A wide variety of instruments are used in volcanology. Some are highly complex, manufactured at large facilities, and very expensive. Others are simple and can be built by hand in a teaching laboratory or a garage.

Speaking of garages, the famous company Hewlett-Packard got its start in a small garage—which, incidentally, still stands today. The initial capital was just $538, and the company's first office was an ordinary garage in Palo Alto, California.

So, you have a great chance of creating your own niche. All you need for that is a pair of hands and a head on your shoulders.

First of all, why is an underwater radiosonde needed in the first place, and what does radiation have to do with it?

Volcanic crater lakes offer an ideal vantage point for getting as close as possible to a volcano. However, the volcano fiercely guards this opportunity by creating a formidable obstacle: the lake water is highly acidic—much like the electrolyte in your car battery—and often exceeds 100 degrees Celsius in temperature. Operating electronic equipment under such conditions presents serious challenges.

In addition to radiation levels, scientists require information on the temperature at the bottom of the volcanic lake, radon gas content, acidity levels, and a host of other parameters. After all, the lakebed is in direct contact with various gases escaping from the volcano.

Why does it make sense to begin work in this area by measuring the gamma background at the bottom of the lake? In this case, there is a simple technical solution available. Once the gamma background measurement process has been refined, additional sensors can be added to the radiosonde.

Such a radiosonde is essential for investigating volcanic lakes that pose a threat to the researcher's life—situations where even standing on the shore is dangerous.

The underwater radiosonde must transmit data wirelessly (via radio waves, electromagnetic signals, or acoustic communication) to a receiver located on the shore or at a safe distance away from it.

The device should be simple and inexpensive, as it is essentially a disposable instrument—much like a meteorological radiosonde launched into the atmosphere to drift with the wind.

In our case, we lower the radiosonde to the lakebed—either using a boat, if the lake is safe, or via a quadcopter or an unmanned radio-controlled boat, if the lake poses a danger to life. The radiosonde then transmits data from the lakebed until its batteries (power sources) run out, a process that takes several months.

The main component of the underwater radiosonde is its housing. The housing must withstand a pressure of 10 to 20 atmospheres, depending on the lake's depth. The best approach is to manufacture the housing specifically for the required depth. The housing material must also be able to withstand the high acidity of the lake water.

Essentially, the operational lifespan of the underwater radiosonde is determined by the service life of its power sources; the lifespan of the housing can be established based on this.

For example, if two high-temperature lithium cells with a capacity of 28 Ah are connected in parallel, the device will operate continuously for three months given an average current consumption of 25 mA by its electronic components. There is potential to extend this operating time by incorporating various auxiliary power sources; essentially, the housing needs to have a service life of approximately six months. Caprolon is a suitable material choice, as it is inexpensive and easy to machine.

The body is cylindrical in shape. This shape allows it to withstand significant pressure.

• Конструкция: Цилиндрический корпус из PTFE (тефлона) или PVDF, усиленный внутренним силовым каркасом из высокотемпературного углеволокна. При толщине стенок всего 8–10 мм такой цилиндр легко выдержит 11 атм.
• Герметизация: Одинарные или двойные уплотнительные кольца из Viton (FKM). При 100 °C этот материал сохраняет полную эластичность и химическую стойкость к кислотам десятилетиями.

A heavy, solid titanium housing is not required for diving to a depth of 100 meters. The optimal solution is a durable composite or polymer housing with external protection.

    *   Construction: A cylindrical housing made of PTFE (Teflon) or PVDF, reinforced with an internal structural framework of high-temperature carbon fiber. With a wall thickness of just 8–10 mm, such a cylinder easily withstands 11 atm.

    *   Sealing: Single or double Viton (FKM) O-rings. At 100°C, this material retains full elasticity and chemical resistance to acids for decades.

• Высокотемпературная электроника : Использование компонентов индустриального класса (Automotive / Industrial Grade), сертифицированных для работы до +125 °C. В этом случае сложная система охлаждения не требуется.

High-temperature electronics: Use of industrial-grade (Automotive/Industrial Grade) components certified for operation up to +125°C. In this case, a complex cooling system is not required.

• Выбор сенсора: Используется высокотемпературный  счетчик Гейгера, работающий при температуре  +100 градусов Цельсия.
• Окно сканирования:  Оптимально использовать пластину из синтетического сапфира толщиной 1 мм с напылением алмазоподобного углерода (DLC). Сапфир выдержит 11 атм, не растворится в кислоте при 100 °C и пропустит жесткое бета- и гамма-излучение.

Radiation detector specifications for 100°C operation

    *   Sensor selection: A high-temperature Geiger counter capable of operating at +100°C is used.

    *   Scanning window: The optimal choice is a 1 mm thick synthetic sapphire plate with a diamond-like carbon (DLC) coating. Sapphire withstands 11 atm of pressure, resists acid dissolution at 100°C, and allows the transmission of hard beta and gamma radiation.

Monocrystalline sapphire retains its strength at high temperatures and exhibits excellent transparency as well as favorable thermophysical properties. It is chemically resistant to alkalis and acids at temperatures up to 1000°C.

Plane-parallel sapphire plates (leucosapphire windows) can be used in harsh environments where optical transparency is required across the visible to near-infrared spectral range.

The second most important component of the underwater radiosonde is the detection unit. In our case, two high-temperature Geiger-Müller tube counters are used.

At a depth of 100 meters, it is impossible to directly measure alpha and beta radiation; only gamma radiation can be measured. Why? The distance alpha particles travel in air is a few millimeters, whereas in water it is practically zero.

To measure alpha radiation in the air at the Earth's surface, specialized counters are used; these feature a window made of an ultra-thin film that allows alpha particles to enter the counter and be detected. No existing film can withstand the pressure at a depth of 100 meters. Consequently, at the bottom of a volcanic lake, we can measure only the gamma background.

Gamma radiation has high penetrating power; however, it can still be significantly attenuated by the walls of the radiosonde housing. To eliminate this attenuation, special sapphire windows can be used. Sapphire is an excellent material: a sapphire window 3 mm thick causes virtually no attenuation of gamma radiation and can withstand the 11-atmosphere pressure found at a depth of 100 meters. In this case, however, it is advisable to use end-window gamma counters positioned almost flush against the sapphire window.

But we have digressed into some specialized issues.

Our task, however, is to produce an underwater radiosonde—a highly cost-effective version, since it is essentially a disposable device. You lower it to the bottom of a hazardous lake, and it simply operates there.

Let us determine the sequence of operations for this task. But first, we will compile a list of the instruments and tools we will need, followed by a list of the components used to assemble the radiosonde.

1. Instruments and tools.

1. A multimeter, commonly referred to as a tester. This is an instrument used to measure voltage, current, and resistance. It is an essential tool; we would be lost without it. A vast array of multimeters is available on the market today. In principle, even the simplest model will suffice for our needs.

3. Soldering iron. This is the most popular tool for working with electronic circuits. In this case, we are using a small, 40-watt soldering iron; it allows you to solder everything from microchips and transistors to wires and contacts.

You also need solder for the soldering iron. This is a tin-lead alloy that is melted to join wires, contacts, and leads together.

To cut and strip wires and contacts, we need side cutters—small wire-cutting pliers—as well as small long-nosed pliers for working with electronic component leads, tweezers, and a scalpel. That covers all the tools needed to assemble the underwater radiosonde.

We’ve covered the tools. If you’ve never held a soldering iron before, don’t worry—there was a time when you didn’t know how to drive a car, either.

To get started, find a book online or buy one that explains how to properly prepare the soldering iron, choose the right solder, and perform the actual soldering. Experience will be your best guide here; once you start assembling a circuit and soldering, you’ll get the hang of it.

Let’s assume you have already learned how to solder two wires together.

Now, let’s move on to the components—the parts that make up the radiosonde. AliExpress will be our main resource here.

You can buy practically all the elements, components, and tools on AliExpress.

As previously mentioned, the key components of the underwater radiosonde are the sensors—devices used to measure specific parameters. In our initial version, we will use Geiger-Müller counters. Later on, we can incorporate other types of radiation sensors, such as scintillation units (comprising NaI(Tl) scintillators and photomultiplier tubes) or semiconductor detectors. For our purposes, high-temperature Geiger-Müller counters are the most suitable choice, offering a balance of simplicity and affordability. They are an excellent starting point for working with radiation.

We will be assembling an underwater radiosonde using Geiger-Müller counters, as they are the most affordable and accessible gamma-radiation sensors. We have a wide range of options to choose from. To the right, there is a photograph showing all the available Geiger-Müller counters. Based on their design, they can be categorized into two types: "pancake" and "hot dog" (tubular).

Each type has its own advantages and disadvantages.

Only high-temperature Geiger-Müller counters—such as the "Gamma-12"—are suitable for operation in a high-temperature volcanic lake where temperatures can exceed 100°C. If the lake temperature does not exceed +60°C, any of the counters shown in the photo can be used.

Let us consider the following question: what is the best way to measure gamma background? That is, how should the Geiger-Müller counter be positioned to ensure its most effective operation? In other words, how can the shielding of the counter by the device's housing be minimized?

It should be noted here that the housing of the underwater radiosonde is made of polyamide, Caprolon, or chemically coated titanium. These materials significantly attenuate ionizing radiation; they completely shield against alpha and beta radiation. In our case, however, we need to detect both hard and soft gamma radiation.

1. Магматические газы (Радон-222): Подводные гидротермальные источники и фумаролы выбрасывают из недр Земли радиоактивный газ радон. При его распаде образуются дочерние изотопы (свинец-214 и висмут-214), которые при переходе в стабильное состояние испускают кванты мягкого гамма-излучения.
2. Вулканический пепел и тефра: Донные осадочные отложения вулканических озер богаты слоями пепла (тефры). Они содержат повышенную концентрацию природных радионуклидов — калия-40, элементов уранового и ториевого рядов.
3. Гидротермальные минералы: Горячие подземные флюиды выносят на дно соли тяжелых металлов. В местах их осаждения (например, около «черных курильщиков» или подводных гейзеров) формируются минеральные аномалии с повышенным радиационным фоном.

Natural low-energy (soft) gamma radiation at the bottom of volcanic lakes serves as an important indicator of geological processes. Water acts as an excellent shield against external cosmogenic radiation; therefore, the gamma background detected at depth is of purely terrestrial origin.

Sources of gamma radiation at the lakebed

   1. Magmatic gases (Radon-222): Subaqueous hydrothermal vents and fumaroles release radioactive radon gas from the Earth's interior. Its decay produces daughter isotopes (lead-214 and bismuth-214), which emit soft gamma-ray quanta as they transition to a stable state.

# Volcanic ash and tephra: Bottom sediments in volcanic lakes are rich in layers of ash (tephra). They contain elevated concentrations of natural radionuclides—potassium-40 and elements from the uranium and thorium series.

# Hydrothermal minerals: Hot subsurface fluids transport heavy metal salts to the seafloor. Mineral anomalies with elevated background radiation levels form at sites where these salts precipitate (for example, near "black smokers" or underwater geysers).

    * Eruption forecasting: A sharp increase in the intensity of soft gamma radiation (due to heightened radon emission) indicates magma ascent and volcanic reactivation.

Radon measurement principle.

Since the housing of the underwater radiosonde is made of Caprolon or a similar material, soft gamma radiation is attenuated as it passes through the housing walls.

To avoid erroneous gamma background measurements, a design solution is required that allows gamma quanta to reach the Geiger-Müller counter without attenuation. The use of sapphire windows provides such a solution.

For a pressure of 10 atmospheres, the sapphire window must be at least 1 mm thick, given a window diameter of 30 mm.

How a sapphire window affects radiation detection