Decay Correction to a Calibration Date Calculator

Q: How do I decay-correct an activity to a calibration time?

Multiply by e−λt going forward in time from the calibration time, and by e+λt going backward to it, with λ = ln2/t½ and t and t½ in the same unit. The only thing to be careful about is the direction: later than the calibration time means less activity, earlier means more. For technetium-99m six hours after calibration the factor is 0.5004, so an 11,100 MBq vial holds 5,554 MBq; six hours BEFORE its calibration time it would hold 22,183. Both calculations are the same arithmetic and the sign is the whole difference.

What a vial assayed for a stated calibration time holds now, what it will hold at administration, and the volume to draw — with the two intervals kept apart and the direction named in every label, because the sign is what goes wrong.

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.

Decay correction to a calibration date

Assay, two intervals → activity now and volume to draw

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. The default is technetium-99m, which is what most of these corrections are for: at six hours a vial loses eleven per cent of its activity every hour, so the difference between 09:00 and 10:00 is not a rounding.

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.

ASSAYED activity, at the calibration timeThe figure on the vial or the delivery note, with the calibration time it belongs to. It is not the activity now unless the calibration time happens to be now.

…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.

Vial volume at that calibration timeNeeded because the answer is a volume. The concentration at calibration is the assayed activity divided by this; the concentration at any later time is that figure decayed. The volume itself does not decay, which is the whole mechanism: a vial’s activity falls and its concentration falls with it, so the volume you must draw for a fixed dose rises through the day.

FROM the calibration time TO nowPositive if the calibration time has passed. NEGATIVE if the vial is calibrated for a time that has not arrived yet, which is the ordinary case for a morning delivery calibrated for noon — and then the activity now is HIGHER than the assayed figure. This is the input whose sign is got wrong, so the page prints the factor for it separately and says which way it went.

…inHours by default, because that is the unit a nuclear medicine department works in. The engine has no date type and cannot subtract 07:15 from 13:40 for you: that subtraction is yours, and it is where the other half of the errors come from.

FROM now TO administrationThe interval people forget. A dose drawn at 13:00 for a patient at 14:30 has decayed by eleven per cent an hour in between if it is technetium-99m, so the volume has to be drawn for the activity wanted at the END of this interval, not at the moment of drawing. Set it to zero if the dose is given immediately.

…inSame ladder. For a fluorine-18 dose this interval is frequently the dominant term: at a 110-minute half-life, thirty minutes between drawing and injection is 17 per cent of the activity.

WANTED activity, at the moment of administrationIn the same unit as the assayed activity above. Read the label carefully: this is the activity in the patient, at injection, not the activity in the syringe when you draw it — the page prints that second figure separately and it is the larger of the two.

Not a circuit: the two intervals drawn as two timelines, which is the whole subject of this page in one picture. The top row is the signed one. NOW is its middle mark and never moves; what moves is the CALIBRATION mark, and it swaps sides. Enter a positive interval and the calibration time sits to the left of now, in the past, the arrow runs forward and the factor is below one. Enter a negative interval, which is the ordinary case for a vial delivered in the morning and calibrated for noon, and the calibration mark jumps to the right of now, the arrow runs backward and the factor goes ABOVE one, because the vial holds more activity now than its label says. That is the sign error this page exists to prevent, and swapping the mark makes it visible rather than arithmetical. The lower row is the second interval, from now to administration; it always runs forward and its factor is always below one. The two rows are kept apart deliberately, because when the calibration time is in the future both marks are on the same side of now and a single axis cannot place them without knowing which interval is longer. The bar at the bottom is the volume you must draw as a share of what is actually in the vial, with both volumes printed: when it fills the box, the vial cannot supply this dose at this time, whatever is left in it.

1.495mLExample

A 10 mL technetium-99m vial assayed at 11,100 MBq (300 mCi), six hours after its calibration time, with a 740 MBq (20 mCi) dose wanted an hour from now

Two intervals, two factors, and a division by a concentration

A_now = A_cal·e^−λt₁ · A_adm = A_cal·e^{−λ(t₁+t₂)} · V = A_wanted·V_vial / A_adm

t₁: calibration time to NOW, and the only signed input on the page. Positive when the calibration time has passed; NEGATIVE when it has not, which is the ordinary case for a vial calibrated for noon and delivered at eight. A negative t₁ gives a factor above one and an activity now higher than the assay, and getting its sign wrong puts the answer out by the factor SQUARED
t₂: now to administration, the interval that gets forgotten. The volume must be drawn for the activity wanted at the END of it. Eleven per cent an hour for technetium-99m, 31 per cent an hour for fluorine-18, and 0.4 per cent an hour for lutetium-177 — which is why the habit of ignoring it survives, and why it should not travel between nuclides
A_adm / V_vial: the concentration at the moment of administration, which is what the volume is divided by. Note which concentration: not the one at calibration and not the one now. The vial’s volume does not decay, so the concentration falls at exactly the rate the activity does
V: the volume to draw. It depends only on the SUM of the two intervals, so swapping them leaves it unchanged — a useful robustness, and the reason the activity-now figure needs more care than this one does
the assay: what the arithmetic is for and not what it replaces. The IAEA’s hospital radiopharmacy guidance requires that “each unit dose should be checked using a dose calibrator” and that “the radioactivity of the individual dose should be measured again before administration”. This page gives the volume to start from

Worked example

A 10 mL technetium-99m vial assayed at 11,100 MBq (300 mCi), six hours after its calibration time, with a 740 MBq (20 mCi) dose wanted an hour from now
THE HALF-LIFE AND THE HOURLY RATE. NPL gives technetium-99m as 0.25028 days, which is 6.00672 hours. λ = ln2/6.00672 = 0.11539 per hour, so the vial loses 10.90 per cent of its activity every hour. That figure is the reason this page exists: an hour's error in the assay time is an eleven per cent error in the dose
FACTOR 1, CALIBRATION TO NOW. Six hours is 6/6.00672 = 0.998881 half-lives, so the factor is e^−λt = 0.500388 — almost exactly a half, because six hours is almost exactly one half-life. The vial now holds 11,100 × 0.500388 = 5,554.3 MBq
FACTOR 2, NOW TO ADMINISTRATION. One more hour gives e^−λ×1 = 0.891014. The two factors multiply: 0.500388 × 0.891014 = 0.445853, which is the whole seven-hour correction and is the same number as e^−λ×7 computed in one step. Multiplying factors rather than adding intervals is the usual hand method and the two agree exactly
THE ACTIVITY AT ADMINISTRATION. 11,100 × 0.445853 = 4,949.0 MBq in the whole vial, and the vial is 10 mL, so the concentration at the moment of administration is 494.90 MBq/mL. Note WHICH concentration that is: not the calibration-time 1,110.0 MBq/mL, and not the present 555.43 either
THE VOLUME. 740 MBq ÷ 494.90 MBq/mL = 1.495 mL. That is the answer, and it is drawn NOW for administration in an hour — which means the syringe contains 1.495 mL × 555.43 MBq/mL = 831 MBq at the moment it is drawn, not 740. Both numbers are correct and they are for different instants; the dose calibrator beside you will read the larger one
THE ARITHMETIC CHECK THAT CATCHES A SIGN ERROR. Correct forward by seven hours and then back by seven hours: 11,100 × 0.445853 × 2.242894 = 11,100 MBq, the number we started with. That round trip is the only cheap test that detects a wrong sign, because a single forward calculation with the sign reversed produces a perfectly plausible number. Checking at zero elapsed time does NOT work — every wrong version returns the assay unchanged at t = 0
AND WHAT THE ERRORS ACTUALLY ARE HERE. NPL knows this half-life to 0.00004 days in 0.25028, which is 0.016 per cent, so over seven hours it contributes 0.01863 per cent — nothing. The dose calibrator is good to a few per cent. And the assay time, if it was written down to the nearest hour, is worth eleven per cent. The order of those three is the practical message: on this timescale, tighten the clock

The decay chart a package insert prints, for seven nuclides at once

This is the table that is printed in the back of a radiopharmaceutical insert, and reproducing it is a fair check on this page: Bracco’s CholeTec prescribing information gives “the fractions that remain at selected intervals after the time of calibration” against a 6.02 hour technetium-99m half-life, and the technetium column here is that chart computed from NPL’s 6.00672 hours instead — which differs in the fifth decimal place and nowhere a dose calibrator could see. The reason to put seven nuclides side by side is the spread. After one hour fluorine-18 has lost 31.5 per cent and lutetium-177 has lost 0.43; after eight hours fluorine-18 is down to 4.8 per cent of the assay while iodine-131 is still at 97.2. So “correct for decay” is a different size of correction for different nuclides, and the nuclides where it is nearly nothing are exactly the ones where people stop doing it and then carry the habit over to one where it matters. Multiply down the column, not across: the factors compose, so four hours is the two-hour factor squared. 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.
Hours after calibration	Tc-99m	F-18	Ga-67	In-111	Tl-201	I-131	Lu-177
0	1.000000	1.000000	1.000000	1.000000	1.000000	1.000000	1.000000
0.5	0.943935	0.827365	0.995582	0.994865	0.995264	0.998202	0.997830
1	0.891014	0.684534	0.991183	0.989756	0.990551	0.996406	0.995664
2	0.793906	0.468586	0.982445	0.979617	0.981192	0.992826	0.991348
3	0.707381	0.320763	0.973783	0.969582	0.971920	0.989258	0.987050
4	0.630286	0.219573	0.965197	0.959650	0.962737	0.985703	0.982770
5	0.561594	0.150305	0.956687	0.949819	0.953640	0.982161	0.978509
6	0.500388	0.102889	0.948253	0.940090	0.944629	0.978631	0.974267
7	0.445853	0.070431	0.939892	0.930460	0.935703	0.975114	0.970043
8	0.397261	0.048212	0.931606	0.920928	0.926862	0.971610	0.965837
9	0.353965	0.033003	0.923392	0.911494	0.918104	0.968118	0.961650
10	0.315388	0.022592	0.915251	0.902157	0.909429	0.964639	0.957481
11	0.281015	0.015465	0.907181	0.892915	0.900836	0.961173	0.953329
12	0.250388	0.010586	0.899183	0.883769	0.892324	0.957719	0.949196
18	0.125291	0.001089	0.852653	0.830822	0.842915	0.937253	0.924770
24	0.062694	0.000112	0.808530	0.781047	0.796242	0.917225	0.900973

The two intervals, their directions, and what goes wrong

Read the second row twice. A vial calibrated for a time that has not arrived yet holds MORE than its assayed activity, the interval to now is negative, and the factor is above one. That is counter-intuitive the first few times and it is the ordinary situation in a department that takes a delivery at eight in the morning calibrated for noon. The failure mode is specific: putting a positive number into a negative interval does not produce a silly answer, it produces an answer wrong by the factor squared — for four hours of technetium-99m that is 1.59 squared, a factor of 2.5 — and every figure on the page will look reasonable. The last row is the consolation: the VOLUME depends only on the sum of the two intervals, so it survives the two being swapped. The activity now does not, and that is the number that goes on the label.
Interval	Sign	Factor it needs	What happens if you get it wrong
Calibration time → NOW, when the calibration time has already passed	positive	e^−λt, less than 1 — the activity now is LOWER	Using the assayed figure as if it were current overstates the activity. For technetium-99m six hours late that is a factor of two: you draw half the volume needed and the patient gets half the dose
Calibration time → NOW, when the calibration time is still in the FUTURE	negative	e^+λ\|t\|, greater than 1 — the activity now is HIGHER	The commonest sign error in the subject, because this is the ordinary case for a morning delivery calibrated for noon. Using the negative interval with the wrong sign gives an answer out by the SQUARE of the factor, and both answers look plausible
NOW → administration	positive	e^−λt, less than 1	Forgetting it entirely is the other common failure. The dose is drawn correct for the moment of drawing and arrives at the patient low — eleven per cent an hour for technetium-99m, 31 per cent an hour for fluorine-18
Calibration → administration, the two together	the sum of the two	e^{−λ(t₁+t₂)}, the product of the two factors	Nothing, if the two were right: the drawn volume depends only on the SUM, so swapping them leaves it unchanged. That is worth knowing because it means the volume is robust to mislabelling while the activity-now figure is not — and the activity-now figure is the one on the syringe label

The one decay correction on a generator eluate that runs the WRONG way

Everything else on this page falls with time. This does not, and the reason is that it is a RATIO of two activities with different half-lives. Molybdenum-99’s 65.95 hours is eleven times technetium-99m’s 6.007, so the molybdenum in an eluate decays eleven times more slowly and its share of the total grows: the ratio multiplies by e^{(λTc−λMo)t}, which is 3.52 over twelve hours and a factor of ten in 22.0 hours. The United States Pharmacopeia limit, quoted in the TechneLite generator’s prescribing information, is “not more than 0.0056 MBq (0.15 microcurie) of Molybdenum 99 per 37 MBq (1 millicurie) of Technetium 99m … AT THE TIME OF ADMINISTRATION” — and the same document says the ratio “is to be determined at the time of each elution prior to administration, and from that ratio, the expiration time (up to 12 hours) of the eluate mathematically determined”. That sentence is this table: an eluate comfortably inside the limit when it is measured can be outside it hours later without anybody touching it, and the expiry is calculated from the measured ratio rather than fixed. The last two columns show the mechanism rather than the consequence. Nothing here is a determination about any eluate: the figures are the published limit and the arithmetic that moves a measurement towards it. This page computes physics. It is not a compliance determination, and where it prints a published limit beside an answer that limit is shown for comparison and never as permission. Radiation work is governed by regulation and by local policy, and a measurement takes precedence over anything calculated here.
Hours after elution	Growth in the Mo-99/Tc-99m ratio	A ratio that started at 0.015 µCi/mCi becomes	…as a percentage of the 0.15 limit	% of the Tc-99m left	% of the Mo-99 left
0	1.0000	0.01500	10.00	100.000	100.000
2	1.2334	0.01850	12.33	79.391	97.920
4	1.5213	0.02282	15.21	63.029	95.883
6	1.8763	0.02814	18.76	50.039	93.889
8	2.3142	0.03471	23.14	39.726	91.936
10	2.8544	0.04282	28.54	31.539	90.023
12	3.5206	0.05281	35.21	25.039	88.151

Two intervals, one of them signed, and a volume at the end of it

Two intervals, and the sign of the first one is where this goes wrong. A vial carries an assayed activity and a calibration time. To know what it holds now you need the interval from that calibration time to now — which is POSITIVE if the calibration time has passed and NEGATIVE if it has not. The second case is not exotic: a delivery at eight in the morning calibrated for noon is the ordinary arrangement, and it means the vial holds more than its label says, by a factor above one. Reversing that sign does not produce a silly number. It produces a number wrong by the factor squared — for four hours of technetium-99m, a factor of two and a half — and every figure on the page still looks plausible. That is why this page prints the two factors separately instead of only their product, and why every label names its direction.

The second interval is the one that gets forgotten entirely. A dose drawn at one o’clock for a patient at half past two has decayed in between, and the volume has to be drawn for the activity wanted at the END of that interval rather than at the moment of drawing. For technetium-99m that is eleven per cent an hour. For fluorine-18 it is 31 per cent. For lutetium-177 it is four tenths of one per cent, which is why the habit of ignoring this interval survives and why it should not be carried from one nuclide to another. One consequence worth noting: the syringe holds MORE than the wanted activity at the moment you draw it, and the dose calibrator beside you will read that larger figure. The page prints it.

The answer is a volume, because a volume is what you can pick up. An activity is not something that can be drawn; it is a rate. What can be drawn is a volume, and the conversion is the concentration — specifically the concentration at the moment of ADMINISTRATION, which is neither the concentration at calibration nor the concentration now. The vial’s volume does not decay, so its concentration falls at exactly the rate its activity does, and the volume required for a fixed dose rises along the reciprocal of the decay curve: for technetium-99m it doubles every six hours. The chart on this page is that reciprocal, hour by hour, with the vial’s own volume to compare it against — and the hour the two cross is the hour that vial stops being able to supply that dose.

What the arithmetic is for, and what it does not replace. The IAEA’s Operational Guidance on Hospital Radiopharmacy requires that the operator drawing unit doses make “appropriate radioactive decay allowance for the respective times of injection”, that “the actual dose at the time of injection should be stated on the label”, that “each unit dose should be checked using a dose calibrator” and that “the radioactivity of the individual dose should be measured again before administration”. A manufacturer’s insert says the same thing in fewer words — Bracco’s CholeTec information directs that “the patient dose should be measured by a suitable radioactivity calibration system immediately prior to administration”. So this page gives the volume to start from. The number that counts is the assayed one, and nothing computed here is a substitute for putting the syringe in the calibrator.

Where the error actually is, on this timescale. It is not the decay data. NPL knows technetium-99m’s half-life to 0.016 per cent, so over a seven-hour correction it contributes nothing measurable at all. A dose calibrator is good to a few per cent. And an assay time recorded to the nearest hour is worth eleven per cent of the activity, which is larger than both of the others put together. The practical conclusion is unglamorous: on a six-hour nuclide, write the time down to the minute. The page prints all three figures — the published half-life uncertainty, what it is worth over your whole correction, and the hourly loss rate — so the comparison can be read rather than assumed.

Frequently asked questions

How do I decay-correct an activity to a calibration time?

Multiply by e^−λt going forward in time from the calibration time, and by e^+λt going backward to it, with λ = ln2/t½ and t and t½ in the same unit. The only thing to be careful about is the direction: later than the calibration time means less activity, earlier means more. For technetium-99m six hours after calibration the factor is 0.5004, so an 11,100 MBq vial holds 5,554 MBq; six hours BEFORE its calibration time it would hold 22,183. Both calculations are the same arithmetic and the sign is the whole difference.

My vial is calibrated for a time that has not happened yet. What do I do?

Enter a negative interval from the calibration time to now, and the activity now comes out HIGHER than the assayed figure. That is correct and it is the ordinary situation for a morning delivery calibrated for the middle of the day. The factor is e^+λ|t|, greater than one. This is the single commonest error in decay correction, and the reason it survives is that reversing the sign gives an answer that is wrong but entirely plausible — out by the factor squared, which for four hours of technetium-99m is a factor of 2.5. The page prints the two factors separately so that one of them being above one is visible.

What volume do I draw for a given dose?

The wanted activity divided by the concentration at the moment of ADMINISTRATION — not at calibration and not now. Work it in three steps: decay the assayed activity from the calibration time to the administration time, divide by the vial’s volume to get the concentration then, and divide the wanted activity by that. For the worked example on this page: 11,100 MBq decayed over seven hours is 4,949 MBq, in 10 mL that is 494.9 MBq/mL, and 740 MBq divided by that is 1.495 mL. The syringe will read about 831 MBq at the moment you draw it, because it still has an hour to go.

Why does the syringe read more than the dose I wanted?

Because the dose was calculated for the moment of administration and you are measuring it at the moment of drawing. The difference is the second decay interval. For technetium-99m that is about eleven per cent an hour, so a 740 MBq dose drawn an hour early reads about 831 MBq in the calibrator; for fluorine-18 half an hour early it reads about 20 per cent high. Both figures are correct and they describe different instants. The page prints the drawing-time activity next to the administration-time one for exactly this reason, because the mismatch otherwise looks like an error in the calculation.

How accurate is a decay correction?

The arithmetic is exact and the inputs are not, and on a short half-life the clock dominates everything. NPL knows technetium-99m’s half-life to 0.016 per cent, so over a seven-hour correction the decay data contribute nothing measurable. A dose calibrator is typically good to a few per cent. An assay time recorded to the nearest hour, on a six-hour half-life, is worth eleven per cent — larger than everything else combined. So the thing to tighten is the time of day, and the page prints the hourly loss rate so you can see what a given sloppiness in the clock is worth.

Does this page work for molybdenum-99 breakthrough?

No, and the reason is worth knowing because the correction runs the opposite way. Breakthrough is a RATIO of two activities with different half-lives: molybdenum-99’s 65.95 hours against technetium-99m’s 6.007. The molybdenum therefore decays eleven times more slowly and its share of the eluate GROWS with time — by a factor of 3.5 over twelve hours, and tenfold in 22 hours. That is why the United States Pharmacopeia limit of 0.15 microcurie per millicurie is specified at the time of administration, and why a generator’s prescribing information asks for the ratio to be measured at each elution and the eluate’s expiry calculated from it. The table on this page shows the growth; it is arithmetic about a published limit and not a determination about any eluate.

Can I enter the calibration time and the administration time as clock times?

Not here. The calculation engine behind this site is numeric only and has no date or time type, so it cannot subtract 07:15 from 13:40 for you — that subtraction is yours, and the page takes the two intervals as numbers with a unit each. That is a real limitation and it is worth naming, because the manual subtraction is the other place errors come from: a correction across midnight, across a daylight-saving change, or between a 24-hour and a 12-hour clock has caught better people than this page will. Write both times down in the same format before you subtract them. For the general one-activity, one-interval case without a vial or a volume in it, the decay and remaining-activity page is the simpler tool.

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.
Operational Guidance on Hospital Radiopharmacy: A Safe and Effective Approach, International Atomic Energy Agency, Vienna, 2008. The practice authority for the drawn-volume page. It requires that “the actual dose at the time of injection should be stated on the label”, that an operator drawing unit doses make “appropriate radioactive decay allowance for the respective times of injection”, that “each unit dose should be checked using a dose calibrator” and that “the radioactivity of the individual dose should be measured again before administration”. That last sentence is why this page describes its own answer as the volume to start from: the activity that counts is the assayed one, not the calculated one.
CholeTec (kit for the preparation of technetium Tc 99m mebrofenin injection), prescribing information, Bracco Diagnostics (read 7 October 2026). A worked example of how the industry actually publishes this arithmetic: “to correct for physical decay of technetium Tc 99m, the fractions that remain at selected intervals after the time of calibration are shown in Table 3” — a printed decay chart against a 6.02 hour half-life — together with the instruction that “the patient dose should be measured by a suitable radioactivity calibration system immediately prior to administration”. The decay chart in an insert is the table this calculator replaces; the assay instruction is the one it cannot.
TechneLite (technetium Tc 99m generator), prescribing information, Lantheus Medical Imaging (read 7 October 2026 through the MedLibrary mirror). Quoted for the breakthrough limit, which is the one decay correction on a generator eluate that runs the WRONG way: the United States Pharmacopeia limit is “not more than 0.0056 MBq (0.15 microcurie) of Molybdenum 99 per 37 MBq (1 millicurie) of Technetium 99m … at the time of administration”, and “the Molybdenum 99/Technetium 99m ratio is to be determined at the time of each elution prior to administration, and from that ratio, the expiration time (up to 12 hours) of the eluate mathematically determined”. Because molybdenum-99 decays eleven times more slowly than technetium-99m, that ratio GROWS with time, so an eluate inside the limit at elution can be outside it later.
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.
Technetium-99m, Wikipedia (read 7 October 2026). Read for the generator description and for the branching figure a single-nuclide decay calculation silently assumes away: “over 87% of the decays lead to the desired ^99mTc”. It also gives the half-life as 6.0066 hours, against NPL’s 6.0067, and notes that a generator “must be replaced weekly, since the half-life of ⁹⁹Mo is still only 66 hours”.
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.