Abstract

Abstract

Ionisation chambers are a good way to know if you have a radiation field. Availability of a radiation detection instrument may not always be possible. It is therefore, necessary to be able to make a detector to detect the availability of ionising radiation. Here, an ionisation chamber was made from material available at home and tested using radioactive consumer goods. The objective of the project was to make and test a basic ionisation chamber. A small but significant current flow was observed as a sign that the ionisation chamber worked. The current flow was detected using a multimeter. The current measured was 0.02 A. The objective of the project was to introduce us to the existing and new technologies for building, testing, and calibrating a radiation detector.

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Introduction of the ion chamber

When quantification of detected ionising radiation started, history points to ionization chambers as being the first equipment to be used, as far back as 1908. Their use has been taken up by newer technology in some situations. But they have not been completely discarded. They still are the type of equipment one would choose when measuring radiation in high radiation fields. In 1910, the existence of cosmic radiation was confirmed using an ionization chamber. Improvements in the chamber were made over the years to make it more user friendly and also to incorporate modern technology.

An ionization chamber gives a response through interactions between the gas filling the chamber and the incident radiation. This results in the gas being ionised and excited by the incident radiation along the trajectory of the incident radiation. The chamber consists of a gas volume in an enclosure (chamber). The body of the chamber is negatively charged (cathode). In the centre of the chamber is a positively charged wire (anode). These create an electric field that is vital to the detection of the incident radiation. An electrometer is used to measure the electrical current that is produced as the gas is ionised and electrons start to move. The design of the chamber depends on the intended use. It can be made to be tissue equivalent or air equivalent by way of material used to make the wall of the chamber.

The design is also dependant on the type of radiation to be detected (charged particles such as alpha particles, uncharged particles in this case neutrons, photons such as gamma rays). Most designs include a thin entrance window where the ionising radiation will enter the chamber in which the gas to be ionised is contained. In this project, the ionisation chamber made will be the window type and it will be air equivalent.

We shall now look at how the ionisation chamber work. As ionising radiation enters the envelope as described above, gas molecules or atoms (depending on the type of gas filling the chamber) physical structure is disrupted. An electron is dislodged and an ion pair is created. The electron is attracted to the anode and the remaining positively charged ion, much heavier than the electron, slowly moves to the cathode. This ionisation can be a result of a primary interaction between the molecule and the incident radiation or can be secondary to the transfer of energy

to an energetic electron. Such an electron is known as a delta ray. The most important thing is the number of ion pairs created in this interaction. The International Commission on Radiological Units (ICRU) defined the Roentgen in 1928, and for a chamber to satisfy the requirements of ICRU, all ion pairs must originate inside the chamber and they must all be collected.

For ionisation to occur, the incident radiation must impart the amount of energy equal to the ionisation energy of the gas molecule that interacted with the radiation. Most lightly bound electrons require 10 – 25 electron volts to ionise. It must be noted that not all interactions lead to ionisation even though energy was imparted. Rather the molecule goes to an excited state. This means an electron is promoted to a higher energy level in the same molecule. This results in average energy imparted to a molecule per ion pair being far more than the ionisation energy of the molecule itself. At the same time, not all ionisations are collected. Due to the tendency of gas molecules to be in motion, thermal motion, other interactions can hinder the electrons from reaching the anode. Collisions between the electron and fill gas molecules can occur resulting in diffusion. The electron can also attach to another atom causing the formation of an anion. The charge can be transferred to another particle (where an ion takes an electron from another atom) and lastly there can be recombination (where the electron combines with an ion.

Because of the existing external electrical field, the charged particles will migrate to oppositely charged electrodes as they will be subjected to an electrostatic force. The movement of the particles is influenced by phenomena, the thermal motion, and drift due to the existence of the electric field. The drift velocity is obtained from the relationship below;

= µԐ 1

Where; µ = mobility constant

Ԑ= electric field

= gas pressure

Measuring of the ionisation events happens in two ways; (1) measurement of ionisation current or (2) collecting the charges.

Measuring the ionisation current involves using a high resistance and a battery connected in series with the chamber and the battery. Since the current is very small, an amplifier is used to enable the electrometer to measure the current. The electrometer measures the voltage drop across the resistance applied.

The other way is where the electrometer is directly connected to the chamber. The signal from the chamber is converted from dc to ac. This conversion makes amplifying the signal easier. The charges collected will change the potential of the electrometer according to the following equation;

= 2

It is ideal for a chamber with a very small capacitance so that a higher voltage can be produced.

Figure 1. Measuring ionisation current using an electrometer connected directly to the chamber.

Creating a capacitance that fluctuates about an average value of C, the resultant ac will have an amplitude proportional to the ionisation current. Where the steady ionisation current is shown by;

= 3

Integration methods are to be used to measure the ionisation current. Capacitors naturally lose charge, if this loss is made very small, then very small ionisation current can be detected.

Materials and Methods

Materials

The following materials will be needed to build an ionisation chamber.

A fusible metal alloy soldering iron (Weller Standard Duty. 25 Watts. 750-degree Fahrenheit. Apex Tool Group LLC. 1000 Lufkin Road. Apex, NC 27539 USA) will be used to melt the solder wire. A 1 mm thick solder wire (Alpha Fry. Item # 51406. Cookson Electronics Assembly Materials, 109 Corporate Blvd. South Plainfield, NJ 07090 USA) will be used to make permanent connections for some components of the circuit. A 1-pint can of coffee, (1811 Mathews Twonship Pkwy, Mathews, NC 28105, USA) will be used as the gas envelope of the ionisation chamber.

The tin size is 6 cm inside diameter and 10 cm height. The inside of the time must be conductive. Since the can used was not conductive, a beauty 360 nail polish remover (CVS Health, Monroeville, PA 15146, USA), was used to clean the plastic coating. A 0.5 mm aluminium foil (W.W. Grainger Company, 2255 Northwest 89th Pl. Miami, FL 33172, USA) will be used to cover the front of the tin.

A transistor, Darlington Transistor (ON Semiconductors, Manufacturer number BC517-D74Z. Phoenix, AZ, USA) with PNP polarity will be used in the circuit to amplify the electrical power

supplied to the tin. A 4.7 kΩ resistor (Manufacturer number MOS1CT528R472J, KOA Speer Electronics, Inc. Bradford, PA 16701 USA.) will be used to reduce the current flow.

If not reduced, the current can damage the other components of the chamber. A 9 V battery (Amazon Basics alkaline battery. 410 Terry Ave North, Seattle WA, 98109 USA) will be used as the power supply of the detector. A 0.7mm copper wire (MSC Industrial Supply Co. 75 Maxess Road Melville, New York 11747-3151, USA) will be used as the anode. An analogue multimeter, (GMT-312 Gardner Bender Instruments. Milwaukee, WI USA) will be used to measure the electrical current, resistance, and voltage of the electrical power produced in the ionisation chamber.

A radioactive source, potassium chloride salt (The French’s Food Company. 445 E. Mustard Way. Springfield, MO 65803-9416, USA) is used to produce the required ionizing radiation that will be detected by the ionization chamber.

Methods

We will make a perforation in the centre of the tin. This perforation will be used for the centre wire that will be our anode. A 9 cm long 22 gauge copper wire will be cut and soldered to the base terminal of the transistor (making the anode). It will be inserted in the hole made in the centre of the tin. This wire will not make contact with the tin at all. The collector terminal of the transistor will be soldered to the wire that is connected to the negative terminal of the battery. The emitter terminal will be soldered to the negative wire from the multimeter.

The positive terminal of the battery will also be connected to the to the positive wire from the multimeter. One end of the resistor will be soldered to the base of the tin and the other side will be connected to the positive terminal of the battery. The remaining 5 cm tin will be soldered at the back of the 10 cm tin to provide shielding of the components soldered at the back of the tin. The foil will be cut and taped to the opening of the 10 cm tin also to shield the inside of the tin from external electric fields. Figure 2 below shows the circuitry of the ionisation chamber.

Figure 2. Schematic of the ionisation chamber

PNP Transistor

Tin Can

4.7 k: resistor

9 V battery

Results and Discussions

All pieces of the ionisation chamber were connected either through soldering or through a loose connection. Since soldering is operator dependant, numerous soldering attempts were done to join the pieces together. Figure 2 below shows the ionisation chamber after it was pieced together. After the ionisation chamber was assembled together, a radioactive source described above was introduced to test if there will be any current flow detected by the connected multimeter.

A small current flow of 0.02 A was measured as shown in figure 4. Since the current flow was very small, the multimeter was disconnected from the ionisation chamber to see if there was any deviation of the needle without any input from the ionisation chamber. Figure 4 shows the multimeter reading when the ionisation chamber was disconnected.

Figure 3. A picture of the ionisation chamber after being assembled. On the plate is a white powder, the potassium salt that was used as a source of radiation to test the ionisation chamber.

Figure 4. Multimeter showing the needle deflected to the right when the radioactive source was placed close to the window of the ionisation chamber.

Figure 5. The multimeter when not connected to the ionisation chamber with is needled resting at zero position indicating that there was no current flow.

Conclusion

An ionisation chamber is the simplest radiation detector to make. It is however important to have a basic understanding of electronics to know how to provide enough power supply to the chamber for effective current flow when a radioactive source is introduced. Once the ionisation chamber is assembled testing it is necessary to be able to see how much current flows when a radioactive source is introduced. The objectives of the project were realised once the current flow was noticed. Due to the Covid 19 pandemic, the ionisation chamber was not calibrated with a source of known activity.

Reference

1. Flakus, F.N. “Radiation Detection: Detecting and Measuring Ionizing Radiation – a Short History.” IAEA BULLETIN, VOL 23, No 4.

2. Knoll, G.F. “Radiation Detection and Measurement. 4th ed.” Wiley Publishing. 2010.

3. Rollo F.D. “Nuclear Medicine Physics, Instrumentation, and Agents.” S.M. Mosby Company. Saint Louis, MO. 1978.

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