Alpha Counter

UltraLo-1800 alpha particle counter
Figure 1: XIA UltraLo-1800 system. The argon-filled ionization chamber uses vertical electric field to drift free electrons created by alpha particle’s interaction with gas molecules. Large sample area and pulse-shape discrimination allow the counter to measure alpha particles as low as 1.0*10-4∕cm2∕hr level. This image is from Ref.

The UltraLo-1800 shown in Fig. 1 from XIA is an ionization chamber system filled with pure argon gas for detecting alpha particles in a sample. The size of the ionization chamber is 1800 cm2 in area and 15 cm in height. A uniform electric field about 70 V/cm is applied between the electrodes and the tray. The detector consists of two positively biased electrodes called anode and guard at the upper part of the chamber and a grounded sample tray at the bottom (see Fig. 2).

The principle of alpha detection in the chamber is based on Schokley-Ramo theorem. When a charged particle ionizes gas molecules in the chamber, ion pairs (Ar+,e) are produced along its path. Then, when the electrons drift through the field, according to the theorem, an induced charge on an electrode is proportional to the potential difference from the start to the end of the passage of the produced electron.

Since a distance from the produced electrons, i.e. alpha particle’s location, to the electrode determines the pulse shape of the signal, the Ultralo-1800 discriminates alpha particles emitted from the sample surface from those emitted from other areas of the chamber volume (see α1, α2, and α3 in Fig. 2).

Figure 2: A simplified view of background discrimination in the chamber. Alpha particles originating from different areas in the chamber can be vetoed by using pulse shapes and amplitudes in two electrodes. α1 is a sample signal showing high anode and flat guard signal if it occured in the middle. α2 started on the ceiling and so it shows little signal due to charge induction. α3 shows large guard signal relative to the anode signal because the event occurred at the sidewall. This image is from Ref.

Specification of the UltraLo-1800 counter

Table 3 shows the physical specification of the counter. The size of the detector is 165 cm × 89 cm × 69 cm (L × W × H, in operation) and 241 cm × 89 cm × 117 cm (fully opened). The detector uses pure argon boiled off from liquid as the ionization gas and consumes about 5 L/min during measurement. Nitrogen gas can be used alternatively without any degrade in performance.

– Table 1: System specifications of the UltraLo-1800 counter. These numbers are from Ref.

System RequirementSpecification
Sample size (min.-max.)
Sample thickness (max.) 
Sample weight (max.) 
Counting gas 
Gas pressure 
Line voltage 
Power consumption (max.) 
Dimension (L×W×H) 
Weight
707-1800 cm² 
6.3 mm 
9 kg 
Argon 
(21±1) psi 
100-240 V, 50/60 Hz
100 W 
(165 cm×89 cm×69 cm) 
163 kg

Detector Test

IBM performed several tests with 210Pb alpha/beta source (~5.3 MeV alpha end point and ~1.2 MeV beta end point) placed at the center of the tray. First of all, an IBM’s test showed the XIA counter is not sensitive to the beta particles. Secondly, Figure 3 shows count rate against horizontal location from the source to evaluate the signal efficiency especially near the edge of the sample area. As can be seen on the left plot, the rate did not decrease until 17 cm from the center. Note that the counter area is a square with 48 cm at one side (roughly 24 cm from the center). Finally, on the right plot, energy resolution using pulse height as a energy proxy is estimated to be better than 9% FWHM at 4.6 MeV.

Figure 3: Count rate using a 210Pb as a function of location for efficiency and pulse height for a resolution estimate. Figure (a) shows decrease in counting efficiency near the edge of the counter area beyond 17 cm. Figure (b) shows the pulse height distribution for 210Pb source. Note that the peak corresponds to 5.4 MeV alpha particles from the source and the spread in the distribution is an energy resolution. Both figures are from Ref.

Performance of UltraLo-1800

The detector is used for screening materials with an emissivity (ϵ) measure defined as alpha particle counts per hour in cm². The UltraLo-1800 applies event classification techniques using both rise time and signal amplitude to reach background rates to ϵ < 1.0 × 10-4∕cm²∕hr. Alpha particles originated from the top and the sidewalls can be discriminated from those in the sample. For a comparison, the conventional proportional counter systems can measure the emmissivity up to ϵ ~ 1.0 × 10-3∕cm²∕hr. Additionally, the background suppression allows the XIA chamber to measure samples faster than other alpha counters. For exammple it measures emissivites in the range ϵ = 0.001 – 0.0005∕cm²∕hr within 10 hours and emissivities ϵ ~ 0.0005∕cm²∕hr within 100 hours.

In 2009, IBM measured alpha emissivities of an ultra-low sample at (6.0 ± 1.0) × 10-4∕cm²∕hr using the XIA detector in a 5-day running. More recently in 2015, the XMASS experiment reported 12-day XIA measurement of ultra-low level Cu at (1.4 ± 0.3) × 10-4∕cm²∕hr.

– Table 2: Performance specifications of the UltraLo-1800 counter. These numbers are from Ref.

PeformanceSpecification
Counting time for ϵ = 0.001 (α∕cm²∕hr) 
, where ϵ is the emissivity 
Efficiency 
Resolution at 4.6 MeV 
Measurement range
(50%) 6-hr run 
(12.5%) 90-hr run (accuracy)
>90% of 2π 
<9% FWHM 
1 – 10 MeV

Plan for alpha measurements at CUP

The alpha counter is scheduled to arrive to CUP at mid-May. We plan to install the counter at the test room at Y2L shown in Fig. 4. Reduced cosmic-ray muon rate, low foot traffics, and low noise sources are the main advantageous for the alpha measurement. The XIA counter can fit in the room (10 m² in area 3 m in height) comfortably.

The detector is mainly for the powder screening, especially for NaI crystal powders. Therefore, a special techniqure should be developed for the powder measurements. We would also like to develop a method about how to improve the current sensitivity. Additional clean facilities including radon free air would allow us to avoid sample contamination to radon-induced alpha particles.

– Figure 4: XIA alpha counter installation layout at Y2L. The room is located at the A5 tunnel end side. The room has an area of about 10 m⊃ with 3 m height. Gray area represents tunnel rock walls. The counter is to be installed on the vibration-free desk at the wall side of the room. Once the counter moves in, the main front door will be sealed to minimize foot traffics. The remaining area will be used for sample preparation and analysis. Radon-free air and/or clean room facilities are expected to be installed.

For the first setup, we plan to run a few calibration measurements using a point alpha source with known energy. This is to obtain efficiency in detection area. We also like to measure several background data without any source or any sample to figure out the intrinsic background radioacitivity. Measurement plan is summarized in Table 3.

– Table 3: Measurement plan

MeasurementsNote
Efficiency scan with 241Am source 
w/ and w/o powder cover 
tray only 
pure Copper 
pure Copper with cleaning 
NaI powder 
NaI film
area calibration 
aluminzed Mylar 5 μm 

PNNL sample 


from NaI powder

Sample Preparation

Although the counter can identify alpha particles of the sample easily, a few steps need to be in place before running the counter. The main challenge is how to prepare the NaI(Tl) powder sample. Since the powder may not be placed in the tray as is, we either make a thin film from NaI(Tl) solution on the tray, make a pallet of NaI(Tl) from powder, or use aluminized Mylar cover to avoid powder particulates from flying and possibly contaminating inside of the detector.