Radiometer RKb4-1eM

A workhorse for β-active isotopes analysis

A cliche picture from a Chernobyl tour: a dosimeter over an apple or a mushroom having some readings on its display.

Beautiful, but what exactly is being measured in this case? If ambient, background radiation is low, then there is a chance you will get readings of surface contamination of the object. But what, if no? Or, even more interesting — what, if there is internal contamination, particles being emitted from are blocked by the apple's skin?

Here comes another way. To make a proper analysis of the activity of various natural materials, you need a device that is capable to check it in a specific volume. A classic example is our today’s hero — a beta-radiometer RKb4-1eM (РКб4-1еМ). Although such sets have been produced in the late 80s (our came from 1987), they are still in use in various laboratories due to their high reliability.

Radiometer RKb4-1eM Soviet dosimeter

It is designed to make express measurements using direct estimation of specific and volume activity of β-emitting radionuclides in dry, liquid, and gas-like probes in a laboratory environment or even straight on-field (despite it is probably hell to carry it). It is capable to work with activities from 1,9 Bq/kg (Bq/l) to 37 MBq/Kg (MBq/l) for isotopes 90Sr+90Y, 144Ce+144Pr, 106Ru+106Rh, 137Cs, 60Co (all in dry or liquid probes), 14C (water only) and also 41Ar, 85Kr, and 133Xe gases.

The instrumental set comes in two massive and perfectly organized grey-colored plywood boxes — there is nearly no free space inside of them. One of them houses two detectors as well as phantoms and various dosage tools, while another one is for electronic blocks, connection cables, and spare parts.

Radiometer RKb4-1eM Soviet dosimeter

Let’s unpack and take a closer look.


The set comes with two detection blocks — a large BDZhB-02 (БДЖБ-02) for gases and liquids and a smaller tabletop BDZhB-07 (БДЖБ-07) for dry or powder-like probes.

Radiometer RKb4-1eM Soviet dosimeter

Both of them are based on the scintillation principle: a particle hits a scintillation panel, which emits photons, that in their turn being registered by photomultiplier — a special vacuum tube that generates signals of current when exposed to light. These signals are then processed by logic boards of the radiometer, being transformed into activity readings.

This is how photomultipliers look like — few of them come in spare parts:

Radiometer RKb4-1eM Soviet dosimeter

Radiometer RKb4-1eM Soviet dosimeter

In BDZhB-02 there are two of them, enclosed in these sturdy grey protective cylinders. In a black central part inside is a chamber with a set of scintillation panels:

Radiometer RKb4-1eM Soviet dosimeter

A chamber has two connectors for injecting a probe and upper and lower caps for washing the internals afterward. The inner space can take up to half a liter of liquid, so to precisely match this volume in the set comes a large metal cup. In the case of gases, a special syringe-like pump should be connected using a rubber pipe (unfortunately, it is absent in our set).

A top handle allows us to conveniently carry the detection block when working on-field. That is an interesting detail, as according to the manual this detector should be used in a shielded lead chamber with at least 100 mm thickness to eliminate background radioactivity influence.

Radiometer RKb4-1eM Soviet dosimeter

Contrary to it, BDZhB-07 consists of one detection block and has a special retractable cassette for small plastic cups where a dry probe should be placed. When a cassette is fully inserted, you can hear a click of a microswitch that prevents measuring when the probe is not inserted.

Radiometer RKb4-1eM Soviet dosimeter

Both detectors have cables with RP-type industrial connectors with specially designed rubber protective covers, to avoid any moisture getting inside.


We have seen many devices, but from our point, the main logic block of the radiometer is one of the most beautiful examples of industrial design of that time — it gives a nice feeling of the epoch. On the backside, there is a slot for accumulator batteries, on the left — a power connector and a large connector for detection blocks, on the right — for a digit-printing device B5-13 (Б5-13). By default, all of them are covered with rotary plastic caps.

Radiometer RKb4-1eM Soviet dosimeter

Radiometer RKb4-1eM Soviet dosimeter

A result of a measurement cannot be directly used. The device will provide the user with a counting rate, which later has to be converted to useful values using a set of formulas and a special table. Why so complex? Because very much depends on the type of isotope measured. So the counting rate is displayed on a 4-digit nixie tube indicator. Below are 3 LEDs — for charging, measurement cycle, and measurement progress.

Below are two calibration knobs to adjust readings when working with check source with known activity, and start/stop buttons that are used if the measurement is being performed until stopped manually.

Radiometer RKb4-1eM Soviet dosimeter

To the right are three more knobs — one is power selector (off/internal/external), the second is a mode of operation (calibration, KBq, or MBq measurements) and the last one is timing (10 seconds, 100 seconds, infinity). A toggle switch sets display mode — internal display or printer. In the second case, readings will be on a paper tape only.

Another block, which is the power supply and charger, comes in the same-sized enclosure but has just a power switch, a fuse slot, and a neon-tube charge indicator.

Let’s assemble and turn on

There is a strict warning in the operations manual that electronic block must not be turned on without the detector connected, otherwise, it can get damaged. So let’s bring it all together — we will use BDZhB-07. Once connected, all together it gives a great visual impression — all elements are precise and high quality. The only tricky thing is to secure a rubber cover from the detector cable, but it was a minor issue.

Once switched on, it requires at least 15 minutes to warm-up, in this period it can give random readings.

Math, math, math

Unfortunately, our set requires quite long and complex calibration, so we can explain only how it should work.

As this device is designed for express testing, a mistake rate varies very much from isotope to isotope and type of measurement. As by manual, it is generally around 40%, with painful 90% for 90Sr+90Y. An operator also has to know with which isotope they are mostly dealing. So let’s keep it in mind and review the method of getting a specific activity for 90Sr+90Y.

The first step is to get a background counting rate — the average quantity of impulses per second. To do this, we should make 5 measurements for 100 seconds each of the empty cassettes and get the numbers from the nixie display. This is the reason why a printer would be handy — during every round of counting, the display remains dark and shows result only once the round was finished:

Round 01: 0005 impulses
Round 02: 0015 impulses
Round 03: 0017 impulses
Round 04: 0009 impulses
Round 05: 0008 impulses

and then get a value using the main formula

ambient = (Σ Ni) / 5 • t; i = [1;5]

where Ni is a number of registered counts on every round and t is the duration of every round. So N̄ambient = (5+15+17+9+8) / (5 • 100) = 0.108.


After this, we have to check a test material. A user’s manual declares different methods of calculations depending on a probe’s type. In the case of dry solid or powder-like probes (e.g. soil, wheat, etc.), they have to be fragmented, mixed, and then packed into the cassette, to be measured 10 times for 100 seconds.

Let’s imagine, that these are our readings:

Round 01: 0015 impulses
Round 02: 0025 impulses
Round 03: 0029 impulses
Round 04: 0039 impulses
Round 05: 0042 impulses
Round 06: 0025 impulses
Round 07: 0041 impulses
Round 08: 0012 impulses
Round 09: 0032 impulses
Round 10: 0012 impulses

then we get a count rate value (N̄probe+ambient) for a probe (with the influence of the ambient radiation) by the formula above, which is 0.272, and deduct from it background count rate:

eff = Nprobe+ambient - N̄ambienteff = 0.272 - 0.108 = 0.164.

Having this value, we can get the specific activity (Q, Bq/kg) with a formula

 Q = N̄eff / P 

using the value of sensitivity of the radiometer per specific nuclide (P, for 90Sr+90Y it is 4.86 * 10-5 Kg / s • Bq) from a table:


Radiometer RKb4-1eM Soviet dosimeter

Q = 0.164 / (4.86 * 10-5 Kg / s • Bq) = 0.164 / 0,0000486 = 3374 Bq/kg


The set requires a skilled lab technician for the full set of operations, as well as features a quite complex protocol of measurements. By memories of those, who worked with it professionally, it was hard to get used to this device as well as learn all small aspects of its functioning; however, once it was done, it became a powerful tool. Apart from the technical side, from our point, it is an interesting historical piece of technology and a good example of early specialized post-Chernobyl devices.

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