1. On measurability of mBq/kg levels of alpha activity
V. V. Kobychev
Institute for Nuclear Research, Kyiv, Ukraine

2. Deff =D ∙ pesc =D ∙∫(Ω/4π)dr ≈ D/4 D
How to measure alpha activity (U, Th, non-family nuclides like 147Sm or 190Pt) of raw materials for crystal growth at the level of mBq/kg?
The problem is that the range of alphas in dense materials D ~ 10 µm. So in “passive” geometries (source≠detector) only the thin surface layer of a target is the source of measurable alpha particles. D
Deff =D ∙ pesc =D ∙∫(Ω/4π)dr ≈ D/4 D
Ω — solid angle
of alpha escape (case of threshold-less detector).
meff = Deff∙S∙ρ — effective mass of the “acting” surface layer of the sample with density ρ and area S.

3. For radioactive families, the activity of a parent nuclide can be estimated by gamma-quanta emitted by many nuclides in the chains (usually γ’s are emitted in beta-transitions with high Q, like 208Tl, 214Bi). As gammas are much more penetrating than alphas, the meff can be equal to all the mass of the sample.
But the secular equilibrium in the chain is usually broken by chemical processing.
For example, Ra is usually removed from reactives during their purification more effectively than U, Th, Pa… Ratio of activities 238U/226Ra can be >100. Measuring of gammas emitted by 214Bi (226Ra daughter) gives activity of 226Ra in the sample but says nothing about activities of 238U, 234U, 230Th.

4. Typical values for a semiconductor alpha spectrometer
Ion-implanted (or surface-barrier) Si detector
Area: mm2 (1-100 cm2)
Dead layer:
150 nm (0.035 mg/cm2, or ΔE = 21 E = 5.3 MeV)
Energy resolution: <30 keV (FWHM)
Background: <1 count/hour/cm2 in 3-8 MeV range
(<0.6 counts/hour within FWHM=30 keV for S=100 cm2)
B=2.5×10-4 Bq/cm2;
Range of alphas for W (at Eα=5.3 MeV, 210Po):
R=17 mg/cm2.
Minimal detectable activity (assuming no spectral info is used):
Amin=Bpesc/R = 60 Bq/kg.

5. Solid-state nuclear alpha track detectors
Most widely used: CR-39
Exposition
Plastic
Etching (NaOH, few h)
Target
Counting tracks
(scanning rate ~ few m2/day)
But typical background:
B ~ 100 tracks/cm2.
For T = 500 h:
Amin=Bpesc/TR ~ 800 mBq/kg

6. Typical values for a gas-filled alpha spectrometer/counter
Area: mm2 ( cm2)
No dead layers.
Energy resolution: few % (FWHM)
Background: <1 count/hour/cm2
But there exist gas-filled chambers with much better background.
Electronic industry needs “low alpha” materials (lead solders etc.) due to problem of so called “soft errors” — switching triggers on chips by alpha particles. Commercially available low alpha lead: decays/h/cm2 (~60 mBq/kg). These low activities are measured by specially designed gas-filled counters.

7. Example of a gas-filled chamber
Gavriljuk et al. Measurement of surface alpha-acrivity of different samples with ion pulse ionization chamber, arXiv:
(Baksan Neutrino Observatory INR RAS, Russia + Karazin Kharkiv Nat. University, Ukraine)
Their chamber (Ion pulse ionization chamber, IPIC) was used for measurement of materials for GERDA.
S=55 cm2 (can be expanded to 400 cm2).
After some enhancing, they estimate the background of their chamber as 1/500 counts/h from 400 cm2 sample. It would correspond to sensitivity of <1 mBq/kg for U and Th. They also claim that this sensitivity can be provided even on the earth surface, but possibly they underestimate (n,α) background in this case.

8. Method of measurement of alpha activity by growth of a test scintillator crystal
Advantages:
1. We use volume (instead of surface) counting => efficiency~100% => high sensitivity.
2. In many cases we can use alpha/beta selection.
Disadvantages:
1. High cost.
2. The raw material purity becames better (or, in principle, worse) when it is turned to form of a single crystal (but, strictly speaking, we need just the purity of single crystal).
For ZWO, T=500 h, m=0.1 kg, low background conditions, alpha/beta selection:
Amin~30 Bq/kg

9. Conclusions:
1. Among different methods for measurements of low alpha activity in passive geometry, gas-filling chambers seem to be the most sensitive method.
2. The 10 mBq/kg (and possibly 1 mBq/kg) level of sensitivity to alpha activity is reachable in passive geometry.
3. In active geometry (source=detector), ~0.03 mBq/kg is reachable.