How to DIY a particle detector?

I first saw an article on Toutiao - Big Data Digest titled "Nuclear Research at Home: How to DIY a Particle Detector" [1]. It described how Steve Foster (a recently retired TI architect at the Bank of England) and his 16-year-old middle school son tinkered with an electronic device at home that could detect environmental radioactive particles amid the ongoing global COVID-19 pandemic.

He saw an article about an external project of the European Organization for Nuclear Research (CERN), which said that one could build his own particle detector for less than £30. The information mentioned in the article could be downloaded from Github [2].

▲ Setve Foster’s DIY particle detector and the observed waveform

In his blog post, Foster vividly recorded the ups and downs he and his son experienced during the three-week production process (the actual production time was about 4 hours), which is very helpful for electronics beginners who want to do the same electronic experiment.

I was interested in their production because I had previously introduced the ionization smoke sensor in the article "Radiation Sources Overhead" [3]. For the detection of americium (Am-241) radioactivity in the smoke sensor , an old Geiger tube [4] was used as the detector. In Foster's particle detector, a silicon semiconductor PIN phototube was used to detect radioactive particles, and the energy spectrum of the particles could be inferred based on the strength of the signal pulses generated.

▲ Two PIN phototubes for detecting radioactive particles

Left: BPW34F; Right: BPX61

Solid particle detection principle

The principle of detecting radioactive particles and their energy distribution based on silicon semiconductor solid-state semiconductors[5] has been introduced in many online articles. Compared with the Geiger tube[6] which uses gas ionization (ionization power of about 15eV), the energy required for ionization in silicon semiconductors, that is, to excite electrons from the covalent band to the conduction band, is very small, about 1.1eV. Therefore, a high-energy particle passing through a silicon crystal can excite free electrons and holes in the semiconductor.

A dissipation zone is formed at the junction of a P-type semiconductor and an N-type semiconductor due to diffusion. The thickness of the dissipation zone increases with the increase of the bias reverse voltage, sometimes reaching several hundred microns. When radioactive particles pass through the dissipation zone, the excited ionized electrons (holes) will be driven out of the dissipation zone by the electric field in the dissipation zone, and finally a pulse current is output from the leads at both ends of the PN junction. Therefore, the dissipation zone at the PN junction is like a solid-state capacitive ionization chamber that can detect high-energy particles passing through. The ability of the particles is related to the strength of the pulse current that is finally formed.

▲ PIN diode and photoelectric detection principle

The BPX61 and BPW32F used in the experiment are PIN-structured phototubes, which have a layer of intrinsic semiconductor (basically undoped silicon) between the traditional P-type and N-type semiconductors. This makes the depletion layer between the PN junction wider and more sensitive to particles passing through the PN junction.

Since the penetrating power of alpha particles is very weak, in order to detect alpha particles, the top package glass window of BPX61 needs to be removed.

Use diagonal pliers to cut a few grooves around the metal shell of the BPX61 package, so that the top glass sheet will break and peel off from the metal shell glass. Be careful not to damage the silicon photoelectric tube chip inside the sensor.

▲ Left: BPX61 photoelectric tube with glass serial port

Right: BPX61 with the window removed

Pulse current amplification

Although a radioactive particle (alpha particle, electron, gamma photon) can ionize many electrons in the dissipation region of a PIN phototube, in order to form an electrical pulse signal that can be measured, it still needs to be amplified many times. The figure below is a signal amplification circuit diagram of a particle detector provided on Github [7].

At the front end of the amplifier circuit, the sensor used to detect alpha particles or beta rays (electrons) can use one BPX61 or four BPW34F(A) in parallel.

When using the BPX61 sensor, it needs to be modified beforehand (cut off the top sealing glass) so that alpha and beta ray particles can be detected.

▲ Schematic diagram of signal processing circuit

Using four BPW34F(A) as sensors can only detect beta rays (electrons) and a small amount of gamma rays (photons). Alpha particles cannot pass through the external package of BPW34 to reach the internal chip and cannot generate electrical pulse signals. Since four BPW24F are connected in parallel, the detection sensitivity of ionizing radiation is increased, so the amplification factor in the circuit can be reduced, which can also reduce the circuit's sensitivity to environmental noise and interference caused by mechanical vibration.

When using BPW34F, the resistors R3, R4, R5, and R9 in the circuit are reduced to 10M, 1K, 100K, and 0Ω respectively, and the overall amplification factor is reduced by about 10 times.

In order to facilitate the understanding of the principle of signal amplification, the previous circuit diagram is reorganized below. It can be seen from the figure that it is actually composed of two independent operational amplifiers in the JFET operational amplifier IC (TLE2072) to form two low-pass filters, C4 and R4 to form a high-pass filter, and R8, C9, C10, and R9 to form a band-pass filter, so the entire circuit is a band-pass amplifier circuit.

▲ Circuit diagram of amplifier circuit

If you buy ready-made circuit boards and components, a high school kid or other engineering and technology enthusiast can assemble a particle detector in two hours, including drilling holes in the outer metal shielding box and installing switches and signal line sockets.

In the electronic course design of the summer semester just past, if students choose to make this content, they can not only practice the production and debugging of analog circuits and weak signal amplification circuits, but also understand the relevant knowledge of digital filtering and signal measurement in the subsequent signal recording and analysis. In particular, some interdisciplinary scientific knowledge can be obtained by using this device.

▲ The finished particle detector equipment

Although the above circuit has amplified and shaped the pulse current generated by the particles, the amplitude is still very weak. The amplified signal can be introduced as a microphone signal into a personal computer, tablet or mobile phone, and the sound card in the computer or mobile phone can be used to further amplify the signal, and the audio signal can be recorded through free software downloaded from the Internet. After that, the recorded signal can be counted and analyzed by the software.

The following shows the pulse waveform generated by alpha particles. By setting the appropriate trigger level, the trigger signal can be separated from ordinary noise. By detecting the amplitude of the pulse signal, the energy of the radioactive particles can be obtained. By statistically analyzing the pulse energy over a period of time, the energy spectrum of the particles emitted by the radiation source can be obtained.

▲ Recorded α-particle pulse waveform

▲ Count the amplitudes of all pulse signals to obtain the energy spectrum of radioactive particles

Perhaps the circuit design above is relatively simple and elementary from the perspective of electronics, or the performance of this particle detector from the perspective of high-energy physics. However, this inexpensive device can help us better understand the various natural radioactive sources in our environment, and even our bodies are undergoing thousands of carbon-14 and potassium-40 decay radiation all the time. This can correct some of our misunderstandings about radioactive phenomena.

For students, if they only see relevant introductions from the Internet, books, and videos, some students may not be interested. Especially for this kind of radioactive phenomenon, which cannot be seen or touched. However, by following certain steps and making simple measuring tools to obtain relevant measurement data, it will stimulate their students' great interest in science and technology.

In addition to stimulating interest, after the Fukushima nuclear leak in Japan, the public used the recommended tools to detect environmental radioactivity and shared and discussed the information, which also had a very important positive significance in avoiding social disaster panic.

The related devices for the experiment are really cheap. When you have some free time at home one day, you might as well kill some time by building it.

References

[1] Doing nuclear research at home: How to DIY a particle detector: https://www.toutiao.com/i6877045217581597191/?tt_from=weixin&utm_campaign=client_share&wxshare_count=1×tamp=1601268327&app=news_article&utm_source=weixin&utm_medium=toutiao_ios&use_new_style=1&req_id=2020092812452701013109907703B3DAE7&group_id=6877045217581597191

[2] The materials involved in the article can be downloaded from Github: https://github.com/ozel/DIY_particle_detector

[3]Radiation source above the head: https://zhuoqing.blog.csdn.net/article/details/108854903

[4] Ancient Geiger tube: https://zhuoqing.blog.csdn.net/article/details/104132819

[5] Principle of detecting radioactive particles and their energy distribution based on silicon semiconductor solid-state semiconductor: https://physicsopenlab.org/2020/06/15/cern-diy-particle-detector/

[6] Geiger tube: https://baike.baidu.com/item/%E7%9B%96%E9%9D%A9-%E7%B1%B3%E5%8B%92%E8%AE%A1%E6%95%B0%E5%99%A8/5866514?fr=aladdin

[7] The signal amplification circuit diagram of the particle detector provided on Github: https://github.com/ozel/DIY_particle_detector/blob/master/hardware/V1.2/documentation/DIY%20particle%20detector%20schematic%20v1-2.pdf