vessel containing the detector. A retractable source arm can be rotated to expose one of two radioactive sources, or the collimated beam of protons can directed to the desired pixel using the electro-static steerer.
  • Figure 2: The silicon detector is affixed to the front of an assembly referred to as the "detector system," the internals of which are depicted above. The detector system is composed of the JFET and amplifier assemblies, the amplifier liquid cooling, and the cryogenic detector cooling system. Each amplifier card contains six channels (amplifier circuits for six detector pixels - see Fig. \ref{['fig:spice_model']}). There are 22 power cables in addition to the 127 coaxial signal cables (not shown). The dashed lines at the bottom left indicate the mounting position for the detector and detector transition board. This assembly is installed in a stainless steel tube to separate the FET vacuum from the ultra-high vacuum detector volume.
  • Figure 3: The front-end amplifier circuit modeled in LT-SPICE ltspice. The trans-impedance amplifier is composed of a common-source BF862 JFET (J1) coupled to an AD8011 video amplifier (U1). The detector, JFET, and feedback components (C3, R3) are all cooled via LN$_2$. T1 and T2 represent the "transition boards" that feed signals between the warm and cold sections. Following the trans-impedance stage, there is an integrator (C2, R4) followed by two amplification stages that act as differentiators (U2, U3). U4 and U5 are protection diodes for the input bias to U1. R9 and R10 are in place for impedance matching.
  • Figure 4: Pixels studied with $^{109}$Cd, $^{113}$Sn, and protons are highlighted in red, whereas pixels only studied with radioactive sources are in blue. Due to amplifier stability constraints, further pixels were not instrumented. The pattern of powered pixels is due to pre-amplifier PCBs having up to six channels per card, and electronic stability constraints (see Sec. \ref{['subsubsec:Amplifier']}).
  • Figure 5: An example of the energy standard deviation ($\sigma$) versus peaking time for the $^{113}$Sn 364k eV conversion electron peak. The data is fit to a bi-logarithmic function: $y=\ln(Ae^{-x/B}+Ce^{x/D})$. The minimum of the bi-logarithmic fit (shown in blue) indicates the minimal noise contribution to the trapezoidal filter energy extraction. Therefore it is used to set the peaking time of the filter algorithm.
  • ...and 16 more figures