Radiation unit converter: Sv ↔ rem, Gy ↔ rad, Bq ↔ Ci

Old units, new units, and why both survive

The SI units — gray (Gy), sievert (Sv) and becquerel (Bq) — replaced the rad, rem and curie decades ago, but US regulations (10 CFR 20), many instrument dials and older source certificates still use the traditional ones. The conversions are exact multiples, so errors come from slipped prefixes rather than from physics: mrem versus µSv, mCi versus MBq. Enter a value once and the converter lists it in every unit of the same quantity.

The fourth mode goes one step further: from absorbed dose (Gy, rad) to equivalent dose (Sv, rem) using ICRP Publication 103 radiation weighting factors. Photons and electrons have w_R = 1, protons 2 and alpha particles 20. For neutrons w_R is a continuous function of energy:

The formula

1 Sv = 100 rem     1 Gy = 100 rad     1 Ci = 3.7 × 10¹⁰ Bq

H = w_R · D

Neutrons (E in MeV):
  E < 1:        w_R = 2.5 + 18.2 · exp(−[ln E]² / 6)
  1 ≤ E ≤ 50:   w_R = 5.0 + 17.0 · exp(−[ln 2E]² / 6)
  E > 50:       w_R = 2.5 + 3.25 · exp(−[ln 0.04E]² / 6)

Worked example

Annual limit. The US occupational whole-body limit of 5 rem per year is 5 × 0.01 = 0.05 Sv = 50 mSv.

Neutron dose. An absorbed dose of 0.2 mGy from 1 MeV neutrons: w_R = 20.7, so H = 0.2 × 20.7 = 4.14 mSv (414 mrem).

Frequently asked questions

How many rem is 1 sievert?

1 Sv = 100 rem exactly, so 1 mSv = 100 mrem and 1 µSv = 0.1 mrem. The same factor links grays and rads: 1 Gy = 100 rad.

What is the difference between Gy and Sv?

The gray measures absorbed dose — energy deposited per kilogram. The sievert measures equivalent or effective dose, which weights absorbed dose by how biologically damaging the radiation type is. For gamma rays and x-rays the weighting factor is 1, so 1 Gy gives 1 Sv; for alpha particles it is 20.

How many becquerels are in a curie?

Exactly 3.7 × 10¹⁰ Bq (37 GBq). A millicurie is 37 MBq and a microcurie is 37 kBq.

Why does the neutron weighting factor depend on energy?

Neutrons deposit energy through recoil protons and nuclear reactions whose biological effectiveness peaks around 1 MeV. ICRP 103 replaced the old step function with a continuous curve: about 2.5 for thermal neutrons, rising to about 20.7 near 1 MeV, and falling again at high energies.

Can I calculate effective dose here?

Not in general. Effective dose sums organ equivalent doses weighted by tissue factors (w_T) and requires organ dose estimates. For a uniform whole-body exposure the effective dose equals the equivalent dose shown here. This site does not do clinical or regulatory dosimetry.

Sources

1. Thompson, A. & Taylor, B. N. (2008). Guide for the Use of the International System of Units (SI). NIST Special Publication 811. www.nist.gov
2. U.S. Nuclear Regulatory Commission. 10 CFR 20.1004, Units of radiation dose. www.nrc.gov
3. ICRP (2007). The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103, Annals of the ICRP 37(2–4). (Table 2 and Eq. 4.3: radiation weighting factors.) www.icrp.org