# Radiation damage caused by cold neutrons in boron doped CMOS active pixel sensors

## TL;DR

This study investigates how cold neutron radiation causes unexpected damage in boron-doped CMOS sensors, revealing effects not predicted by existing models and highlighting the need for revised damage assessments.

## Contribution

The paper provides the first experimental data on cold neutron damage in boron-doped CMOS sensors, challenging existing NIEL-based damage predictions and revealing new damage mechanisms.

## Key findings

- Cold neutrons cause significant acceptor removal in sensors.
- Experimental results contradict NIEL model predictions.
- Damage from cold neutrons is stronger than expected.

## Abstract

CMOS Monolithic Active Pixel Sensors (MAPS) are considered as an emerging technology in the field of charged particle tracking. They will be used in the vertex detectors of experiments like STAR, CBM and ALICE and are considered for the ILC and the tracker of ATLAS. In those applications, the sensors are exposed to sizeable radiation doses. While the tolerance of MAPS to ionizing radiation and fast hadrons is well known, the damage caused by low energy neutrons was not studied so far. Those slow neutrons may initiate nuclear fission of $^{10}$B dopants found in the B-doped silicon active medium of MAPS. This effect was expected to create an unknown amount of radiation damage beyond the predictions of the NIEL (Non Ionizing Energy Loss) model for pure silicon. We estimate the impact of this effect by calculating the additional NIEL created by this fission. Moreover, we show first measured data for CMOS sensors which were irradiated with cold neutrons. The empirical results contradict the prediction of the updated NIEL model both, qualitatively and quantitatively: The sensors irradiated with slow neutrons show an unexpected and strong acceptor removal, which is not observed in sensors irradiated with MeV neutrons.