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 a surface contamination of the object. But what, if no? Or, even more interesting — what, if there is an internal contamination, particles being emitted from are blocked by the apple's skin?

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

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 laboratory environment or even straight on-field (despite it is probably a 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.

Let’s unpack and take a closer look.

Detectors

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

Both of them are based on 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 then processed by logic boards of the radiometer, being transformed to activity readings.

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

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:

A chamber has two connectors for injecting a probe and upper and lower caps for washing the internals afterwards. 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 case of gases, a special syringe-like pump should be connected using rubber pipe (unfortunately, it is absent in our set).

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

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 probe is not inserted.

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

Electronics

We have seen many devices, but from our point, a main logic block of radiometer is one of the most beautiful examples of industrial design of that time — it gives a nice feeling of epoch. On a back side 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.

A result of measurement cannot be directly used. The device will provide the user with counting rate, that later has to be converted to useful values using set of formulas and a special table. Why so complex? Because very much depends on a type of isotope measured. So the counting rate being 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 measurement is being performed until stopped manually.

To the right are three more knobs — one is power selector (off/internal/external), 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 second case, readings will be on a paper tape only.

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

Let’s assemble and turn on

There is a strict warning in operations manual that electronic block must not be turned on without 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 detector cable, but it was a minor issue.

Once switched on, it requires at least 15 minutes for 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 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.

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 empty cassette and get the numbers from the nixie display. This is the reason why 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 number of registered counts on every round and t is a 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 case of dry solid or powder-like probes (e.g. soil, wheat, etc.), they have to be fragmented, mixed and then packed into 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 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 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:

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

Conclusion

The set requires a skilled lab technician for the full set of operations, as well 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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