General

I cannot guarantee the functionality of the device. The user is responsible for complying with all relevant safety regulations and for the use of the measurement results. Please make sure to follow my guidelines provided on the “Zielstellung”, .

Preface:

There are various proposals for building radiation measurement devices for hobbyists available online, often using an Arduino. My assembly instructions aim to adapt existing electronic circuits to a specific measurement task, namely the detection of radon decay products. I may have described some aspects in too much detail, but I wanted to document my experiences and provide a basic version of the setup. Of course, everyone is free to apply their own expertise and potentially find better and faster solutions. What was important to me was the measurement side, particularly the air intake system, including flow rate measurement, as well as a detailed explanation of how to evaluate the measurements.

The entire measurement system can be divided into three modules, which can also be set up separately:

Scheme of Measuring System

Detektor Amplifier Unit

Electronic circuit

Commercially available detectors used in measurement devices are very expensive and practically unaffordable for hobbyists. However, in literature and on the internet, I have found interesting and well-functioning DIY solutions that are also suitable for a device to measure radon and thoron decay products. I have implemented two variants in practice:

• Variant 1: Published in the magazine Elektor 06-2011, by author Burkhard Kainka
• Variant 2: A solution available online.

Both variants use the BPX 61 photodiode as a radiation detector, but the glass must be removed to avoid shielding the alpha radiation from the diode. This, however, is not entirely “risk-free.” Occasionally, one may end up damaging the diode, which is frustrating, especially since it is the most expensive component, costing around €4-6 per piece.

I used a rather crude method by clamping the diode (with the glass facing down) in a vise and carefully, very slowly, squeezing it until the first crack appeared in the glass. Then, using a pointed tool, I cautiously tried to enlarge the crack and, ideally, “pick out” the glass. If necessary, I would carefully squeeze it once more. Throughout this process, I always hold the diode in a way that prevents any glass shards from falling onto the silicon wafer.

Version 1 uses a two-stage amplifier with bipolar transistors, whereas Version 2 operates with an operational amplifier. In both versions, the first stage functions as a charge-sensitive preamplifier, while the second stage generates a usable voltage pulse.

Version 1: Amplifier with bipolar transistors

electronic circuits

Version 1:  Amplifier with bipolar transistors

Version 2: Amplifier with operational amplifier

The module 272/1 consists of 2 operational amplifiers.

I built both versions and obtained fully functional amplifiers with each. Using Version 1 with bipolar transistors, I achieved pulses of approximately 2V, while Version 2 with operational amplifiers produced signals with an amplitude of around 500 mV. The signal-to-noise ratio was sufficiently good in both versions to allow for reliable noise discrimination.

The supply voltage UbUb​ can range between 9V and 12V, but it should remain constant within +/- 1V to avoid significant changes in pulse amplitude.

The detector-amplifier module is the most sensitive part. The signals from the diode are so small that electromagnetic interference (EMI) must be completely prevented. Therefore, the module is fully enclosed in an aluminum case.

Here is a proposal for the housing design:

A bracket for a brass tube will be attached to the housing, which will facilitate air intake through a filter (see “Air Intake System” to be published later).

Mechanics

An aluminum housing measuring 70 mm x 55 mm x 30 mm was used for the detector-amplifier unit (DAU), which still needs to be mechanically adjusted.

The following modifications are necessary:

• Opening in the housing for the PIN diode (window for measuring alpha radiation)
• Bracket for the intake device with filter
• Cable conduit for the power supply of the DAU and signal coupling
• BNC socket for easy connection of an oscilloscope

The mechanical setup will be explained below in bullet points and with images:

1. Hole for PIN Diode

Fit the PIN diode into a non-conductive plate; thickness D of the plate: 1.5 mm≤D<3 mm1.5mm≤D<3mm.

I used an acrylic plate with a thickness of 1.5 mm, which fit the width of the lower part of the housing and thus already provided a certain level of stability. This plate was temporarily fixed to one side of

Figures: Acrylic plate with diode (defective unit used as a dummy) fixed to the housing

2. Mounting Bracket for the Suction Device

A bracket is installed on the back of the housing to securely attach the suction filter device in a reproducible manner. Suction is performed through a brass tube with a diameter of 6 mm (see description below under “Suction Device”).

To hold the brass tube, two identical aluminum blocks are used, each with a thickness of more than 6 mm and a 6 mm diameter hole (matching the diameter of the suction tube), as shown in Figure 3 (width less than the housing width).

Figure 3: Aluminum block with hole

Two through-holes of 2.5 mm are drilled into this block (see next Figure).

After drilling, the blocks are split lengthwise in the middle.

Figure 5: Separated Aluminum Block

One part will be provided with 2 additional 2.5 mm holes (symmetrically and at a sufficient distance from the existing holes). This part, now with 4 holes, serves as the lower section of the bracket that needs to be mounted on the housing. Threads for M3 screws are then tapped into all 4 holes. The other part with 2 holes will be drilled out to a diameter of 3.2 mm (3.5 mm is also acceptable). The same procedure applies to the second block.

With the completed brackets, a tube (a sample piece of sufficient length) can be fixed with two M3 screws each and temporarily aligned on the back of the housing. The tube axis should ideally be aligned so that it is positioned directly above the existing opening for the PIN diode. Inaccurate work led to my prototype showing the two brackets not quite aligned in parallel. However, this is merely a cosmetic issue. The important thing is that the tube runs neatly parallel over the housing and is centered above the opening. Once aligned, the assembly can be fixed to the housing using super glue. After the glue has set, the tube can be removed, and markings can be made on the housing through the holes intended for mounting the bracket’s lower part. The (not very strong) super glue fixation can now be released, and a 3.5 mm hole can be drilled in each marked spot. This allows the lower parts of the brackets to be securely screwed to the housing. For this, countersunk screws (M3) should be used.

Figure: Fixed Bracket on the Back of the Housing

3. Holes for Cable Routing and BNC Connector

Both holes are made on the side opposite the PIN diode. The anodized (non-conductive) layer around the hole for the BNC connector must be removed using sandpaper to ensure that the housing is grounded.

Figure: Grommet and BNC Connector on the Bottom Part of the Housing

Installation and Commissioning

I assembled the amplifier on a universal circuit board. Figures 8a and 8b show the amplifier (here, version 2 with an operational amplifier) mounted in the enclosure with the connection cable. The connection cable provides the power supply and transmits the pulses from the amplifier to the data acquisition system. Additionally, the pulses are routed to a BNC connector, allowing them to be monitored with an oscilloscope (preferably a digital storage oscilloscope). This is also important for quality assurance, as described later.

Figures Amplifier and housing

Once the amplifier and the (functional) diode are installed in the enclosure, the diode opening in the housing should be sealed light-tight with as thin aluminum foil as possible. I tested standard household foil, which causes the signal amplitude to be approximately halved due to the slowing down of the alpha particles. Given the expected favorable signal-to-noise ratio, this is not problematic. The exact position of the center of the diode opening should be marked so that the measurement filter can be positioned as concentrically as possible over the diode.

Before installation, the proper function of the diode should be tested by measuring its dark current.

The operating voltage is supplied via the cable; in the final version, this is done through the connection cable between the DVE and the “Pulse Measurement, Data Acquisition, Data Storage with Arduino Nano” module. I used a SUB-D9 connector for this. The pulses from the amplifier output are also coupled out via the cable. The amplifier output is also routed to the BNC connector. The enclosure is grounded; however, the diode should not be in contact with ground.

The completed DVE can be tested by exposing the measurement filter, as described under point “Preliminary Measurement” in the side “Pulse Measurement, Data Processing, and Data Storage with Arduino Nano” section, to radon decay products (or using a material that emits alpha radiation) and observing the generated pulses with an oscilloscope.

Figure:  Pulse after the amplifier, here after version 1 with bipolar transistors

The scaling of the ordinate is 1 V, and the abscissa is 100 μs.

Note: The DVE may exhibit a so-called microphonic effect, meaning that vibrations can also generate pulses that distort the measurement results. This effect must be tested practically and taken into account during measurements; therefore:

• Do not place the DVE on vibrating components, and
• Try not to change the position of the DVE during the measurement.

Proceed to the pages with the description of the Pulse M