DURRIDGE RAD8 Radon Detector

• DURRIDGE RAD8 is a compact, high-sensitivity electrostatic radon detector designed for direct measurement of short-lived thoron using PIPS α-spectrometry.
• It employs an in-chamber methodology with a stainless-steel chamber and high voltage to channel decay products onto the detector, eliminating sample transfer losses.
• Enhanced sensitivity with helium carrier gas and precise calibration enables robust detection at low mBq levels, essential for rare-event physics applications.

The DURRIDGE RAD8 Electrostatic Radon Detector is a compact, high-sensitivity instrument designed for the direct measurement of radioactive radon isotopes, with a particular emphasis on quantifying short-lived radon-220 (thoron) emanation in low-background physics experiments. Its operation uses electrostatic collection and α-spectrometry within a stainless-steel chamber to identify and count specific radon decay products, enabling both absolute and relative emanation assays from materials relevant for rare-event searches (Gregorio et al., 18 Jan 2026).

1. Construction and Detection Principle

The RAD8 consists of a ≃0.6 L cylindrical stainless-steel chamber with hemispherical endcaps. The chamber walls are maintained at a negative high voltage (–2 to –3 kV), while the entrance window of a passivated implanted planar silicon (PIPS) detector at one end is held at ground. This configuration creates a radial electric field, channeling positive ions towards the detector. The measurable signal arises when gaseous 220Rn (thoron) introduced into the chamber decays, generating short-lived positively charged daughters such as 216Po+. These polonium ions drift onto the PIPS surface and undergo α-decay, with the resulting α-particles detected at an energy resolution of 50–80 keV FWHM (5–9 MeV range).

The RAD8 is firmware-configured for spectrometric readout in four energy windows:

• Window A: 218Po (6.00 MeV) and 212Bi (6.15 MeV, overlapping);
• Window B: 216Po (6.88 MeV, the primary thoron tracer);
• Window C: 214Po (7.69 MeV, for 222Rn);
• Window D: 212Po (8.78 MeV).

Counting rates in these windows allow separation of different radon isotopes and extraction of their respective activities (Gregorio et al., 18 Jan 2026).

2. Mathematical Formalism and Calibration Parameters

Thoron decays according to $N(t) = N_0 e^{-\lambda t}$, with $\lambda_{220} = (\ln 2)/55~\mathrm{s}$ for 220Rn. When emanated into a previously evacuated chamber, the activity buildup follows $A(t) = A_\infty [1 - e^{-\lambda t}]$, reaching ≳0.97 $A_\infty$ within ~5–10 minutes due to the short half-life.

Electrostatic collection efficiency $\eta$ can be approximated (not direct from the RAD8 firmware) as:

$$\eta \simeq \frac{\mu E}{\lambda R} \left[ 1 - e^{-\frac{\lambda R}{\mu E}} \right]$$

where $\mu$ is ion mobility, $E$ is field strength, $R$ is chamber radius, and $\lambda$ is the decay constant. In the RAD8, field distortions induced by bulk samples typically reduce $\lambda_{220} = (\ln 2)/55~\mathrm{s}$0 to ~80–90%.

Factory calibration delivers sensitivity $\lambda_{220} = (\ln 2)/55~\mathrm{s}$1 in counts per minute per Bq m⁻³. For flow-through operation,

$\lambda_{220} = (\ln 2)/55~\mathrm{s}$2

where $\lambda_{220} = (\ln 2)/55~\mathrm{s}$3 is the concentration in Bq m⁻³ determined by Window B, $\lambda_{220} = (\ln 2)/55~\mathrm{s}$4 the flow (L min⁻¹), and $\lambda_{220} = (\ln 2)/55~\mathrm{s}$5 the decay constant. All activities are determined after background subtraction: $\lambda_{220} = (\ln 2)/55~\mathrm{s}$6.

3. In-Chamber Methodology for Thoron Emanation

The in-chamber measurement protocol involves inserting the sample directly into the active volume, within a low-density 3D-printed holder. Eliminating gas transfer losses is particularly crucial for 220Rn (half-life 55.6 s). The method was validated using 31 g of 2 % thoriated-tungsten rods, machined to fit a 20×50 mm² cavity, thickness ≤5 mm, to ensure reproducible and minimally perturbative field geometry.

Carrier gas selection substantially affects sensitivity:

• Air, RH ≲ 15 %: In-chamber sensitivity is ≃3× the flow-through configuration.
• Helium (99.99 %): An additional ≃1.7× increase, for a total of ≃5.3× gain compared to flow-through in air.

The principal enhancement is attributed to the higher mobility of Po ions in He and their reduced neutralization at lower humidity and in noble carrier gas environments.

Method	Carrier gas	Window B net rate (cpm)	Relative sensitivity
Flow-through	Air	0.32 ± 0.09	1.0
In-chamber	Air	0.98 ± 0.15	3.1 ± 1.0
In-chamber	He	1.71 ± 0.19	5.3 ± 1.6

4. Experimental Performance and Quantitative Results

The RAD8, using in-chamber methodology, has demonstrated the capacity to measure thoron emanation at low-mBq levels. For thoriated-tungsten rods:

• Standard flow-through (3 h): $\lambda_{220} = (\ln 2)/55~\mathrm{s}$7 (95 % C.L.).
• In-chamber, air: Agreement with flow-through, but with tripled statistical power due to improved sensitivity.
• In-chamber, helium: Window B net count (1.7 cpm) yields further SNR gain.

Calibration for absolute activity compensates for 222Rn collection suppression, with a measured factor $\lambda_{220} = (\ln 2)/55~\mathrm{s}$8. The activity is given by

$\lambda_{220} = (\ln 2)/55~\mathrm{s}$9

where $A(t) = A_\infty [1 - e^{-\lambda t}]$0 L is the active volume minus the sample, $A(t) = A_\infty [1 - e^{-\lambda t}]$1 cpm/(Bq m⁻³). The resulting value $A(t) = A_\infty [1 - e^{-\lambda t}]$2 mBq is consistent with the standard method.

Approximate minimum detectable activity (MDA) for 220Rn in a 3 h in-chamber He run is ≃15 mBq, set by background fluctuations (3σ criterion, $A(t) = A_\infty [1 - e^{-\lambda t}]$3 on $A(t) = A_\infty [1 - e^{-\lambda t}]$4 cpm for $A(t) = A_\infty [1 - e^{-\lambda t}]$5).

Dominant uncertainties are statistical for Window B (15 % in 3 h), with calibration and suppression-factor uncertainties ≲5 % and ≃17 %, respectively. Maintenance of low RH (≲15 %) is essential to prevent charge-neutralization losses.

5. Comparison with Conventional Flow-Through Methodology

The flow-through mode involves circulating carrier gas (air, RH ≲ 15 %) at $A(t) = A_\infty [1 - e^{-\lambda t}]$6 through an external emanation vessel, followed by concentration measurement in the RAD8. The in-chamber mode bypasses transfer losses and shortens the measurement circuit, essential for short-lived 220Rn and enabling complete recovery of emanated species.

Parameter	Flow-through	In-chamber, air	In-chamber, He
220Rn sensitivity gain	1×	3×	5.3×
Transfer loss	Present	Eliminated	Eliminated
Sample location	External vessel	In active volume	In active volume
Signal statistics	Low	Moderate	Highest

A plausible implication is that, for materials with very low 222Rn/220Rn activities—typical in rare-event searches—the in-chamber He protocol provides both superior detection thresholds and more robust absolute quantification against calibration artifacts (Gregorio et al., 18 Jan 2026).

6. Recommended Operational Protocols for Low-Background Measurements

Optimal results require:

• Electric Field: Apply –2.5 kV to chamber, PIPS at ground; check for field distortions when using large or complex samples.
• Gas Handling: For in-chamber: evacuate chamber, flush to RH ≲ 10 %, fill to 1 bar with air or He; no continuous flow necessary.
• Counting Time: 3 h is typically used, yielding ≃15 % statistical error; longer times reduce error as $A(t) = A_\infty [1 - e^{-\lambda t}]$7.
• Sample Preparation: Fit assemblies to 20×50 mm² cavity, ≤5 mm thickness; use passivated or electropolished holders.
• Chamber Integrity: Maintain leak rate < 10⁻⁵ mbar L s⁻¹.
• Background/Mitigation: Perform empty-chamber runs pre/post sample; flush chamber (5 min in flow-though, 3 min in He) to remove Po residues; maintain RH ≲ 15 % using desiccants; avoid geometric field distortions.
• Calibration: Employ periodic cross-checks with 222Rn reference sources to track suppression effects and instrument stability.

7. Use Cases and Significance in Low-Background Physics

Direct in-chamber 220Rn emanation measurements with the RAD8 provide a reliable, expedient alternative for quantifying thoron release from materials slated for rare-event detectors, accelerating material screening and background characterization. The method achieves sensitivity of a few × 10 mBq in 3 h, a regime relevant for next-generation double-beta decay, dark matter, and solar neutrino experiments, where radon progeny are a dominant background. Compatibility of absolute activity with conventional flow-through calibrations, after appropriate suppression correction, validates the method for both relative reduction-factor studies and primary material qualification (Gregorio et al., 18 Jan 2026). The protocol capitalizes on the short half-life of 220Rn to screen out reservoir and transfer effects, and its robustness against humidity and field artifacts suggests broad applicability across low-radon environments.