Electrostatic Air Precipitator Film Imaging with iQID

5.3.1. Introduction

Ambient air particles were electrostatically collected onto a conductive tape sample and imaged on the iQID imager. For this imaging test, interested ambient air particles included decay products from ²²²Rn. ²²²Rn is in the uranium series and the immediate alpha-decay product of radium-226 (²²⁶Ra). The half-life of ²²⁶Ra and ²²²Rn is 1.62 x10³ years and 3.82 days,

respectively. The alpha decay of ²²²Rn from ²²⁶Ra results in three crucial decay products. These decay products consist of polonium-218 (²¹⁸Po) via alpha and beta decay, lead-214 (²¹⁴Pb) via beta decay, and bismuth-214 (²¹⁴Bi) via beta decay. These decay products have a short half-life time, which will be vital for iQID imaging of the electrostatic tape sample. This will be achieved by using the short half-lives of these beta decay products. The half-life of ²¹⁴Pb and ²¹⁴Bi is 26.8 minutes and 19.7 minutes, respectively. For completeness, the half-life of ²¹⁸Po is 3.05 minutes [68].

The objectives of this imaging study were divided into two parts. The first objective was proof of concept to determine if the conductive tape sample can be imaged on the iQID. If iQID imaging was successful, then the electrostatically deposited air particles' pattern could be studied further. This became the second task of the first experiment. The trajectory of the different air particles and location on the tape sample is based on the electric field and the various air

particles' mass. Therefore, through iQID imaging, the pattern and trajectory of these air particles could be determined. This study's second objective was to determine if the iQID imager can quantitatively detect the beta decay products of ²²²Rn.

5.3.2. Methods

iQID imaging of the electrostatic tape sample was conducted with two different iQID imagers. The imaging area is the significant difference between iQID imagers: the large-area iQID system and a smaller iQID system (ø 40mm imaging area). This difference is due to the lack of a fiber optic taper, hence reduced imaging area. All measurements with these iQID systems were imaged in beta mode using the TranScreen LE phosphor screen. The tape sample consisted of a blue painter’s tape. Using an iQID imaging method called time slicing, the number of detected air particles within 5-minute intervals will be collected. Based on the decay rates and initial abundances, the detected abundances and activities within a decay chain can be calculated as a function of time. In nuclear physics, this mathematical model is called the Bateman

equation. Equation 27 below demonstrates changes in isotope (i) amounts within the decay chain as a function of time (t) when there are Ni (t) isotope atoms that decays into isotope (i+1) at a rate of λi.

𝑑𝑁₁(𝑡)

𝑑𝑡 = −𝜆₁𝑁₁(𝑡)

𝑑𝑁_𝑖(𝑡)

𝑑𝑡 = −𝜆_𝑖𝑁_𝑖(𝑡)+ 𝜆_𝑖−1𝑁_𝑖−1(𝑡) Equation 27 𝑑𝑁_𝑘(𝑡)

𝑑𝑡 = 𝜆_𝑘−1𝑁_𝑘−1(𝑡)

Coupling time-slicing iQID imaging technique with the Bateman equation, the detected activities for decay products ²¹⁸Po, ²¹⁴Bi, and ²¹⁴Pb will be calculated. These two methods will determine the usefulness of iQID imaging with an electrostatic tape sample.

5.3.3. Results

An image of the tape sample with electrostatically deposited ²²²Rn air particles can be observed in Figure 80. This tape sample is expressed on top of the input window of the large- area iQID.

Figure 80: Image of the blue painter’s tape electrostatic tape sample on the large-area iQID

Next, the time slicing iQID imaging technique and Bateman equation was applied to analyze the electrostatic tape sample. In Figure 81, a plot of the count-rate vs. elapsed time with a corresponding three-component Bateman fit was calculated based on the decay products of

218Po, ²¹⁴Bi, and ²¹⁴Pb. In Figure 81, a high drop in count-rates over the first 5 minutes was detected that matched the decay rate of ²¹⁸Po.

In Table 13, the signature, detected activity on the large-area iQID, and associated uncertainty were calculated.

Table 13. Imaging Results of Electrostatic Air Sample on the Large-area iQID

Signature Detected Activity Uncertainty [%]

214Pb 2.06 0.03

214Bi 1.28 0.14

218Po 47.99 0.81

Figure 81: Decay curves of count rate vs. elapsed time obtained from the electrostatic precipitator tape sample on large-area iQID. Beta/gamma decay products measured for 2 hours with TranScreen LE. ²¹⁸Po ( red curve) , black curve is the fit used to calculate uncertainty, ²¹⁴Bi (green line), and ²¹⁴Pb (blue line).

5.3.4. Discussion

In Figure 81, count-rates exponentially decreased at a decay rate of ²¹⁸Po during the first 5- minutes with an uncertainty of 0.81 %. This is an unexpected finding because iQID was placed in beta imaging mode, and the emission of ²¹⁸Po is primarily via alpha decay. This finding was compared to the event-rate/count-rate instability observed and discussed in Chapter 4.

Therefore, the high drop in count-rate was determined not to be related to the emission of ²¹⁸Po but rather to count-rate instability within the first 5-minutes of acquisition. This finding may be caused by electronic noise within the iQID components when the power supply is turned. This observation is supported because count-rates seem more stable as the elapsed time increases after 5-minutes. Therefore, the system’s count-rate instability decreases the longer the acquisition time. This instability is essential to be aware of when doing short-lived half-life measurements with the large-area iQID. During long half-life measurements, count-rate instability is not a factor. This informs the LAiQID should be on for several minutes before acquiring data to stabilize.

Chapter 6