Radiation Total Dose for PRIMA: Cold Exposure with Alpha Particles

1. Problem Statement

The Probe far-Infrared Mission for Astrophysics (PRIMA) is a proposed space observatory designed to operate at the Sun-Earth L2 Lagrange point. It aims to map the far-infrared spectrum (24–261 µm) using kilopixel arrays of Kinetic Inductance Detectors (KIDs). These detectors utilize thin-film aluminum inductors and niobium interdigitated capacitors to achieve extreme sensitivity (NEP < 0.1 aW Hz$^{-1/2}$).

The primary concern for PRIMA's success is the radiation environment at L2, characterized by a flux of energetic particles (primarily solar and galactic protons). Previous studies (e.g., Barends et al., 2009) suggested that ion bombardment could reduce the quasiparticle lifetime ($\tau_{qp}$) in aluminum KIDs by creating lattice defects that act as trapping sites. However, those experiments were conducted at room temperature, raising concerns that:

1. The damage mechanism differs from that caused by solar protons (which dominate the L2 environment).
2. Lattice defects might anneal (heal) at room temperature during transport or irradiation, leading to an underestimation of the damage required to degrade performance at cryogenic temperatures.

The paper addresses the need to characterize the Total Ionizing Dose (TID) and Displacement Damage Dose (DDD) effects on KIDs under conditions that accurately replicate the L2 environment (sub-Kelvin temperatures and proton-like damage).

2. Methodology

A. Mission Dose Calculation

The authors first calculated the expected radiation dose for a 5-year mission at L2:

• Models Used: JPL Solar Proton model (solar max) and CREME96 (galactic protons, solar min).
• Shielding: Modeled the spacecraft's aluminum metering structure and detector enclosure using GEANT4.
• Result: The calculated total mission DDD is $2.9 \times 10^7$ MeV g$^{-1}$, dominated by solar protons. Galactic protons contribute negligibly.

B. Experimental Setup: Cryogenic Irradiation

To replicate the L2 environment, the team developed a fully cryogenic irradiation system inside a BlueFors LD400 dilution refrigerator (base temperature ~10 mK).

• Mechanism: A stepper motor (mounted on the ~4 K stage to avoid magnetic interference and thermal load) controls a copper screen via a nylon thread. The screen can block or reveal an alpha particle source located near the detector array on the 10 mK stage.
• Source: A custom Am-241 source emitting 5.486 MeV alpha particles.
• Dose Calibration: The alpha particles were used as a proxy for protons. Using the SR-NIEL-7 calculator, the team determined that the alpha particles produce a DDD profile across the array.
• Irradiation Level: The array was irradiated for 97 hours, delivering a median DDD of $1.8 \times 10^7$ MeV g$^{-1}$, which corresponds to 62% of the predicted 5-year L2 mission dose.

C. Measurement Protocol

• Pre- and Post-Irradiation: The team measured the quasiparticle lifetime ($\tau_{qp}$), resonant frequency ($f_r$), and internal quality factor ($Q_i$) for 312 functional KIDs.
• Readout: Used a single-tone homodyne readout system with a 1550 nm laser for photon pulse generation.
• Analysis: $\tau_{qp}$ was extracted by fitting a single-pole exponential decay to the mean photon pulse. $f_r$ and $Q_i$ were derived from forward transmission ($S_{21}$) fits.

3. Key Results

• Quasiparticle Lifetime ($\tau_{qp}$):

  • The mean lifetime shifted slightly from 0.37 ms (pre-irradiation) to 0.36 ms (post-irradiation).
  • The average shift per pixel was $\langle \delta\tau_{qp} \rangle = -0.004$ ms.
  • Conclusion: This shift is within the statistical measurement error and is not considered statistically significant.

• Resonant Frequency ($f_r$):

  • The mean fractional frequency shift was negligible ($-6$ kHz).
  • Scatter Analysis: The scatter in frequency shifts was analyzed to ensure resonators would not "collide" (overlap) after a full mission dose. Even scaling the observed scatter by the factor required to reach 100% mission dose (1.6x), the resulting frequency shift ($4.2 \times 10^{-6}$) is 380 times smaller than the spacing between adjacent resonators.

• Internal Quality Factor ($Q_i$):

  • The mean $Q_i$ showed a slight increase (+40) rather than a decrease.
  • Conclusion: Since radiation damage typically increases disorder and reduces $Q_i$, the positive shift suggests the irradiation had no meaningful degrading effect on the film quality.

4. Significance and Contributions

1. Validation of Radiation Hardness: The study provides the first experimental evidence that aluminum KID arrays, when held at sub-Kelvin temperatures, are highly resistant to the displacement damage expected from a 5-year mission at L2.
2. Methodological Advancement: The paper introduces a novel fully cryogenic irradiation setup using a stepper motor-controlled screen. This eliminates the "room temperature annealing" variable present in previous studies, offering a more accurate proxy for space-based radiation effects.
3. Mission Assurance: The results confirm that the FIRESS instrument's KID arrays will likely maintain their sensitivity (NEP) and spectral multiplexing capability (no frequency collisions) throughout the PRIMA mission.
4. Future Work: The authors plan to irradiate arrays to 100% (and beyond) of the mission dose to definitively map the degradation curve, though current data suggests the mission is safe from total dose effects.

Summary Conclusion

The paper concludes that the displacement damage dose expected at the L2 orbit is insufficient to significantly degrade the performance of PRIMA's aluminum KIDs. The observed changes in quasiparticle lifetime, resonant frequency, and quality factor after receiving 62% of the mission dose are statistically insignificant, suggesting the detectors are robust enough for the proposed 5-year mission.