Study of CMOS Sensing System for Radon and Alpha Radiation
Daniel Husson
Round 1
Reviewer 1 Report
This manuscript is about a very interesting and current topic: the radon measurement in dwellings is an important task. I have only some comments and questions.
Line 64-72+figure 2.: the absorbed energy and alpha energy loss was used in the same meaning. Please use one of them (I recommend the absorbed energy instead of energy loss)
Figure 6: something is wrong, please correct it.
Please add the manufacturer of the devices (Radon Eye+ and Airthings Wave)
What is the measurement range of this sensor? What is the upper and lower limit?
Did you investigate the influence of different parameters (for example temperature, humidity, etc.)? What was your experience? For example, if I want to measure in a cave, is it possible?
Author Response
Dear reviewer, thank you for your reviews, suggestions and requesting minor revisions for our manuscript.
To facilitate your review of our revisions, the following is a point-by-point response to the questions and comments listed in your review dated August 23, 2021.
Line 64-72+figure 2.: the absorbed energy and alpha energy loss was used in the same meaning. Please use one of them (I recommend the absorbed energy instead of energy loss)
Fixed in figure 2 caption + line 71:
“The total absorbed energy within the STI layer is 53.7KeV”.
Figure 6: something is wrong, please correct it.
Figures 5 and 6 somehow were inserted into a table - fixed.
Please add the manufacturer of the devices (Radon Eye+ and Airthings Wave)
The manufacturers are reported in REFs. 17 and 18:
- Airthings Wave detector. Available online: https://www.airthings.com/wave-radon (Accessed on 1st August 2021)
- Radon Eye detector. Available online: http://radoneye.com/ (Accessed on 1st August 2021)
What is the measurement range of this sensor? What is the upper and lower limit?
The device range is configurable:
The device can be configured for a short term operation where a single alpha particle can be detected, and it could be configured to a long term, where only several particles can be detected.
About the upper limit, as an experiment, the device was exposed to alpha source, so that about 45 alpha particles per second were hitting the device.
As for the lower limit - during a 9 days with average concentration of 500 Bq/m^3, 4900 alpha particles were reaching the device - 22 particles/hour rate.
Did you investigate the influence of different parameters (for example temperature, humidity, etc.)? What was your experience? For example, if I want to measure in a cave, is it possible?
We didn't investigate the device sensitivity to temperature or humidity.
We do know that as temperature is reduced, the performance of CMOS circuits and devices improves.
Furthermore, the device is packaged as in the standard CMOS technology thus it should not be affected by humidity.
We made several measurements in a deep basement. We are therefore certain that it can be used in a cave.
Reviewer 2 Report
Please answer the 9 questions as preciseley as possible. Thank you.
The paper is written in good english, the subject is about pertinent scientific concerns, it is correctly referenced and the presented device can lead very soon to practical applications. The first part which describes the electronic structure and its basic response to radiation is completely satisfactoring, there is little to say about that and the 200 mBq level is really a good achievement for radon purposes.
However :
1) the actual size of the chip is not clear, on Fig4 we can read 5 « m », METERS really ? Please correct this (certainly MILLI-meters everywhere !)
2) It seems that the chip is to be used in integration mode/CURRENT mode only, with no individual counting of alpha particles : if this is true please say it more clearly.
3) The factor 3 DISPERSION on Fig.10 has to be commented ! Straightforward !!
4) At the end the new device is compared to existing systems for radon monitoring: is the new detector only EQUIVALENT or really better ? If is is better please tell us to what extent. TowerJazz is a low power consuming technology and something much more precise must be said about batteries and/or autonomy)
5) A rate of 45 particles/s is quite high : what is the MINIMUM rate in hits/s ?
The main concern is about spectroscopic capabilities. Given the thinness of the sensing layer it is certainly not possible to be sure that an alpha particle has been stopped (Bragg peak detection), on the other hand it is not a minimum ionising particle, so what about ENERGY ? Radon is in fact a mix of THREE charged species (including two polonium ions), adsorbed on small aerosols, therefore some additional questions :
6) Is it possible to separate the three energies of Rn daughters ?
7) For radon gas do you use a forcing system to collect some of these aerosols ? If the answer is yes, which one do you collect ?
8) Is the sensing surface to be cleaned after some days (because adsorption)?
9) For general alpha detection the energy measurement is mostly straightforward : if you do NOT measure energy do you see any application where your device can be useful despite of this ?
I think these 9 questions can be answered easily by the authors, if they do so I would say the paper can be published with these few complements/corrections)
Author Response
Dear reviewer, thank you for your reviews, suggestions and requesting minor revisions for our manuscript.
To facilitate your review of our revisions, the following is a point-by-point response to the questions and comments listed in your review dated August 30, 2021.
1) the actual size of the chip is not clear, on Fig4 we can read 5 «m », METERS really ? Please correct this (certainly MILLI-meters everywhere !)
The actual size is 5mm X 5mm. We fixed it in figure 4.
2) It seems that the chip is to be used in integration mode/CURRENT mode only, with no individual counting of alpha particles : if this is true please say it more clearly.
That's true. And it is added in lines 111-112:
"The change in the output voltage over time, meaning the output voltage derivative reflects the amount of alpha particles incidents"
3) The factor 3 DISPERSION on Fig.10 has to be commented ! Straightforward !!
Added in lines 165-167:
"All instances present the same trend, while the distribution of the results reflects each sensor’s sensitivity, which is configured by the sensor’s bias current."
4) At the end the new device is compared to existing systems for radon monitoring: is the new detector only EQUIVALENT or really better ? If is is better please tell us to what extent. TowerJazz is a low power consuming technology and something much more precise must be said about batteries and/or autonomy)
Regarding the power consumption, we added in lines 104-106:
"Since the power consumption of the device is dominated by the bias current of the sub-pixels, working in subthreshold will results in low power consumption which will enable the device to be operated by standard AAA batteries."
We believe that lines 188-193 from the conclusion section describes the advantages of the proposed sensor:
"In summary, the main advantages of the new Sensing System are:
Low cost compared to commercial radon sensors, enabling to apply it to every smart home, low power - may be battery operated using standard AAA batteries, integrated digital sensor for continuous sampling of radon gas in indoor air - providing hourly resolution of Radon levels. The CMOS die is passivated, and therefore insensitive to humidity."
5) A rate of 45 particles/s is quite high : what is the MINIMUM rate in hits/s ?
That's true - 45 particles/s is high rate. Since the device range is configurable - it can be configure for short term measurement and detect a single alpha particle. In the described experiment of radon exposure, the rate of alpha incidents was 22 particles/hour.
6) Is it possible to separate the three energies of Rn daughters ?
No. Actually most the absorbed energy comes from Rn daughters.
7) For radon gas do you use a forcing system to collect some of these aerosols ? If the answer is yes, which one do you collect ?
No collection system was used.
8) Is the sensing surface to be cleaned after some days (because adsorption)?
We have measured the devices periodically along one year, without cleaning them.
We do believe that improved packaging will be needed.
We are considering a low-cost package that may be easily changed.
9) For general alpha detection the energy measurement is mostly straightforward : if you do NOT measure energy do you see any application where your device can be useful despite of this ?
The main concept of the sensor is to convert the absorbed energy which affects the FG transistor threshold voltage into readout voltage. We believe that by making some adjustments (and keeping the same main concept), the device can be used for measuring other types of ionizing radiation as well.
