Cosmogenic activation in detector materials at shallow depths

This paper presents the first detailed study of cosmogenic activation in detector materials at shallow depths (<100 m.w.e.), calculating specific isotope production rates and suppression factors to address the unique multi-process background challenges faced by sensitive dark matter and neutrino experiments.

The Gist

Imagine you are trying to listen to a single, tiny whisper in a room that is currently filled with the roar of a jet engine. This is the challenge for scientists trying to detect dark matter or neutrinos. These particles are so elusive that they rarely interact with anything. To hear their "whisper," scientists need detectors made of ultra-pure materials (like Germanium, Silicon, and Copper) that are completely silent.

However, there's a problem: Cosmic rays.

The Problem: The "Rain" from Space

Think of cosmic rays as a constant, invisible rain of high-energy particles falling from space. When this "rain" hits the Earth's atmosphere, it creates a splash of secondary particles, mostly neutrons.

If you leave your detector materials sitting on the surface of the Earth (like in a warehouse), these neutrons hit the atoms in the metal and crystals. It's like a billiard ball hitting a cluster of other balls; it knocks them apart and creates new, radioactive "debris." This debris is long-lived and radioactive. It acts like static noise in your radio, drowning out the faint signals the scientists are trying to find.

The Solution: Going Underground

To stop this "rain," scientists put their detectors underground. The rock above acts like an umbrella.

• Deep underground (like in a mine): The rock is so thick that almost all the cosmic rays are blocked.
• Shallow underground (like a parking garage or a small tunnel): The rock is thick enough to block the big, energetic neutrons from the atmosphere, but not thick enough to stop everything.

This paper focuses specifically on these shallow depths (about 15 to 60 meters of rock). The scientists wanted to know: Is this "shallow umbrella" good enough to stop the noise, or does it still let too much in?

The Three Main Ways "Noise" Gets In

The researchers discovered that at these shallow depths, the "noise" doesn't just come from one source. It's a mix of three different mechanisms, like three different types of intruders trying to break into a house:

1. The Neutron Intruders (The "Bouncers"):
   Even underground, some neutrons are created when cosmic rays hit the rock above the tunnel. These neutrons bounce down into the tunnel and hit the detector materials.

   • The finding: At very shallow depths, these neutrons are still a major problem, especially for creating Tritium (a radioactive form of hydrogen) in Silicon and Germanium.

2. The Muon Stoppers (The "Heavy Hitters"):
   Cosmic rays also create particles called muons. These are like heavy, fast-moving bullets. At shallow depths, the rock isn't thick enough to stop them completely, but it is thick enough to slow them down until they stop dead inside the detector material. When a muon stops, it gets captured by an atom and causes a nuclear reaction.

   • The finding: This is a huge source of noise, especially for Copper. In fact, at shallow depths, "stopping muons" are often the biggest culprit for making radioactive Copper, even more so than neutrons.

3. The Gamma Rays (The "Flashbangs"):
   When muons interact with the rock, they also produce high-energy light particles called gamma rays. While these are usually less dangerous than neutrons, there are so many of them at shallow depths that they also contribute to the noise.

The Experiment: Testing the "Umbrellas"

The team used powerful computer simulations (like a virtual physics lab) to calculate exactly how much radioactive "debris" would be created in Germanium, Silicon, and Copper at three specific shallow locations:

• SUF (Stanford Underground Facility): A tunnel about 15–20 meters deep.
• PNNL SUL: A lab about 30 meters deep.
• SLC Adit: A storage area about 50–60 meters deep.

They compared these results to what would happen if the materials were left on the surface (sea level).

The Results: How Much Better is Underground?

The paper provides a "suppression factor," which is like a volume knob. If the surface noise is at 100%, how much is it turned down underground?

• For Silicon and Germanium (The Detectors):

  • At the shallowest site (SUF), the radioactive "noise" (specifically Tritium) is reduced by a factor of 250 to 400 compared to the surface.
  • The Catch: Even at 20 meters deep, the "stopping muons" are still creating a significant amount of noise. It's not a perfect silence yet, but it's much quieter.

• For Copper (The Shielding):

  • Copper is used to build the boxes that hold the detectors. The study found that at shallow depths, the "stopping muons" are the main reason copper becomes radioactive (creating a isotope called Cobalt-60).
  • The noise is reduced significantly, but the researchers found that the type of rock above the tunnel matters. If the rock is made of limestone (which has more Calcium), it creates more neutrons than standard rock, leading to more radioactive copper.

The Bottom Line

This paper tells us that shallow underground facilities are useful, but they aren't a magic cure-all.

• Good News: Storing materials in these shallow tunnels (like the ones used by the SuperCDMS experiment) reduces the radioactive noise by hundreds of times compared to storing them on the surface. This is essential for building sensitive detectors.
• Reality Check: At these shallow depths, the "stopping muons" are still a major problem. You can't just ignore them. The researchers provided a detailed map of exactly how much noise to expect at different depths so that future experiments can plan accordingly.

In short: Going underground is like putting on noise-canceling headphones. At shallow depths, they cancel out most of the jet engine roar, but you can still hear a faint hum. Scientists now know exactly how loud that hum is, so they can design their experiments to hear the whisper of dark matter over it.

Technical Summary

Technical Summary: Cosmogenic Activation in Detector Materials at Shallow Depths

Problem Statement
Direct-detection dark matter and neutrino experiments rely on rare-event searches where background events from radioactive decay can severely limit sensitivity. A significant source of background arises from cosmogenic activation: the production of long-lived radioactive isotopes in detector and shielding materials due to exposure to cosmic ray secondaries (primarily neutrons) while stored or fabricated above ground. Key isotopes of concern include tritium ($^3$H, $\tau_{1/2} = 12.32$ y) in Germanium (Ge) and Silicon (Si), and Cobalt-60 ($^{60}$Co, $\tau_{1/2} = 5.3$ y) in Copper (Cu). While extensive studies exist for above-ground activation and deep-underground sites (typically $>1000$ mwe), there has been no systematic study of cosmogenic activation at shallow depths ($<100$ mwe). As experiments scale up, the use of shallow-depth facilities for crystal growth, material fabrication (e.g., electroformed copper), and detector assembly is becoming increasingly common, necessitating a quantitative understanding of activation rates in these environments.

Methodology
The authors developed a comprehensive cosmogenic activation model to estimate production rates for tritium in Ge and Si, and $^{60}$Co in Cu at specific shallow-depth sites: the Stanford Underground Facility (SUF) Tunnels A/C (15–20 mwe), the Pacific Northwest National Laboratory Shallow Underground Lab (PNNL SUL, $\sim$30 mwe), and the Stanford Linear Collider (SLC) Adit Storage (50–60 mwe).

The methodology involved:

1. Flux Simulation: Using the Cosmic-Ray Shower generator (CRY) to model primary cosmic rays at sea level (New York City, 2003) and FLUKA simulations to propagate muons and secondary particles through a realistic crustal rock composition (density 2.7 g/cm³) to various depths.
2. Process Decomposition: The study isolated and calculated contributions from four primary mechanisms:
   • Muon-induced Neutrons: Both from hadronic showers and negative muon capture.
   • Stopping Negative Muons: Direct capture of negative muons in the detector material.
   • Photonuclear Interactions: Activation induced by energetic gammas produced by muon interactions.
   • Other Secondary Processes: Contributions from protons, pions, and heavy ions.
3. Cross-Section Modeling: Production rates were calculated using a mix of nuclear models (TALYS for $<100$ MeV, INCL for $\ge100$ MeV) and scaled to match available experimental yield measurements where possible (e.g., for Si tritium yield and Cu $^{60}$Co yield).
4. Site-Specific Adjustments: The model accounted for back-injected neutrons from cavern walls and floors, estimating an additional 40% contribution to neutron-induced activation compared to infinite rock geometries.

Key Contributions and Results
The paper provides the first detailed calculation of cosmogenic activation rates at shallow depths, breaking down the relative contributions of different physical processes:

• Activation Rates: The study presents specific production rates (atoms/kg/day) for the three sites. For example, at SUF (20 mwe), the total tritium production in Si is calculated at 0.48 atoms/kg/day, and in Ge at 0.24 atoms/kg/day. For Cu at the same depth, $^{60}$Co production is 0.53 atoms/kg/day.
• Process Dominance:
  • Neutrons vs. Muons: At sea level, neutrons dominate activation. At shallow depths, the relative importance shifts. In Ge, neutron-induced tritium production remains comparable to negative muon capture even at 20 mwe. In Si, neutron-induced production only becomes comparable to muon capture at deeper shallow sites ($\sim$50 mwe).
  • Copper Activation: Negative muon capture dominates $^{60}$Co production in copper at shallow depths (up to at least 80 mwe), contributing roughly 50–60% of the total rate, while neutrons contribute significantly less than at sea level.
  • Photonuclear Contributions: Despite smaller cross-sections, the high flux of muon-induced gammas results in photonuclear activation contributing $\sim$10% to tritium production in Si/Ge and $\sim$20% to $^{60}$Co production in Cu.
• Suppression Factors: The study calculates suppression factors relative to sea-level production. For 20 mwe depth, the suppression factor is approximately 250 for Si and 400 for Ge.
• Material Dependence: The authors highlight that rock composition (e.g., limestone vs. crust) can alter neutron flux and activation rates by up to 30% for $^{60}$Co in copper, due to the sensitivity of muon-capture neutrons to the overburden's atomic composition.

Significance and Claims
The paper claims that its results provide essential data for experiments like SuperCDMS, DAMIC, and OSCURA, which utilize Ge and Si detectors and are sensitive to low-energy backgrounds. By quantifying activation at shallow depths, the study validates the use of facilities like SUF and PNNL for:

• Long-term storage of detector crystals and components to mitigate cosmogenic activation.
• Underground fabrication of low-radioactivity materials (e.g., electroformed copper).
• Detector assembly and testing.

The authors emphasize that while shallow-depth sites do not offer the same level of background suppression as deep underground laboratories (e.g., SNOLAB), they provide a viable and necessary intermediate solution for large-scale experiments where above-ground exposure would otherwise generate prohibitive background levels. The study concludes that shallow-depth facilities can effectively control tritium and $^{60}$Co production, with induced activities in stored materials (e.g., after 6 years at SUF) dropping to the $\mu$Bq/kg range, significantly lower than unmitigated sea-level exposure.