Guides / Radiation safety math

Stay time and dose budgets: working from dose rate and time

Work through illustrative dose budgets and rate-times-time examples, with federal limits and the measurements and procedures that govern real work.

Practical guide

By Steven Michael Barber · Published October 17, 2026 · Updated November 3, 2026 · 7 min read

A dose rate on its own does not tell you whether a job is safe. At 30 mR/h, one minute gives approximately 0.5 mrem under the photon convention used here; an afternoon gives much more. Whether a task is authorized also depends on the licensee's procedures, measurements and dose controls. This guide takes the dose rates from our distance guide and shows how rate multiplied by time uses a task's dose budget.

The limits, and where they come from

The U.S. Nuclear Regulatory Commission's radiation protection rules, 10 CFR Part 20, set the ceilings. Agreement States adopt equivalent rules for the licensees they regulate. The numbers that matter for planning a job are these:

• Occupational: 5 rem (5,000 mrem, 50 mSv) total effective dose equivalent per year to an adult worker (10 CFR 20.1201), with separate, higher limits for the skin, the extremities and the lens of the eye.
• Members of the public: 100 mrem (1 mSv) per year, and no more than 2 mrem in any one hour in an unrestricted area from external sources (10 CFR 20.1301). Both apply together. Where a licensee puts the boundary of a restricted area, and how access to it is controlled, comes from surveys and the expected occupancy, not from either number alone (10 CFR 20.1302).
• Monitoring: an adult worker likely to receive more than 10 percent of the occupational limit from external sources, 500 mrem in a year, has to be monitored with a dosimeter (10 CFR 20.1502). That is one criterion of several: the rule also covers the skin, extremities and eyes, minors, declared pregnant workers, entry into high radiation areas and internal dose, which a badge does not measure.
• ALARA: whatever the limit, doses are to be kept as low as is reasonably achievable (10 CFR 20.1101). Most licensees set their own administrative limits well below 5 rem, often a tenth of it, and plan jobs against those.

For gamma rays of these energies, one roentgen of exposure is approximately one rem of dose equivalent, and this guide uses that convention so a survey meter reading in mR/h can stand in for mrem/h in the arithmetic. It is a convention for illustration, not how a dose of record is assessed; our units guide explains why.

Stay time is budget divided by rate

Pick the dose you are willing to receive for the task, the budget, and divide it by the dose rate where you will be standing:

stay time (hours) = budget (mrem) ÷ dose rate (mrem/h)

The budget is a planning choice, and in practice the licensee's radiation protection program makes it for each task. The three budgets below are illustrative numbers picked to make the arithmetic concrete. None of them is a limit in the rules, and dividing an annual limit by days or weeks does not create a daily or weekly limit:

• 2 mrem, a small one-off budget. It is the same number as the public hourly limit, but it is not permission for anyone to go near a source.
• 20 mrem, the occupational annual limit divided by 250 working days. A worker who used it every day would end the year at the limit, which is not the aim.
• 100 mrem, the same limit divided by 50 working weeks. Licensees that use weekly administrative levels set their own.

Applied to the sources from the distance guide, with the rates computed by our calculator:

The table is why radiography crews work from behind a collimator and a barrier, and why nobody handles a bare curie-level cobalt source. At 1,320 mR/h, the rate one meter away, the 100 mrem budget is gone in four and a half minutes of standing still at that distance. That figure says nothing about the dose to the hands or body of anyone who goes closer, which a one-meter rate cannot estimate. Never touch or try to recover a dropped or unshielded source: move away, keep others back without approaching it, and call the radiation safety officer or emergency responders. Recovering a source is work for trained, authorized people with measurements and a plan.

Worked example: a gauge operator's year

Take a technician who runs a portable moisture-density gauge on compaction jobs. Suppose the source is in its measuring position with the technician about a meter away for two hours in a working day, 200 days a year. At 2.64 mR/h that is 2.64 × 2 × 200 = 1,056 mrem in the year: a fifth of the occupational limit, and twice the 500 mrem adult external-dose level at which a worker likely to exceed it must be monitored. If the same technician instead spends one hour a day at half a meter, where the rate is 10.6 mR/h, the year comes to 2,120 mrem, which is over 40 percent of the limit from one habit of standing too close.

Two things soften this in practice. The source is in its shielded stored position for most of the working day, with the rod retracted into the housing, and the exposed-source figure applies only while a reading is taken. And the inverse-square law is on the technician's side: the dose is dominated by the minutes spent closest to the source, so the cheapest reduction is to extend the rod, step back, and not lean over the gauge while it counts. Licensee survey records and dosimeter results give the real annual figures; typical gauge operators record far less than this worked case because of exactly those habits. The point of the arithmetic is to show which habit costs what.

The 2 mrem hour and the public boundary

The public limits work the other way around: the question is how long someone could be at a given spot. At the modeled 2 mR/h contour, which the distance guide's point-source arithmetic places at 1.15 meters from the exposed gauge source and about 110 meters from the radiography source, a bystander standing there for an hour would receive about 2 mrem, and would reach 100 mrem after 50 hours. Both the hourly and the annual public limits apply together, and a contour from a formula is not a boundary: the licensee sets the restricted area from surveys, the expected occupancy and the cumulative dose, and controls access to it (10 CFR 20.1301, 20.1302).

Inside the boundary the arithmetic reverses again. A member of the public who walks up to the exposed gauge for a look, at half a meter, receives 2 mrem in eleven minutes. That is the case the rope and the sign are for.

Time, distance and shielding together

The three controls multiply. Halving the time halves the dose. Doubling the distance cuts it to a quarter. One tenth-value layer of shield cuts it to a tenth. A job planned with all three, a shorter task from farther away behind something, gets the product of the three reductions, and the cheapest of them is nearly always distance. When the time cannot be shortened, because the task takes what it takes, and the distance is fixed by the work, shielding is what is left, and our shielding comparison sizes it.

A planning habit that follows from the table: before a task near a source, write down the expected rate, the expected time and the budget, and check the product. If the product is more than the budget, change one of the three before starting, not after. The calculation takes thirty seconds; the dose, once received, cannot be given back.

What the estimate leaves out

These stay times assume the dose rate from the point-source formula at a fixed distance, which is an estimate: real sources are shielded by their housings, real rooms add scatter, and real people move. A calibrated survey meter reading at the working position replaces the estimate and should be used whenever one is available. Stay times also assume whole-body exposure; a hand on a source holder gets the extremity dose, which has its own, higher limit and its own dosimeter. And the limits are ceilings set by regulation, not targets: the ALARA rule means a job that could be done at a tenth of the budget should be. None of this is training, a qualification or a substitute for a licensee's radiation safety program and its radiation safety officer.