Calibration of Solid State Nuclear Track Detectors for Rare Event Searches

TL;DR

MoEDAL's search for highly ionizing particles requires precise calibration of CR39 and Makrofol solid-state track detectors. The paper exposes detector stacks to heavy-ion beams across a wide energy range, etches them under controlled conditions, and measures track dimensions to map the detector response to REL and $Z/β$ using Monte Carlo energy-loss simulations. It reports thresholds at $Z/β$ ~ 7 for CR39 and ~50 for Makrofol, with the response described by 4th-degree polynomial fits of $p-1$ versus REL. The calibrated detectors enhance MoEDAL's sensitivity to heavy ions and exotic charged particles, improving background rejection and the ability to set limits or potentially discover new physics in rare-event searches.

Abstract

The calibration of the CR39 and Makrofol Nuclear Track Detectors of the MoEDAL experiment at the CERN-LHC was performed by exposing stacks of detector foils to heavy ion beams with energies ranging from 340 MeV/nucleon to 150 GeV/nucleon. After chemical etching, the base areas and lengths of etch-pit cones were measured using automatic and manual optical microscopes. The response of the detectors, as measured by the ratio of the track-etching rate over the bulk-etching rate, was determined over a range extending from their threshold at Z/$β\sim7$ and $\sim50$ for CR39 and Makrofol, respectively, up to Z/$β\sim92$

Figures (11)

• Figure 1: Scheme of calibration stacks: a) stack composed of several CR39 foils; b) stacks with one or two 9.5 mm thick aluminum slabs interleaved to groups of SSNTDs foils. In blue are CR39 foils 0.5 or 1 mm thick, in purple are CR39 foils 1.5 mm thick and in grey are the aluminum slabs.
• Figure 2: Kinetic energy (a) and (b) Z/$\beta$ ratio of Kr ion across a 40 mm thick CR39 foils
• Figure 3: Kinetic energy (a) and (b) Z/$\beta$ of Fe ion across 70 mm thick stack composed of CR39 foils and aluminum slabs. The change in slope corresponds to transitions from a group of SSNTD foils to an Aluminum slab.
• Figure 4: Ion tracks in CR39 after 15 hours etching in 6 N NaOH solution at 70 $^{\circ}$C as seen under an optical microscope with $20\times$ objective, coupled to a CCD camera: from top left clockwise: Oxygen ions, Z/$\beta$ = 9; Silicon ions, Z/$\beta$ = 24; Krypton ions, Z/$\beta$ = 52 and Iron ions, Z/$\beta$ = 39.
• Figure 5: Tracks of (a) Pb and (b) Xe ions, along with their nuclear fragments, observed in Makrofol after 15 hours of etching in a 5.5N KOH solution with 20 % ethyl alcohol at 45$^{\circ}$C. The dimensions of the image frames are 0.80 mm by 0.64 mm.