1 MeV Fast Neutron Irradiation of MPPC Sensors

Multi-Pixel Photon Counters (MPPCs), also known as Silicon Photomultipliers (SiPMs), are compact solid-state photodetectors capable of detecting extremely low light levels with high gain, fast timing response, and relatively low operating voltage. These characteristics make MPPC technology attractive for particle physics instrumentation, radiation monitoring, medical imaging, space systems, and other applications that require reliable photon detection in challenging environments. An important consideration for these applications is the response of MPPC devices to fast-neutron radiation. Neutrons can interact with the silicon lattice indirectly through nuclear collisions, producing energetic recoil atoms and causing displacement damage within the semiconductor. Such defects may introduce additional generation–recombination centers and can modify important MPPC characteristics, including dark current, dark-count rate, noise, gain stability, photon-detection performance, and breakdown behavior.

Experimental Study with 1 MeV Fast Neutrons

This work investigates the behavior of an MPPC detector exposed to a 1 MeV fast-neutron irradiation environment. The objective is to characterize the sensor's electrical and detection performance before, during, and/or after irradiation, and to determine how accumulated neutron exposure affects its operation. The experimental hardware shown above incorporates the MPPC sensor and its associated signal interface, along with RF/coaxial connections for biasing, signal extraction, and connection to external characterization instrumentation. This configuration enables controlled measurements of the MPPC response while maintaining a practical interface between the irradiated detector and the acquisition electronics.

Particular attention can be given to changes in parameters such as:

• Dark current and leakage current

• Dark-count rate (DCR)

• Breakdown voltage

• Operating overvoltage

• MPPC gain

• Pulse amplitude and pulse shape

• Signal-to-noise ratio

• Photon-detection efficiency and stability

• Recovery behavior following irradiation

Why 1 MeV Neutrons Are Important

Fast-neutron testing is commonly used to investigate displacement damage in semiconductor devices. A neutron with energy around 1 MeV can transfer sufficient momentum to silicon atoms to produce lattice defects and defect clusters. For MPPC devices with many Geiger-mode avalanche microcells, radiation-induced defects can be especially important, as additional carrier-generation sites may trigger unwanted avalanches. One of the most visible consequences is therefore expected to be an increase in the detector's background activity. In simplified terms, the measured dark-count rate can be represented as

For neutron irradiation studies, detector degradation is normally evaluated as a function of the accumulated neutron fluence,

When radiation-damage results obtained with different neutron-energy spectra need to be compared, the exposure can additionally be expressed using a 1 MeV neutron-equivalent fluence in silicon, Φ based on the non-ionizing energy loss or displacement-damage equivalence of the irradiation field.

Expected Radiation Effects

A useful experimental comparison is obtained by recording MPPC characteristics before irradiation and repeating the same measurements after successive neutron exposures. The change in a parameter X can be represented as

Increasing neutron fluence will generally produce additional silicon defects, and a corresponding increase in dark current and dark-count activity is therefore an important quantity to monitor. At sufficiently high radiation levels, the resulting noise can eventually limit the MPPC's ability to resolve weak optical signals or individual photoelectron peaks. Temperature must also be carefully controlled during these measurements because MPPC dark noise is strongly temperature-dependent. Consequently, separating genuine radiation-induced degradation from temperature-related variations is essential for a reliable irradiation campaign.

Toward Radiation-Tolerant Photon Detection

Understanding MPPCs' response to 1 MeV fast neutrons helps determine whether these detectors can maintain acceptable performance in radiation-intensive environments. Experimental characterization also provides information to develop mitigation approaches, including temperature control, optimized operating overvoltage, shielding, calibration procedures, annealing studies, and signal-processing techniques. The 1 MeV Fast Neutron Irradiation MPPC experiment therefore provides a practical platform for studying the relationship between neutron displacement damage and the performance of SiPMs/MPPCs. The resulting measurements can support the design and qualification of more robust photon-detection systems for scientific instrumentation, nuclear and high-energy physics experiments, radiation monitoring, and other applications requiring reliable optical sensing in the presence of neutron exposure.

Application of MPPC/SiPM Technology in ALICE at CERN

The characterization of MPPC devices under 1 MeV fast-neutron irradiation is particularly relevant to detector development for high-energy physics experiments such as ALICE (A Large Ion Collider Experiment) at CERN’s Large Hadron Collider (LHC). ALICE studies strongly interacting matter and the properties of the quark–gluon plasma produced in ultra-relativistic heavy-ion collisions.

For the proposed ALICE 3 detector, planned as a next-generation experiment for LHC Run 5, silicon photomultipliers are being considered for several scintillator-based detector systems.  In particular, the ALICE 3 electromagnetic calorimeter (ECal) design includes scintillator layers whose optical signals are transported through wavelength-shifting fibers to photodetectors. The ALICE 3 scoping documentation identifies the Hamamatsu S14160-3010PS MPPC, with a 3 × 3 mm² sensitive area and 10 µm pixel pitch, as a suitable candidate for sampling cells in the ECal outer barrel and endcap. An R&D program is evaluating SiPM/MPPC devices with the required optical response and dynamic range. MPPC/SiPM technology is also being investigated for the ALICE 3 Muon Identifier (MID). During the 2025 CERN test-beam campaign, one MID prototype used approximately 1 m-long scintillator bars coupled to wavelength-shifting fibers and read out with silicon photomultipliers. This prototype demonstrated the applicability of SiPM-based optical readout to large-area particle-detection systems envisioned for ALICE 3.

Conclusion

The study of MPPC/SiPM performance under 1 MeV fast-neutron irradiation is important for understanding the long-term reliability of silicon photodetectors operating in radiation-intensive environments. Neutron-induced displacement damage can increase the dark current and dark-count rate, reduce the signal-to-noise ratio, and progressively degrade the detector's stability and sensitivity as the accumulated fluence increases. By comparing parameters such as dark current, DCR, breakdown voltage, gain, pulse amplitude, and signal stability before and after irradiation, it is possible to quantify radiation-induced degradation and identify suitable operating conditions for the detector. These studies are especially relevant to high-energy physics instrumentation, including future detector developments such as ALICE 3 at CERN, where SiPM/MPPC devices are being considered for scintillator-based readout systems. Radiation characterization therefore contributes to the appropriate selection of photodetectors, the optimization of thermal and bias conditions, and the definition of mitigation strategies to maintain reliable detector performance over extended experimental campaigns.