Unit of Radioactivity

Introduction

Radioactivity, as the name introduces, is the emission of energy waves by unstable atomic nuclei of certain elements. There are 38 radioactive elements in the periodic table. The radioactive elements are highly unstable and try to become stable through successive disintegration. During this process, it emits radioactive waves. Essentially these unstable atomic nuclei hardly try to become stable while in this process, it emits radioactive waves.

The alpha, beta and gamma rays are harmful to our body and out of these radiations the gamma rays are the most harmful radiation. The emission of radioactive rays is called radioactive decay. If it is beta rays emitted, then it is beta decay. Curies (Ci) is the unit used to measure this type of activity of radioactive materials.

What Is Radioactivity?

The alpha, beta, and gamma rays which happen naturally and artificially are dangerous to health. We will try to understand the ability of these rays. The ionising power is the ability of radiation to damage molecules. Penetration power is the ability of radiation to pass through matter. The table below shows some properties of the radioactive particles.

Particle	Atomic number, Mass number	Penetrating Power	Ionising Power	Shielding Material
Alpha (α)	2, 4	Very Low	Very High	Paper
Beta (β)	-1, 0	Intermediate	Intermediate	Wood
Gamma (γ)	0, 0	Very High	Very Low	Lead

Table-1: Radioactive particles and their characteristics

Now we know that these rays are labelled as radioactive radiations. It is normal to have the interest to know how these radiations are generated. The isotopes of radioactive elements like Thorium, Radium etcetera release alpha, beta, and gamma rays to attain a stable nucleus. These rays cause radioactive decay.

The common radioactivity-capable materials are Uranium, Thorium, Radium etcetera. There are many more and these elements form a series or chain in the transmutations from one to another releasing rays. The radiation decay series of Uranium-238 reaching the stable nucleus of Pb-206 after going through 14 cycles is shown below.

Uranium → Protactinium → Thorium → Actinium → Radium → Francium → Radon → Astatine → Polonium → Bismuth → Lead → Thallium → Mercury. (Ending in Lead)

SI Units of Radioactivity

Curie and Becquerel are the units of radioactivity in disintegration per second.

$$\mathrm{One\:curie = 1\:Ci = 3.7\times 10^{10}\:disintegrations\:per\:second}$$

$$\mathrm{1\:Ci = 3.7\times 10^{10}\:Bq = 37\:GBq (37\:GigaBecquerels)}$$

SI unit for measuring the radioactivity is Becquerel.

$$\mathrm{1\:Bq = 1\:disintegration\:per \:second}$$

Other Units of Radioactivity

Some of the other units that define radioactivity measurement and conversion methods are

• Rutherford (Rd) - Activity corresponding to 1 million nuclei decay. So we can say 1 Becquerel = 1 MicroRutherford

• Ambient radiation levels are measured in Gray per hour (Gy/h) or Sievert per hour (Sv/h), which are international units. In the US, Roentgen per hour (R/h) or Rem per hour (rem/h) are also used.

• 1 Bq = 1 disintegration/s,

• 1Gy = 1 Joule per kilogram,

• 1 Sv = 1Gy × FQ,

where FQ = 1 for X Rays, Beta, and Gamma rays.

FQ=10 for neutrons and protons.

FQ=20 for Alpha rays.

• $\mathrm{1\:Rad=10^{−2}\:Gy}$

Types Of Radioactivity

Radioactivity is the existence of three types of rays alpha, beta, and gamma. The alpha rays are helium ions with 2 protons and 2 neutrons. It can be represented as He(2,4) where 2 indicates the 2 positive charges which are protons and 4 is the mass of the ion. Since atomic mass is the sum of protons and neutrons and since it has 2 protons, there are 2 neutrons in the alpha particle.

Alpha Decay

Let us discuss and find how a nucleus that will undergo disintegration by emitting alpha particles. One such reaction is

$$\mathrm{_{92}^{238}U\rightarrow_2^4 He+_{90}^{234}Th}$$

Here Uranium transmutes to thorium releasing alpha particles. U-238 and Th-230 are radioactive elements capable of producing alpha decay.

Beta Decay

Similar to the alpha ray, the beta ray is a high-energy electron and is represented as e (-1, 0) or B (-1, 0). The convention used here is X (atomic number, mass number). So here the atomic number of the electron is given -1 and the mass number is 0 since the electron does not have protons or neutrons. The reaction releasing beta radiation is

$$\mathrm{_{90}^{234}Th\rightarrow\:_{−1}^{0} e+_{91}^{234}Pa}$$

Pa is Protactinium and Pa-234 is also a beta emitter transmuting to Uranium-234.

Gamma Decay

Gamma decay happens in almost all nuclear reactions but is not always shown. In the alpha decay of U-23, gamma rays are released as shown below.

$$\mathrm{_{92}^{238}U\rightarrow_{2}^4 He+_{90}^{234}Th+2y}$$

Law of Radioactive Decay

The law states that the probability per unit time that a nucleus will decay is a constant, independent of time. This constant is called the decay constant, denoted by λ.

$$\mathrm{A\:number\:of\:nuclei = N\times\:e^{−\lambda t}}$$

$$\mathrm{Activity = A\times e^{−\lambda t}}$$

$$\mathrm{Mass = M\times e^{−\lambda t}}$$

where N = the total number of particles in the sample

A = the number of decays per unit time

M = the remaining mass of radioactive material.

One more relation we can find like

$$\mathrm{\lambda(s^{−1})=ln(2)/ t^{1/2}}$$

where $\mathrm{t^{1/2}}$ is the half time and ln(2) is the natural log of 2.

Conclusion

The unit of radioactivity, becquerels, and curies and their conversion which is the primary topic, is all clarified in this tutorial. Special attention is given to the SI unit and alternate units used to measure radioactivity. The abilities of radioactive rays and the shielding material that can block their abilities are studied together.

Radioactivity is essentially the decay of alpha, beta, and gamma rays as the radioactive material try to transmute to a stable material. These three decays and the laws governing them are discussed to conclude the topic.

FAQs

Q1. How can radioactive materials help the medical industry in tracing internals?

Ans. Iodine isotopes are used as tracers to determine cardiac output and fat metabolism due to the property of this isotope accumulating in such areas.

Q2. How do smoke detectors work?

Ans. The ionisation of air between two plates in the smoke detector creates a smooth but constant electric flow. When smoke enters the chamber, this electric flow is stopped which will trigger an alarm. One microcurie or less of Americium-241 can work for smoke detectors.

Q3. What is Gamma radiography? How is it used?

Ans. Gamma rays coming from radioactive material inside a radiography device are used for scanning and inspecting places inaccessible due to process or challenging nature. It can be used to scan inside pipes, ships and places of complex design.

Q4. What is Lead Pig? How is it related to radioactive materials?

Ans. Lead pig is a colloquial term indicating container. This lead container has a thick wall and is designed to avoid spillage of material inside and is rightly and safely locked.

Q5. Which material has the fastest half-life?

Ans. Francium-223 has the fastest half-life which is 22 minutes.