Specific Activity and Source Mass Calculator

Q: What is the formula for specific activity?

a = ln2·NA/(t½·M), giving becquerel per gram when the half-life is in SECONDS. Equivalently a = λ·(NA/M): the decay constant per second multiplied by the number of atoms in a gram. Three places it goes wrong. The half-life must be in seconds — leaving it in days is a factor of 86,400 and the answer still looks like a number. M must be the isotopic mass, not the element's standard atomic weight, which for caesium-137 is a 3.0 per cent error. And the result is a theoretical maximum for a pure sample, not the specific activity of anything you can buy.

a = ln2·N_A/(t½·M), and therefore how many milligrams a 37 GBq cobalt-60 source is and how many nanograms of technetium are in a dose — checked against twelve published specific activities, ten of which reproduce to better than one per cent.

Last updated 7 Oct 2026

The decay arithmetic here is exact; what you put into it is not. A half-life, an activity quoted for a stated date and the assay time itself all carry uncertainty, and an activity projected far from the calibration date compounds it. Nothing here tells you whether a source is safe to handle or to move, and nothing here replaces the certificate the source came with.

Specific activity and source mass

Nuclide → Bq/g, and activity ↔ mass both ways

NuclideTwenty-two nuclides, with their half-lives from NPL Report IR 6. Picking one fills the half-life field below and locks it; choosing the last option unlocks that field so any nuclide at all can be used. The half-lives carry published uncertainties — NPL’s own, which run from 0.0096 per cent on manganese-54 to 0.44 per cent on radium-223, a factor of forty-five between the best and the worst known in this list — and the sources do not fully agree in the last figures. Selecting a nuclide also fills in its ISOTOPIC mass, which is the one specific activity needs. It is not the element’s standard atomic weight and the two are not interchangeable: caesium-137 is 136.907 against caesium’s 132.905, a 3.0 per cent difference that lands directly on the answer. Technetium has no standard atomic weight at all, having no stable isotope.

Half-lifeLocked while a nuclide is selected above. Choose “other nuclide” and this becomes the input, which is how an unlisted nuclide — a short-lived cyclotron product, a daughter, anything at all — still works here. The decay arithmetic does not care where the half-life came from.

…inHalf-lives here are stored in days, which is how NPL tabulates them. A year means 365.25 days, the Julian year; with 365 instead, a fifty-year projection of a thirty-year half-life moves by about a third of a per cent.

Molar mass of the nuclideLocked while a nuclide is selected. Use the mass of the ISOTOPE, which to three figures is just its mass number — 60 for cobalt-60, 137 for caesium-137 — and differs from it by under half a per cent because of the nuclear binding energy. Using the element’s standard atomic weight instead is the usual mistake and is worth up to several per cent.

An activity, to turn into a massThis is the direction the question is usually asked in: “how many grams is a 37 GBq cobalt-60 source” or “how much technetium is in a dose”. The answers are under a milligram and a few nanograms respectively, which is why a sealed source is a disc of metal in a capsule rather than a lump of anything.

…inOne curie is 3.7×10¹⁰ becquerel by definition, not by measurement, so the curie columns on this page are a rename of the becquerel ones and carry no extra uncertainty of their own.

A mass, to turn into an activityThe other direction, and the one that shows how unforgiving the arithmetic is: one milligram of cobalt-60 is 41.9 GBq, and one milligram of technetium-99m would be 195 TERAbecquerel, which is about 264,000 patient doses and does not exist anywhere.

…inPicograms are at the bottom of the ladder because for the short-lived nuclides that is where the answers are: a fluorine-18 PET dose of 370 MBq is 105 picograms of fluorine, and a dose a thousand times smaller would be a tenth of one.

41.87TBq/gExample

Cobalt-60, with a 37 GBq (1 curie) source and one milligram of material

One line, two divisions, and the word that has to be said out loud

a = ln2·N_A / (t½·M) · m = A / a · N = A / λ · m = N·M / N_A

a: the specific activity: becquerel per gram of the pure nuclide. A property of the nuclide and nothing else, so it is a constant and not a measurement — but it is the constant for a hypothetical sample containing nothing but that nuclide, which is the thing to keep hold of
N_A: the Avogadro constant, 6.02214076×10²³ per mole. Exact by definition since the 2019 revision of the SI, so it contributes no uncertainty at all
M: the ISOTOPIC mass in g/mol, not the element’s standard atomic weight. To three figures it is just the mass number; the difference from it is the nuclear binding energy and is under half a per cent. Using the element’s atomic weight is the standard mistake and for caesium-137 it is a 3.0 per cent error, because natural caesium is caesium-133
t½: the half-life, IN SECONDS if the answer is to be in becquerel per gram. This is where the arithmetic goes wrong most often: NPL tabulates half-lives in days, and leaving them in days gives an answer 86,400 times too large
carrier-free: the assumption, and the international consensus radiochemistry nomenclature guidelines say it may be used only “in the rare case where the theoretical maximum specific or molar activity is unambiguously proven”. That is exactly this number, so the page calls it a theoretical maximum. A real preparation is below it — sometimes by orders of magnitude — because of stable isotopes of the same element, the nuclide’s own accumulating daughter, and whatever the chemistry left behind

Worked example

Cobalt-60, with a 37 GBq (1 curie) source and one milligram of material
THE HALF-LIFE IN SECONDS, which is the step to be careful about. NPL gives cobalt-60 as 1,925.2 days. That is 1,925.2 × 86,400 = 1.6634×10⁸ s. Leaving it in days gives an answer 86,400 times too big, and the answer still looks like a number
THE DECAY CONSTANT. λ = ln2/1.6634×10⁸ = 4.167119×10⁻⁹ per second
ATOMS PER GRAM. N_A/M = 6.02214076×10²³/59.933817 = 10,050,000,000 Tper g. Note the mass used: 59.9338, the ISOTOPIC mass of cobalt-60, not 58.933 which is cobalt's standard atomic weight. That would be a 1.7 per cent error here, and 3.0 per cent for caesium-137
SPECIFIC ACTIVITY. Multiply the two: λ × N_A/M = 41.87 TBq/g, which is 1,131.7 Ci/g or 41.871 GBq/mg. Wikipedia's specific activity table gives 41.884 PBq per KILOGRAM, which is 41.884 TBq/g — so the derivation is 0.03 per cent from the published value, and reading that table as per gram would be a factor of a thousand
THE MASS OF A CURIE, which is the answer people actually come for. 37×10⁹ Bq ÷ 41.87 TBq/g = 0.8837 mg. Under a milligram. A curie of cobalt-60 needs a shielded container, a transport licence and a written procedure, and it weighs less than a grain of sugar. For comparison, the same curie of technetium-99m is 190 NANOGRAMS and a 740 MBq patient dose is 3.79 ng
AND THE ATOM COUNT, by two routes that must agree. From the activity: N = A/λ = 37×10⁹/4.167119×10⁻⁹ = 8,879,000 Tatoms. From the mass: 0.8837 mg × 10,050,000,000 Tper g = the same figure. Eight and a half billion billion atoms, in a milligram, giving 37 billion decays a second — which is one atom in 240 million per second, and is why a half-life of five years and a hazard of this size live together comfortably
WHAT THE NUMBER IS NOT. It is the CARRIER-FREE maximum: the activity of a gram containing nothing but cobalt-60. A real cobalt-60 source is cobalt metal that was irradiated, so most of it is cobalt-59 that never activated, and its actual specific activity is a few hundred Ci/g rather than 1,132 — the US National Isotope Development Center quotes “>11 TBq/g (>300 Ci/g)” for its product, about a quarter of the theoretical figure. The consensus nomenclature guidelines reserve the term carrier-free for the rare case where the theoretical maximum is “unambiguously proven”, which is why this page says theoretical maximum and means it

The derivation against twelve published specific activities

This is the verification, printed rather than claimed. Twelve published specific activities from six independent sources — two university radiation safety services, a health physics chapter’s nuclide data sheets, a commercial safety data sheet and two Wikipedia articles — against the same one-line derivation, and TEN of the twelve reproduce to better than one per cent. Iridium-192 agrees to 0.03 per cent, sodium-22 to 0.02, americium-241 to 0.05. The two that miss are named: iodine-125 by 1.41 per cent and thallium-201 by 1.08. The last two columns are what makes the comparison fair, because specific activity goes as 1/t½ and the two sides often used different half-lives: recomputed with the source’s OWN stated half-life, iodine-125 closes from 1.41 per cent to 0.24, which accounts for five sixths of the gap, and every row in the table lands inside 1.1 per cent. Thallium-201 does not close — it stays about one per cent out either way, which means the published figure itself is not exactly ln2·N_A/(t½M) — and it is left in the table saying so rather than quietly dropped. The decay arithmetic is exact. The inputs are not: a quoted activity carries the calibration laboratory’s uncertainty, the assay time is often recorded only to the nearest hour, and a half-life itself has a published uncertainty. Projected far from the calibration date those compound — and the published half-lives themselves disagree in the last figures, caesium-137 being 10976 days to NPL and 11000 to another published decay sheet.
Nuclide	NPL half-life	Isotopic mass	Derived here	…in Ci/g	Published, as printed	…in Bq/g	Difference, %	Half-life the source used	Difference on THAT half-life, %	Source
Tc-99m	6.0067 h	98.906251	195,200 TBq/g	5,274,860.9	5,243,820 Ci/g	194,000 TBq/g	0.592	6.0200 h	0.370	U. Michigan EHS RSDS
F-18	1.8288 h	18.000938	3,522,000 TBq/g	95,194,266.0	3.52e18 Bq/g	3,520,000 TBq/g	0.062	1.8300 h	-0.003	NCHPS Nuclide Safety Data Sheet
Tl-201	3.0421 d	200.970820	7,902 TBq/g	213,577.0	2.159e5 Ci/g	7,988 TBq/g	-1.076	3.0400 d	-1.008	U. Michigan EHS RSDS
Lu-177	6.6470 d	176.943758	4,108 TBq/g	111,019.7	4.1e15 Bq/g max	4,100 TBq/g	0.188	6.6500 d	0.143	NCHPS Nuclide Safety Data Sheet
I-131	8.0223 d	130.906126	4,600 TBq/g	124,337.5	124,068 Ci/g	4,591 TBq/g	0.217	8.0500 d	-0.128	U. Michigan EHS RSDS
I-125	59.4070 d	124.904630	651.1 TBq/g	17,597.2	17,353 Ci/g carrier-free	642.1 TBq/g	1.408	60.1000 d	0.238	U. Michigan EHS RSDS
Ir-192	73.8270 d	191.962602	340.9 TBq/g	9,213.6	341 TBq/g	341 TBq/g	-0.028	73.8200 d	-0.019	Wikipedia, Iridium-192
Na-22	2.6026 y	21.994437	231.1 TBq/g	6,245.3	6,244 Ci/g	231 TBq/g	0.020	2.6020 y	0.043	U. Michigan EHS RSDS
Co-60	5.2709 y	59.933817	41.87 TBq/g	1,131.7	41.884 PBq/kg	41.88 TBq/g	-0.031	5.2704 y	-0.020	Wikipedia, Specific activity
Sr-90	28.8022 y	89.907730	5.108 TBq/g	138.1	5.143 PBq/kg	5.143 TBq/g	-0.681	28.8000 y	-0.673	Wikipedia, Specific activity
Cs-137	30.0507 y	136.907089	3.215 TBq/g	86.9	3,220 GBq/g	3.22 TBq/g	-0.153	30.2200 y	-0.712	Stuart Hunt & Associates RMSDS
Am-241	432.5804 y	241.056829	126.8 GBq/g	3.4	126.91 TBq/kg	126.9 GBq/g	-0.048	432.6000 y	-0.053	Wikipedia, Specific activity

All twenty-two: specific activity, and the mass of a curie

The mass columns are the point of this table and they span nine orders of magnitude. A curie of americium-241 is a third of a gram — 292 milligrams — which you could see and weigh. A curie of cobalt-60 is 0.88 of a milligram. A curie of technetium-99m is 190 nanograms, and a curie of fluorine-18 is ten and a half nanograms — about 350 million million atoms, which sounds enormous and is an invisible smear. The ordering is the whole lesson: specific activity goes as 1/t½, so the shorter the half-life the less material a given activity is — more than seven orders of magnitude between the two ends of this table — and the nuclides that need shielding and a transport licence are frequently the ones with no weighable mass at all. It also explains why a sealed source is a disc of inert alloy — the radioactive part could not hold itself together — and why the chemistry of a radiopharmaceutical is done at nanomolar concentrations where ordinary reaction kinetics do not apply. One caution on reading this table against a supplier’s: these are CARRIER-FREE theoretical maxima and a real preparation is below them, sometimes far below. This is a first-pass calculation on an idealised geometry — a point source, a uniform slab, no self-absorption in the source, no container, no floor and no walls. Real sources have extent and encapsulation, and real rooms scatter. A closed-form answer cannot see any of that.
Nuclide	Half-life	Isotopic mass	Specific activity	…in Ci/g	…in TBq/g	Mass of 37 GBq (1 Ci)	Mass of 1 GBq	Atoms per gram
Tc-99m	6.0067 h	98.906251	195,200 TBq/g	5,274,860.9	195.2 kTBq/g	189.6 ng	5.124 ng	6,089,000,000 Tper g
F-18	1.8288 h	18.000938	3,522,000 TBq/g	95,194,266.0	3.522 MTBq/g	10.5 ng	283.9 pg	33,450,000,000 Tper g
Ga-67	3.2613 d	66.928202	22,130 TBq/g	598,220.1	22.13 kTBq/g	1.672 µg	45.18 ng	8,998,000,000 Tper g
In-111	2.8049 d	110.905108	15,530 TBq/g	419,751.2	15.53 kTBq/g	2.382 µg	64.39 ng	5,430,000,000 Tper g
Tl-201	3.0421 d	200.970820	7,902 TBq/g	213,577.0	7.902 kTBq/g	4.682 µg	126.5 ng	2,997,000,000 Tper g
Mo-99	2.7479 d	98.907708	17,780 TBq/g	480,429.7	17.78 kTBq/g	2.081 µg	56.26 ng	6,089,000,000 Tper g
Y-90	2.6684 d	89.907152	20,140 TBq/g	544,271.7	20.14 kTBq/g	1.837 µg	49.66 ng	6,698,000,000 Tper g
Lu-177	6.6470 d	176.943758	4,108 TBq/g	111,019.7	4.108 kTBq/g	9.007 µg	243.4 ng	3,403,000,000 Tper g
I-131	8.0223 d	130.906126	4,600 TBq/g	124,337.5	4.6 kTBq/g	8.043 µg	217.4 ng	4,600,000,000 Tper g
Ra-223	11.4300 d	223.018502	1,895 TBq/g	51,224.0	1.895 kTBq/g	19.52 µg	527.6 ng	2,700,000,000 Tper g
I-125	59.4070 d	124.904630	651.1 TBq/g	17,597.2	651.1 TBq/g	56.83 µg	1.536 µg	4,821,000,000 Tper g
Ir-192	73.8270 d	191.962602	340.9 TBq/g	9,213.6	340.9 TBq/g	108.5 µg	2.933 µg	3,137,000,000 Tper g
Co-57	271.8000 d	56.936291	312.2 TBq/g	8,437.7	312.2 TBq/g	118.5 µg	3.203 µg	10,580,000,000 Tper g
Mn-54	312.1300 d	53.940359	287 TBq/g	7,755.5	287 TBq/g	128.9 µg	3.485 µg	11,160,000,000 Tper g
Zn-65	244.0100 d	64.929241	304.9 TBq/g	8,241.6	304.9 TBq/g	121.3 µg	3.279 µg	9,275,000,000 Tper g
Na-22	2.6026 y	21.994437	231.1 TBq/g	6,245.3	231.1 TBq/g	160.1 µg	4.328 µg	27,380,000,000 Tper g
Co-60	5.2709 y	59.933817	41.87 TBq/g	1,131.7	41.87 TBq/g	883.7 µg	23.88 µg	10,050,000,000 Tper g
Ba-133	10.5399 y	132.906002	9.443 TBq/g	255.2	9.443 TBq/g	3.918 mg	105.9 µg	4,531,000,000 Tper g
Eu-152	13.5222 y	151.921745	6.439 TBq/g	174.0	6.439 TBq/g	5.746 mg	155.3 µg	3,964,000,000 Tper g
Sr-90	28.8022 y	89.907730	5.108 TBq/g	138.1	5.108 TBq/g	7.244 mg	195.8 µg	6,698,000,000 Tper g
Cs-137	30.0507 y	136.907089	3.215 TBq/g	86.9	3.215 TBq/g	11.51 mg	311 µg	4,399,000,000 Tper g
Am-241	432.5804 y	241.056829	126.8 GBq/g	3.4	126.8 mTBq/g	291.7 mg	7.883 mg	2,498,000,000 Tper g

The caesium-137 figure two sources do not agree on

Printed in full rather than resolved silently, because the rule on this site is that where two sources disagree both appear. The caesium-137 specific activity is quoted as 83 Ci/g in one widely read place and about 87 in another, and the two cannot both be right. The test that settles it is not authority but internal consistency: take each source’s OWN stated half-life, put it through ln2·N_A/(t½M), and see whether it reproduces the specific activity the same source prints. The safety data sheet’s 30.22 years gives 86.4 Ci/g against its printed 87.0, which is agreement. Wikipedia’s 30.17 years gives 86.6 Ci/g against its printed 83.0, which is not — the row disagrees with itself by 4.3 per cent. So this site uses NPL’s half-life and the 87 Ci/g family of values, and says why. Note also the fourth row: Wikipedia’s own article on the nuclide gives a half-life 0.4 per cent different from the one in its specific activity table, which is a reminder that a single publication is not a single source. The decay arithmetic is exact. The inputs are not: a quoted activity carries the calibration laboratory’s uncertainty, the assay time is often recorded only to the nearest hour, and a half-life itself has a published uncertainty. Projected far from the calibration date those compound — and the published half-lives themselves disagree in the last figures, caesium-137 being 10976 days to NPL and 11000 to another published decay sheet.
Source	Half-life it states	Specific activity it prints	…in Ci/g	What that half-life implies	Self-consistent?
NPL Report IR 6 (the half-life used throughout this site)	10,976 ± 30 d (30.0507 y)	— (not tabulated)	—	3.215 TBq/g (86.9 Ci/g)	n/a — it publishes no specific activity
Stuart Hunt & Associates, Radioactive Material Safety Data Sheet	30.22 y (11,038 d)	3,220 GBq/g	87.0	3.197 TBq/g	YES — its own half-life reproduces its own figure to 0.7 per cent
Wikipedia, Specific activity	30.17 y (11,020 d)	3.071 PBq/kg	83.0	3.202 TBq/g	NO — its own half-life implies 86.6 Ci/g, 4.3 per cent above the 83 it prints
Wikipedia, Caesium-137	30.04 y	—	—	3.216 TBq/g	n/a — a fourth published half-life, for the record

A curie of cobalt-60 weighs 0.88 of a milligram, and the two divisions that get it wrong

A curie of cobalt-60 weighs 0.88 of a milligram. That is the sentence this page exists for. Thirty-seven billion decays a second, a shielded container, a transport licence and a written procedure, and the radioactive material itself weighs less than a grain of sugar. In the other direction a 740 MBq technetium-99m patient dose is 3.8 nanograms of technetium, and a 370 MBq fluorine-18 PET dose is about a tenth of a microgram — 105 picograms — of fluorine. There is no material there. What there is, is a decay rate — and specific activity is the exchange rate between the two.

a = ln2·N_A/(t½M), and both divisions matter. Dividing by the half-life says the shorter-lived a nuclide is, the less of it a given activity needs: that is why the mass column of the table on this page spans more than seven orders of magnitude between americium-241 and fluorine-18, and why the nuclides that most need shielding are often the ones with no weighable mass. Dividing by the molar mass is the quieter trap, because it must be the ISOTOPIC mass and not the element’s standard atomic weight. For cobalt the difference is 1.7 per cent; for caesium-137 against natural caesium-133 it is 3.0; and technetium has no standard atomic weight at all, having no stable isotope. The third trap is the unit of the half-life, and it is the one that produces a confident answer off by a factor of 86,400.

This was checked against twelve published figures and the working is on the page. Six independent sources — two university radiation safety services, a health physics chapter’s nuclide data sheets, a commercial safety data sheet and two Wikipedia articles — and ten of the twelve reproduce to better than one per cent, several of them to better than 0.05. The two that miss are named rather than dropped: iodine-125 by 1.41 per cent and thallium-201 by 1.08. For iodine-125 five sixths of the gap is the half-life each side used — specific activity goes as 1/t½, so the two factors multiply exactly, and recomputing with the source’s own 60.1 days closes it to 0.24 per cent. Thallium-201 does not close, which means that published figure is itself not quite ln2·N_A/(t½M), and the table says so.

Carrier-free is an assumption and a real preparation is not. The number at the top of this page is the activity of a gram containing nothing but the nuclide, and the international consensus radiochemistry nomenclature guidelines are blunt about the term: carrier-free may be used only “in the rare case where the theoretical maximum specific or molar activity is unambiguously proven”, and no-carrier-added is a qualitative description rather than a figure. Three things push a real sample below the maximum. Stable isotopes of the same element that the production route left behind — a cobalt-60 source is irradiated cobalt metal, most of which is still cobalt-59, and the National Isotope Development Center quotes over 300 Ci/g for its product against a theoretical 1,132. The nuclide’s own daughter, which is chemically identical and accumulates: a technetium-99m eluate is diluted by the technetium-99 that grew in since the last elution, and only 87 per cent of molybdenum-99 decays produce the metastable state in the first place. And whatever the chemistry introduced. The chart on this page shows the first mechanism quantitatively: the specific activity of a MIXTURE falls by the decay factor while the theoretical maximum stays where it is.

Why anybody needs this number. For a radiopharmacy it sets whether a labelling reaction is possible at all, because at nanomolar concentration the radionuclide competes with trace metals and with the walls of the vial. For a source manufacturer it sets how much material a given activity requires and therefore how big the source is and how much self-absorption it has. For waste and transport it converts between the activity on the paperwork and the mass in the drum. And for anyone doing dosimetry it is the sanity check that catches a misplaced factor of a thousand: if a calculation says a patient dose is a gram of anything, it is wrong. For the activity itself over time rather than its mass, the decay and remaining-activity page is next door, and the decay constant this page needs in its per-second form is set out on the half-life and decay constant page.

Frequently asked questions

How many grams is a 37 GBq cobalt-60 source?

0.884 of a milligram. Cobalt-60’s carrier-free specific activity is ln2·N_A/(t½M) with t½ = 1,925.2 days = 1.663×10⁸ s and M = 59.9338 g/mol, which gives 4.187×10¹³ Bq/g, or 1,132 Ci/g; 37×10⁹ divided by that is 8.84×10⁻⁴ g. Two cautions. That is the carrier-free figure, and a real cobalt-60 source is irradiated cobalt metal that is mostly still cobalt-59 — the National Isotope Development Center quotes over 300 Ci/g for its product, about a quarter of the theoretical value, so the real source is several times heavier. And the mass of the SOURCE is larger again: it is a disc of alloy inside a welded capsule.

How much technetium is in a nuclear medicine dose?

About four nanograms. A 740 MBq (20 mCi) dose of technetium-99m is 3.79×10⁻⁹ g of technetium, which is 2.3×10¹³ atoms, or 0.038 nanomoles. That is why a radiopharmaceutical is a chemical system unlike any other: at nanomolar concentration the labelling reaction competes with adsorption onto the vial and with trace metal impurities present at similar levels, and mass-action intuitions from ordinary chemistry do not transfer. It is also why the pharmacological effect of the technetium itself is nil — there is not enough of it to have one — and the whole of the biology is in whatever it is attached to.

What is the formula for specific activity?

a = ln2·N_A/(t½·M), giving becquerel per gram when the half-life is in SECONDS. Equivalently a = λ·(N_A/M): the decay constant per second multiplied by the number of atoms in a gram. Three places it goes wrong. The half-life must be in seconds — leaving it in days is a factor of 86,400 and the answer still looks like a number. M must be the isotopic mass, not the element’s standard atomic weight, which for caesium-137 is a 3.0 per cent error. And the result is a theoretical maximum for a pure sample, not the specific activity of anything you can buy.

What does carrier-free actually mean, and is it ever true?

It means the sample contains no non-radioactive isotope of the same element, so its specific activity is the theoretical maximum. The international consensus radiochemistry nomenclature guidelines restrict the term to “the rare case where the theoretical maximum specific or molar activity is unambiguously proven”, and recommend no-carrier-added as a qualitative description otherwise. In practice it is rarely true: a reactor-produced nuclide sits in a target full of its own stable isotope, a generator eluate contains the long-lived daughter that grew in since the last elution, and every nuclide accumulates its own decay product. Technetium-99m is the clean illustration — one published data sheet gives 5.24×10⁶ Ci/g carrier-free and 3.4×10⁶ for the pertechnetate form it is actually supplied as.

Why do shorter-lived nuclides have higher specific activity?

Because specific activity is the decay RATE per gram, and a shorter half-life means each atom decays sooner, so the same number of atoms gives more decays per second. The relationship is exactly inverse: a goes as 1/t½, so halving the half-life doubles the specific activity. The practical consequence is the one that surprises people — the nuclides that need the most shielding are frequently the ones with the least material. A curie of americium-241, half-life 432 years, is 292 milligrams, which you could weigh on a balance. A curie of fluorine-18, half-life 110 minutes, is ten and a half nanograms.

Should I use the isotopic mass or the element’s atomic weight?

The isotopic mass, always. The standard atomic weight is an average over an element’s natural isotopes and has nothing to do with a single nuclide. For caesium-137 the isotopic mass is 136.907 against caesium’s standard atomic weight of 132.905, because natural caesium is caesium-133 — a 3.0 per cent error straight into the answer. For cobalt-60 it is 59.934 against 58.933, which is 1.7 per cent. Technetium and promethium have no standard atomic weight at all, having no stable isotope, so for those the question does not even arise. To three significant figures the isotopic mass is just the mass number, and the difference from it is the nuclear binding energy.

Why does the specific activity on my certificate not match this page?

Almost certainly because the certificate describes a real preparation and this page computes a theoretical maximum, and the real figure is lower — usually by a factor of a few, occasionally by orders of magnitude. The three mechanisms are stable isotopes of the same element left over from production, the nuclide’s own accumulated daughter, and carrier added deliberately. A certificate also states a reference time, because specific activity of a mixture falls as the sample ages: the activity decays and the chemical mass does not. The consensus nomenclature guidelines ask for the time of measurement to be stated with any specific or molar activity for exactly that reason. If the mismatch is a factor of a thousand rather than a factor of a few, check whether the figure you are comparing against is per gram or per kilogram — Wikipedia’s table is per kilogram.

Related calculators

References

A. Pearce, NPL Report IR 6: Recommended Nuclear Decay Data, National Physical Laboratory. Cited, not reproduced — Crown copyright. It is the source of every half-life on this page and, more usefully, of every half-life’s UNCERTAINTY, which most published tables drop. Those uncertainties are the reason the nuclide table here prints a ± column: manganese-54 is known to 0.03 days in 312.13, which is 0.0096 per cent, and technetium-99m to 0.000 04 days in 0.250 28, which is 0.016; caesium-137 is known to 30 days in 10 976, which is 0.27 per cent, and radium-223 to 0.05 days in 11.43, which is 0.44 and is the widest in the set — a factor of forty-five across the list, and a projection over many half-lives inherits whichever end it lands on.
Commission on Isotopic Abundances and Atomic Weights (CIAAW) / IUPAC, atomic masses, as carried in _nuclide_data.py. Specific activity needs the mass of the particular ISOTOPE, not the standard atomic weight of the element, and the two are not interchangeable: caesium-137 is 136.907 against caesium’s standard atomic weight of 132.905, a 3.0 per cent difference that would land directly on the answer. Technetium and promethium have no standard atomic weight at all, having no stable isotope.
The 2019 revision of the SI, as published by the BIPM and tabulated in NIST’s Fundamental Physical Constants. Two exact values are used here and both are exact by definition rather than by measurement: the Avogadro constant N_A = 6.022 140 76×10²³ mol⁻¹, and the elementary charge, through which 1 eV = 1.602 176 634×10⁻¹⁹ J. One curie is likewise 3.7×10¹⁰ Bq by definition, so every curie figure on these pages is a rename of a becquerel figure and adds no uncertainty of its own.
Specific activity, Wikipedia (read 7 October 2026). Used for the statement of the formula a = ln2·N_A / (t½·M) and for four published figures the derivation here reproduces: cobalt-60 at 41.884 PBq per kilogram, americium-241 at 126.91 TBq/kg, strontium-90 at 5.143 PBq/kg and iodine-131 at 4.625 EBq/kg. Note the units: that table is per KILOGRAM, and misreading it as per gram is a factor of a thousand. Its caesium-137 row is the one entry this batch could not reproduce — see the module docstring and the table caption, where both values are printed.
Iridium-192, Wikipedia (read 7 October 2026). Quoted for one number: “a specific activity of 341 TBq·g⁻¹ (9.22 kCi·g⁻¹)” against a half-life of 73.82 days. The derivation from NPL’s 73.827 days and the isotopic mass gives 340.9 TBq/g, which is 0.03 per cent away — the closest agreement of any row in the check.
Cobalt-60, Wikipedia (read 7 October 2026). Quoted for “the radioactive activity of a gram of ⁶⁰Co is close to 42 TBq (1,100 Ci)”, which the derivation reproduces at 41.87 TBq/g. Also the source for the half-life cross-check at 5.2714 years.
Radioisotope Safety Data Sheets, University of Michigan Environment, Health & Safety, Radiation Safety Service (read 7 October 2026). Four published specific activities come from this series, each with the half-life the sheet itself used, which is what makes the comparison fair: iodine-125 17 353 Ci/g at 60.1 days and explicitly labelled “theoretical/carrier free”; iodine-131 124 068 Ci/g at 8.05 days; technetium-99m 5 243 820 Ci/g at 6.02 hours carrier-free, with 3.4×10⁶ Ci/g given separately for the pertechnetate form; thallium-201 2.159×10⁵ Ci/g at 3.04 days; and sodium-22 6 244 Ci/g at 2.602 years. Where the derivation here differs by more than a per cent — iodine-125 by 1.4, thallium-201 by 1.1 — the difference is the half-life each side used, not the arithmetic.
Nuclide Safety Data Sheets, published by the North Carolina chapter of the Health Physics Society (nchps.org) and mirrored by the University of Virginia Environmental Health & Safety radiation safety service (read 7 October 2026). Two maximum specific activities are taken from them: fluorine-18 at 9.51×10⁷ Ci/g [3.52×10¹⁸ Bq/g], and lutetium-177 at 1.1×10⁵ Ci/g [4.1×10¹⁵ Bq/g] max. Both reproduce to better than a quarter of a per cent. The lutetium sheet is also a small lesson in half-life disagreement: it prints two values, 6.73 days and 6.65 days, side by side in one line.
Radioactive Material Safety Data Sheet: Cs-137, Stuart Hunt & Associates Ltd. (read 7 October 2026 through the University of Florida radiation safety document library). It gives 3 220 GBq/g against a half-life of 30.22 years. This is the second, self-consistent caesium-137 figure that the derivation reproduces to 0.15 per cent, and it is why the Wikipedia row of 83 Ci/g is treated here as the outlier rather than as the answer.
H. H. Coenen, A. D. Gee, M. Adam, G. Antoni, C. S. Cutler, Y. Fujibayashi, J. M. Jeong, R. H. Mach, T. L. Mindt, V. W. Pike and A. D. Windhorst, “Open letter to journal editors on: International Consensus Radiochemistry Nomenclature Guidelines”, EJNMMI Radiopharmacy and Chemistry (2019), CC BY 4.0. The authority for the wording this page uses about carrier-free. It defines specific activity as “the measured activity per gram of compound” and molar activity as “the measured activity per mole of compound”, requires the time of measurement to be stated for both, calls no-carrier-added and carrier-added “non-quantitative terms”, and restricts carrier-free to “the rare case where the theoretical maximum specific or molar activity is unambiguously proven”. The number at the top of the specific-activity page is that theoretical maximum, which is exactly the thing the guideline says is rarely proven — so the page says so rather than calling the figure the specific activity of anything real.
G. J. Morrissey, “Lest We Forget Generator Technology”, Journal of Nuclear Medicine Technology, September 1996. The source for the two timings this batch quotes about a molybdenum-99/technetium-99m generator: “the maximum amount of activity is achieved at approximately 23 hr after the previous elution” — 22.83 hours in its own table — and “transient equilibrium is attained with the ⁹⁹Mo/^99mTc generator system at approximately 48 hr following the last elution”, at which point “the ^99mTc activity approximates the ⁹⁹Mo activity”. It also makes the point that the technetium mole fraction of the eluate is about 70 per cent after a four-hour ingrowth and about 27 per cent after twenty-four, which is why the chemistry and the activity do not keep step.