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Stasi Wolke
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.
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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.
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Also known as a Geiger-Müller counter or Geiger-Mueller counter.
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It is known that the device can also detect gamma radiation (γ),
but with less sensitivity.
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Scandium-46 (46Sc) is an unstable isotope of scandium with a
half-life of 83.8 days [5].
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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.
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The 25053 was suitable for a wide range of applications, including:
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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.
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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:
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- 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.
- 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.
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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
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A dose of 100 mSv or more is clearly linked to increased cancer risk [6].
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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.
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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.
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The actual detector measures 150 x 115 x 60 mm and weighs 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.
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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.
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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.
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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.
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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.
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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.
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AOH-411 Geiger-Müller tube
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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
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AOH-411 Geiger-Müller tube
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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
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Document kindly provided by Detlev Vreisleben [1].
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© Crypto Museum. Created: Wednesday 05 June 2019. Last changed: Wednesday, 05 November 2025 - 11:43 CET.
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