Long-Range Imaging of Alpha Emitters Using Radioluminescence in Open Environments: Daytime and Night-Time Applications
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThere is no much novelty in this work compared to previous paper published by the same authors in references [21].
The only significant information obtained is that it has been tested in more lighting conditions.
I also see a lack of the main goal of the paper. It is easy to follow what they do, this investigation is a straight-forward approach, but it is not clear why they do it. The introduction is rather long but the objective is not found. It should be stated clearly.
The results might be interesting for certain circumstances such as nuclear safety or forensic science, but not for environmental monitoring, as they say. The case studied, that of an Am-241 source of 3 MBq, cannot be found in the environment. It is a non-exempted source. In such case, the level of intensity of gamma rays from that source at 1 m would be enough to detect them by any conventional radiation monitoring device.
Moreover, they assume that the 280-440 nm photons are produced by ionization by alpha particles, not considering the possibility of ionization by X-rays and gamma-rays of Am-241, with lower ionization possibilities but better penetration. This should be demonstrated.
Therefore, the technical results are worth to be published if these issues are resolved.
There are some corrections and comments and are listed here.
Line 46: replace by [5-18].
Figure 2: Use filter names or wavelength specification, but mixing is confusing.
Line 219 and figure 13 (b): “clear” and “clearly” is a bit of an exaggeration. Remove it.
Therefore, the document should be subject to minor improvements.
Author Response
Thank you for all the comments, they are valuable for our manuscript. Please find the responds below.
Comments1: There is no much novelty in this work compared to previous paper published by the same authors in references [21]. The only significant information obtained is that it has been tested in more lighting conditions.
Respond1: References [21] discussed our novel sandwich filter structure for reducing the visible and infrared light, which enables us to detect alpha sources under environment without UV background. However, for outdoor detection, the UV background is another challenge. We have introduced several innovative solutions in this paper to address these complexities: 1. UV background subtraction background for night time imaging. 2. New design of lens system for stacking filters and reducing blue shift. 3. Stack of tilting UVC filters enabling detection under sunlight.
For better clarification, we added:
Line 83: However, for outdoor imaging, the UV background cannot be ignored. Using only the 337 nm filter system is insufficient to effectively block all ambient background light.
Line 90: However, RL emissions in this spectrum are weak, constituting 0.42\% of total emissions \cite{wu2023physical}, which is 61 times lower than the main emission at 337 nm.
Line 277: It is important to note that the use of stacked UVC filters is also effective for nighttime detection, as the UV background at night is orders of magnitude lower than during the day. The preference for the 337 nm filter system at night stems primarily from the relatively low RL emission in the UVC region, which is 61 times lower than the signal in the 337 nm region. Additionally, the use of a stack of UVC filters diminishes the signal to just 3.1\%. Thus, in conditions where background UV light is minimal, using the 337 nm filter system substantially enhances the signal. This strategy optimizes the detection capabilities of the system under various ambient light conditions.
Comments2: I also see a lack of the main goal of the paper. It is easy to follow what they do, this investigation is a straight-forward approach, but it is not clear why they do it. The introduction is rather long but the objective is not found. It should be stated clearly.
Respond2: Agree. Therefore, we added:
Line 101: Therefore, the primary objective of this research is to develop an imaging system capable of accurately mapping the distribution of alpha emitters from a distance, eliminating the need for close contact or extensive scanning processes. This system is designed to perform effectively in open environments, including under sunlight, thereby enhancing the efficiency of alpha radiation surveys and significantly reducing health risks to workers. This technology aims to enhance nuclear safety and facilitate faster decommissioning processes. Additionally, it will allow for more effective assessment of nulcear waste packages from a safe distance and enable real-time monitoring of inventories within gloveboxes, contributing substantially to safer and more efficient operational practices in radioactive environments.
Comments3: The results might be interesting for certain circumstances such as nuclear safety or forensic science, but not for environmental monitoring, as they say. The case studied, that of an Am-241 source of 3 MBq, cannot be found in the environment. It is a non-exempted source. In such case, the level of intensity of gamma rays from that source at 1 m would be enough to detect them by any conventional radiation monitoring device.
Respond3: Thanks for the comments, we agree that this needs more explanation. First, the lowest detection limit for night time imaging is actually 75kBq, much lower than 3MBq. For this We added:
Line 237: The signal intensity is calculated as the mean pixel value in the area of the alpha source, while the noise is determined by the standard deviation of the pixel values in the background area. The signal-to-noise ratio (SNR) is calculated by dividing the signal by the noise, which results in an SNR of 40. Given that the alpha RL signal is proportional to the activity of the alpha source \cite{sand2016stand}, we can establish the lowest detection limit. By setting the minimum detectable SNR to 1, the lowest activity that can be detected at 1 meter with a 1-minute exposure is calculated as \( \frac{3 \text{ MBq}}{40} = 75 \text{ kBq} \)
Second, the Am-241 is very special, with 36% gamma emission, where common alpha emitters have less than 1% gamma emission, hard to detect using gamma detector. And even for Am-241, the gamma energy is very low and can be easily overwhelmed by the high gamma background in real applications. For this we added:
Line 43: Although most alpha emitters also emit gamma rays, which can be detected at long distances, the probability of gamma emission is typically low. This lower likelihood can be attributed to the mechanism of alpha decay, specifically the quantum tunneling effect. If an alpha decay is also accompanied by gamma emission, the kinetic energy of the alpha particle is reduced by the energy of the emitted gamma. Consequently, this increases the thickness of the potential barrier the alpha particle must overcome to exit the nucleus, making its escape even more challenging \cite{gammaalpha}. For example, Polonium-210 is a pure alpha emitter. Other isotopes such as Plutonium-239, Plutonium-238, and Uranium-238 predominantly decay through alpha emission, with more than 99\% of their decays proceeding via this mode, making these isotopes difficult to detect with gamma detectors. In the context of nuclear waste, minor alpha emitters that exhibit high rates of gamma emission, such as Americium-241, which emits a main gamma line of 59.54 keV with a probability of 35.64\% \cite{terada2016measurements}, also pose detection challenges. Detecting such materials using gamma detectors is difficult because the low-energy gamma emissions are easily overwhelmed by the high gamma background commonly encountered in practical applications, due to the high penetrating power of gamma rays.
Comments4: Moreover, they assume that the 280-440 nm photons are produced by ionization by alpha particles, not considering the possibility of ionization by X-rays and gamma-rays of Am-241, with lower ionization possibilities but better penetration. This should be demonstrated.
Respond4: Gamma induced RL is orders of magnitude lower than alpha RL. X-rays is similar.
For better clarification, we added:
Line 285: It is also important to discuss the effect of gamma rays on the imaging system. Gamma rays emitted from Am-241 cannot be focused by the fused silica lens and will directly impact the camera sensor, resulting in intense peaks in individual pixels. As previously discussed, these effects can be mitigated using a median filter. As for the gamma induced RL, according to the model proposed by Thompson et al. (2016) \cite{thompson2016predicting}, the gamma rays from Am-241 are relatively weak and do not significantly contribute to RL emissions, making them a less useful parameter for RL emission considerations. Moreover, even for gamma background from strong gamma emitters like Co-60 and P-32, the intensity of gamma induced RL, with activity levels similar to that of Am-241, is orders of magnitude lower compared with alpha induced RL. This is because gamma rays can travel long distances, causing gamma-induced RL to be distributed over a much larger volume. Consequently, we can conclude that our imaging system is largely unaffected by gamma rays due to these factors.
Comments5: Line 46: replace by [5-18]. Corrected
Figure 2: Use filter names or wavelength specification, but mixing is confusing. Corrected by using filter names
Line 219 and figure 13 (b): “clear” and “clearly” is a bit of an exaggeration. Remove it. Removed
Reviewer 2 Report
Comments and Suggestions for AuthorsIt was a pleasure to read this well-written and interesting manuscript on alpha imaging using radioluminescence. Performing radioluminescence imaging outdoors during both daytime and nighttime is a novel topic that has not been published before. I highly recommend this manuscript for publication with the following minor suggestions:
Lines 146-147: The very high light collection efficiency of f/0.5 sounds a bit unrealistic. Therefore, please double-check if the value given is correct. Also, please specify if the f-value given includes reflection/attenuation losses on the filters.
Lines 214-226: It would be good to discuss how the stack of interference filters impact the image quality. Is there any possibility that the signal recorded in Figure 13 a could just be partially caused by reflections from white plastic ring? Including a third image that shows the same measurement in complete darkness would be helpful to address this concern.
Author Response
Thank you very much for the encouragement for our project and the comments. Please find the responds below.
Comments1: Lines 146-147: The very high light collection efficiency of f/0.5 sounds a bit unrealistic. Therefore, please double-check if the value given is correct. Also, please specify if the f-value given includes reflection/attenuation losses on the filters.
Respond1: We added the method for calculating the f-number:
Line 178: The effective focal length (EFL) is estimated using the lens combination formula \cite{hecht2012optics}:
\begin{equation}\label{eq:efl}
\frac{1}{\text{EFL}} = \sum_{i=1}^{n} \frac{1}{f_i} = \frac{1}{150\text{mm}} + \frac{1}{-75\text{mm}} + \frac{1}{25\text{mm}}
\end{equation}
After calculating the values in the equation, the EFL is determined to be 30 mm. The aperture \(D\) was determined from simulations using Zemax 2016 and is found to be 60 mm. Consequently, the f-number is calculated as:
\begin{equation}
f\text{-number} = \frac{\text{EFL}}{D} = \frac{30\text{ mm}}{60\text{ mm}} = 0.5
\end{equation}
We also checked that the angular field of view is measured to be 7.2 degrees.
And yes, there are losses on the filters. We added:
Line148: For 337nm imaging, the transmission of the filter arrangement within the signal wavelength band is 67\%.
Line160: For UVC daylight imaging, the cumulative transmission of the stack of five filters is calculated as 3.1%
Comments2: Lines 214-226: It would be good to discuss how the stack of interference filters impact the image quality. Is there any possibility that the signal recorded in Figure 13 a could just be partially caused by reflections from white plastic ring? Including a third image that shows the same measurement in complete darkness would be helpful to address this concern.
Respond2: Thanks for the comments. The stack of filters might affect the image quality, but the most important thing is that the final image accurately showed the location of alpha sources, as demonstrated in the overlayed images. As suggested, we added anther image of capturing the alpha RL signal in dark room to show that the signal is not reflection of sunlight:
Line 266: To confirm that the detected signal originated from the alpha RL and not just from reflected sunlight, a controlled experiment was conducted in a dark room under similar settings, shown in Figure~\ref{sunlight}(a). The similarity between the signal detected in a dark room and that under sunlight confirms the successful detection of alpha RL in sunlight conditions.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsNone.
