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Stasi
Wolke
  
25053
Radioactivity detector

25053 was a radioactivity detector — also known as Geiger-counter 1 — developed during the Cold War in the former DDR (East Germany), for the repressive state security service Ministerium für Staatssicherheit (MfS), also known as the Stasi. The device detects the presence of ionizing radiation by using an end-window mica Geiger-Müller tube to count alpha and beta particles. 2

The Stasi also used the device for the detection of human movement behind a wall – for example to count people walking through a corridor – like a photoelectric sensor does in a security system, but using a radioactive source instead of light.

The device can be used with the built-in AOH-411 Geiger tube, but also has a socket to which an external probe with six such tubes can be connected, for increased sensitivity. Readout is via a built-in meter or (acoustic) via an earpiece. The image on the right shows the basic device with external probe and leather carrying case.
  
Geiger-counter with external probe and accessories

The Stasi also used the device for tracking objects and people – such as dissidents and suspected foreign agents – by invisibly marking them, their vehicle, documents or money, with a nuclear radiating substance that could be traced with the 25053. In some cases Scandium-46 3 — a gamma and beta-radiating metal isotope — was used, as a result of which the subject could be exposed to as much as 150 mSv per day, which is three times the allowed annual dose of a US radiation worker. It is known that a dose of 100 mSv or more gives an increased cancer risk [4].

It is currently unknown when the 25053 was developed, but judging from the date codes on some of the internal components, the device shown here was manufactured around 1980. As it has a rather low serial number (0S 022), it is likely that it was introduced around that time.

  1. Also known as a Geiger-Müller counter or Geiger-Mueller counter.
  2. It is known that the device can also detect gamma radiation (γ), but with less sensitivity.
  3. Scandium-46 (46Sc) is an unstable isotope of scandium with a half-life of 83.8 days [5].

Leather storage and carrying bag All parts stowed in the leather carrying bag Complete kit Geiger counter Rear side Detector with control panel uncovered External probe with cover removed Spare AOH-$11 Geiger-Müller tube
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Spare AOH-$11 Geiger-Müller tube

Features
The diagram below shows the connections, controls and features on the body of the main unit. At the rear are the ON/OFF switch, a socket for connection of an earpiece, a socket for connecting an external DC power source, and a push-button for checking the battery voltage. A battery pack, with six 1.5V AA-size batteries, is installed behind the removable rear panel. At the top is a clear meter, which displays the number of pulses per minute, plus a red alarm indicator LED that lights up when a certain threshold is exceeded. The threshold can be adjusted on the control panel.


At the right side of the device is a removable panel, behind with the settings are located. The device can be set to simple pulse counting (W) or switch mode relay operation (S), and the detection level is fully adjustable. Also present on the control panel is a BNC socket at which the contacts of the internal relay are available. It can produce a pulse when an existing beam is interrupted or when a subject — marked with a radioactive substance — passes the detector.


Applications
The 25053 was suitable for a wide range of applications, including:

Radiation detection
Like most other Geiger-counters, the 25053 was used for detecting radioactive alpha (α) and beta (β) particles and, to some extent, also gamma radiation. The tube has the hightest sensitivity for beta particles though. Due to the operating principle of the Geiger-Müller tube used as the probe of the 25053, the device can only count particles and can not measure their energy.

When counting particles, the meter on the body of the main unit gives an indication of the pulse-rate (in pulses per minute). If the pulse-rate exceeds a preset maximum, an alarm will be raised, and the red LED indicator aside the meter will light up. The alarm threshold can be adjusted.

Gate counter
In addition to the basic function described above, the 25053 can also be used as a so-called gate counter, or passage counter, much like a photocell is used in an access control or alarm system. In Stasi parlance, this application was known as Strahlungsschranke (radiation gate). Inside the device is a relay that will be activated as soon as the pulse-count exceeds a given threshold. As the relay function can be reversed, there were two possible applications of the gate counter:

  1. Active detection
    This application allows, for example, to invisibly count people walking through a corridor. By placing a fixed radioactive source behind one wall of the corridor and aiming it at the 25053 detector which is located behind the opposite wall, anyone walking through the corridor will momentarily interrupt the 'beam' and cause the relay to generate a pulse.

  2. Passive detection
    By reversing the operation of the relay, is becomes possible to count people or objects that are marked with a radioactive substance. By placing the detector at a stategic place, the relay will generate a pulse as soon as the marked person or object passes its probe. Tracing by means of radioactive markers, was known within the Stasi as Project Wolke.

Surveillance
For surveillance purposes — in particular for tracing the whereabouts of suspects, dissidents and foreign agents — the Stasi often used radioactive isotopes – known as markers – to deliberately contaminate people, vehicles, documents, money, equipment, etc. For example, when using Scandium-46 — a popular Stasi marker — the subject could be exposed to as much as 150 mSv 1 per case, which is as much as 3 × the allowed annual dose for a nuclear worker in the US [6].

The radioactive markers came in many flavours: strong, weak, with short half-life, with long half-life, as foils that could be attached to clothing, as ballpoints – so that written document could be traced – as sprays and even as radioactive needles, allowing a person to be marked in passing.

The Stasi also used markers to contaminate the floor of a room in which dissidents would meet, so that each individual visitor could be traced afterwards. The Stasi-operative who followed the nuclear trace, would keep sufficient distance to stay under the limit of 1 mSv 2 per week [4]. The use of nuclear markers in surveillance, was known within the Stasi as Project Wolke (cloud). For a good understanding of radiation levels absorbed in the human body, please refer to this chart [6].

 More about Projekt Wolke

  1. A dose of 100 mSv or more is clearly linked to increased cancer risk [6].
  2. The Stasi kept a safety limit for their own personnel of 1 mSv per week. Although this may not seem much, it is equivalent to staying in the Fukushima Exclusion Zone (after the 2011 nuclear disaster) for two weeks.




Parts
Leather storage and transport case Geiger-Müller counter type 25053 External probe with six Geiger-Müller tubes Earpiece for acoustic detection Leather strap for carrying the device around the neck
Carrying case
The complete set – without any spare parts or consumables – is neatly stowed in the strong leather carrying bag shown in the image on the right. Inside the case are three compartments: one for the detector, one for the external probe and one for the probe cable.

There is also room for the earpiece.
  
Leather storage and carrying bag

Detector
The actual detector measures 150 x 115 x 60 mm and weights 1260 grams, batteries included. It is a completely self-contained device that can be used on its own as a bare minimum. Optical indication of any radiation is via the meter on the top surface and the red LED alarm indicator.

Any radioactive particles are captured by a single Geiger-Müller tube, that is mounted behind a sliding aluminium cover at the front panel.
  
Geiger counter

External probe
For situations in which the single internal Geiger-Müller tube of the main unit does not provide sufficient sensitivity, the external probe shown in the image on the right can be used. It contains six AOH-411 Geiger-Müller tubes.

The external probe should be connected to the 5-pin 270° DIN socket at the front panel, which becomes visible when the cover over the internal Geiger-Müller tube is closed.
  
External probe with cover removed

Earpiece
In addition to optical indication of the presence of nuclear radiation – via the meter and the red LED – it was possible to get acoustic feedback, by connecting the earpiece shown in the image on the right to the phones socket at the rear. This allows the device to be used unobtrusively, for example when tracing footsteps.

The earpiece 'ticks' every time a particle hits the probe. The more ticks are heared, the more radiation is present.
  
Earpiece

Leather strap
This leather strap can be attached to the two metal stubs at the sides of the detector, close to the rear panel. It allows the device to hang from the shoulder, with the probe facing the floor, so that the footsteps of contaminated shoes can be traced throughout the city.

It can also be used for hanging the device around the neck, so that it is effectively carried on the chest.
  
Leather carrying strap

Leather storage and carrying bag Geiger counter Probe active Socket for connection of external probe Meter and LED indicator External probe (conter tubes uncovered) Earpiece
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Earpiece

Interior
The interior of the 25053 can be accessed by opening the battery compartment (remove the two large screws in the rear panel and remove the battery pack), plus the two metal stubs to which the leather carrying strap can be attached. This allows the interior to be removed from the shell.

The image on the right shows the interior after it has been removed from the black case shell. The interior is well-constructed and consists of a metal frame with three circuit boards (PCBs), the front panel and the meter panel. At the side is a small relay that delivers the pulse output.

It is worth noting that the internal power supply of the device is extremely efficient. Although it has to provide the 600V voltage for the Geiger-Müller tube, it only consumes a few milli-amps from the batteries, which is a great achievement.

  
Interior - top side

AOH-411 Geiger-Müller tube
The probe used in the 25053 device, is an AOH-411 Geiger-Müller counter, made by OBRTT-ZD in Warsaw (Poland). It was imported in the DDR by Isocomerz GmbH in Berlin-Buch and sold by VEB Maschinenbauhandel Dresden [A]. It was (and still is) one of the most sensitive end-window counter tubes available. It's mica window has a diameter of 25 mm and a density of 2 mg/cm2 .

According to the datasheet, the AOH-411 is sensitive to α-particles (3 MeV) and β-particles (40 MeV). Although not specified in the datasheet, it will also pickup γ-radiation, albeit with a reduced sensitivity, as one user demonstrates in this YouTube video [3].  AOH-411 datasheet

Battery compartment Cover removed Interior - top side Interior - bottom side Earpiece connected at the rear AOH-411 Geiger-Müller tube Spare AOH-$11 Geiger-Müller tube
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Spare AOH-$11 Geiger-Müller tube

Specifications
AOH-411 Geiger-Müller tube
  • Type
    End-window
  • Window
    Mica
  • Density
    2 mg/cm2
  • Diameter
    25 mm
  • Gas
    Neon (halogen)
  • Range α
    4 MeV
  • Range β
    40 MeV
  • Voltage
    580 - 680 V
  • Plateau slope
    0.08 %/V
  • Recommended
    600 V
  • Background
    50 cpm
  • Lifespan
    109 counts
  • Temperature
    -40°C to +50°C
  • Dimensions
    55 x 34 mm
  • Weight
    40 grams
Documentation
  1. User Manual of a similar device (German)
    Date unknown. 1

  2. AOH-411 Fensterzählrohre, datasheet
    OBRTT-ZD Warsaw (Poland). 18 October 1979.
  1. Document kindly provided by Detlev Vreisleben [1].

References
  1. Detlev Vreisleben, Projekt 25053
    Personal correspondence, June 2019.

  2. Wikipedia, Geiger counter
    Retrieved June 2019.

  3. YouTube, SV-500 with α, β, γ AOH-411 GM probe
    User: som3e. 28 December 2012.  More  AOH-411

  4. Iris Schaper, Stasi markierte Opfer mit radioaktiven Stoffen
    Website wissenschaft.de. 8 January 2001.

  5. Wikipedia, Isotopes of scandium
    Retrieved June 2019.

  6. Wikipedia, Randall Munroe — Radiation dose chart
    Retrieved June 2019.
Further information
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© Crypto Museum. Created: Wednesday 05 June 2019. Last changed: Monday, 10 June 2019 - 21:37 CET.
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