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Bq
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Ci
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dps
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s⁻¹
Enter values to see results
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Bq
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Ci
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dps
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s⁻¹
The Radioactive Activity Calculator determines the activity of a radioactive sample — the number of nuclear disintegrations occurring per unit time. Activity is the most practical measure of a radioactive source's intensity, directly determining the radiation dose rate and the sample's usefulness in applications from medical imaging to industrial gauging.
Activity $$A$$ is defined as $$A = \lambda N$$, where $$\lambda$$ is the decay constant and $$N$$ is the number of radioactive atoms. This calculator computes activity from either a known atom count or from sample mass and molar mass, converting to standard units of becquerels (Bq) and curies (Ci). It also handles time-dependent activity after a specified elapsed period.
Activity is the rate of radioactive disintegrations:
$$A = \lambda N = \frac{\ln(2)}{t_{1/2}} \cdot N$$
When starting from mass, the number of atoms is calculated using Avogadro's number:
$$N = \frac{m}{M} \cdot N_A$$
where $$m$$ is sample mass in grams, $$M$$ is molar mass, and $$N_A = 6.022 \times 10^{23}$$ mol⁻¹. If elapsed time $$t$$ is specified, the activity at that time is:
$$A(t) = \lambda N_0 e^{-\lambda t} = A_0 \cdot e^{-\lambda t}$$
Activity units: 1 Becquerel (Bq) = 1 disintegration per second (dps). 1 Curie (Ci) = 3.7 × 10¹⁰ Bq, originally defined as the activity of 1 gram of Ra-226.
Activity in Bq gives the number of decays per second — the SI standard unit. Activity in Ci uses the older curie unit still common in medicine and industry; typical medical sources range from microcuries to millicuries. Disintegrations per second is numerically identical to Bq but emphasizes the physical process. The Decay Constant in s⁻¹ is provided for reference. Higher activity means more radiation emitted per second and requires greater shielding precautions.
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One gram of pure C-14 has an activity of ~165 GBq (~4.46 Ci), which is significant and would require careful handling in a laboratory setting.
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After 12 hours (~2 half-lives), the Tc-99m activity drops to about 25% of its initial value, demonstrating rapid clearance ideal for medical imaging.
Activity is the rate at which radioactive atoms in a sample are decaying, measured in disintegrations per second. It quantifies the intensity of a radioactive source. One becquerel (Bq) equals one disintegration per second; one curie (Ci) equals 3.7 × 10¹⁰ Bq.
The becquerel (Bq) is the SI unit equal to 1 disintegration per second. The curie (Ci) is an older unit equal to 3.7 × 10¹⁰ Bq, originally defined as the activity of one gram of radium-226. Medical applications often use millicuries (mCi) or microcuries (µCi) due to convenient magnitudes.
Activity decreases exponentially: $$A(t) = A_0 e^{-\lambda t}$$. After one half-life, activity is halved. After 10 half-lives, it drops to about 0.1% of the original value. Activity and the number of remaining atoms decrease at exactly the same rate.
Not necessarily. Danger depends on the type of radiation emitted (alpha, beta, gamma), the energy of the radiation, the distance from the source, shielding, and duration of exposure. A high-activity alpha emitter sealed in a container may be less hazardous than a low-activity gamma emitter with no shielding.
Specific activity is the activity per unit mass of a substance (Bq/g or Ci/g). For a pure isotope, $$SA = \lambda N_A / M$$ where $$N_A$$ is Avogadro's number and $$M$$ is the molar mass. It depends only on the half-life and atomic mass, not the sample size.
Activity (Bq) measures disintegration rate, while dose (Sv or rem) measures biological effect. The conversion requires knowing the radiation type, energy, geometry, and tissue type. The relationship involves absorbed dose (Gy), radiation weighting factors, and tissue weighting factors as defined by the ICRP.
Typical diagnostic imaging doses range from 100-1000 MBq (3-27 mCi) for PET scans using F-18 FDG, and 400-1100 MBq (11-30 mCi) for SPECT scans using Tc-99m. Therapeutic doses for I-131 thyroid treatment can reach 3700-7400 MBq (100-200 mCi).
Activity depends on the number of atoms ($$A = \lambda N$$), not the mass directly. Converting mass to atom count requires knowing how many atoms are in each gram, which is $$N_A/M$$ where $$M$$ is the molar mass. Different isotopes of the same element have different molar masses and different activities per gram.
The human body contains about 140 g of potassium, of which 0.012% is radioactive K-40. This gives approximately 4,400 Bq of activity, making potassium-40 the largest source of internal radioactivity in the body, contributing about 0.17 mSv per year to natural background dose.
For a single isolated isotope, activity always decreases. However, in decay chains, a daughter isotope's activity can initially increase as it is produced by the parent until secular or transient equilibrium is reached. Also, neutron activation in a reactor produces new radioactive atoms, increasing activity until production balances decay.
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