-Tim Bean

In the mid-1960s, men in dark suits from the Atomic Energy Commission (AEC) visited the folks at the Institute of Forest Genetics in Rhinelander, Wisconsin. The AEC had an odd request: slow cook 1440 acres of gorgeous northern Wisconsin forest with 10,000 Curies of radiation for about six months.

On the surface this might seem a brutal request, but for a country knee-deep in the Cold War, knowing if our forests could withstand prolonged and intense radiation was useful information. Humanity-saving information.

The AEC’s request was tempting for the Institute. It included a comfortable stipend, the promise of continued employment, and the prestige of heading a landmark study. In the circles of US Forestry, the Rhinelander outfit would become rock stars, or the academic equivalent of rock stars.

The Institute agreed, and by 1967, the ball was rolling.

The “slow cooking” would come from a borrowed chunk of cesium-137, the same source once used in many radiation therapy machines. Those carefully-constructed medical machines had layers of containment protecting patients and personnel from exposure. There’s good reason for caution; the radioactive cesium salts can and have killed before (there are too many incidents to list here, but see the 1987 Goiânia accident).

This Wisconsin forest would be exposed to a much larger “hunk” than that contained in any medical machine. This 14-ounce source was the equivalent of scooping out the glowing cesium salts of TEN external beam radiation machines, molding this mess into a pile, slapping a metal jacket on it (affectionately called a lead pig), and dangling the whole mess from a Rube Goldberg-looking contraption in the middle of a Midwest forest. This oddity would be controlled entirely by a remote clicker with a 400′ to 500′ length of cable as a failsafe.

Honestly, it looked like something Wile E. Coyote would buy from Acme.

How deadly was the cesium-137 source?

Distinguishing between systems of radiation measurement is almost a science unto itself. There’s a reason: the more you learn about radiation, the stranger it is. There are so many variables involved in measuring radiation (material activity, exposure, absorbed dose, equivalent dose, effective dose…), with each instance using its own system of measurement.

When you measure radiation, you might use Grays or Curies or roentgens or rems or rads or sieverts or becquerels or CPM (counts per minute)…You get the point. I’ll ignore all that  and do my best to keep this scenario simple*. Don’t worry about the different systems of measurement. Just pay attention when I get to the BIG, BOLD sentence, all right?

*And for the science folks that are clutching their pocket protectors and know I’m painting with a wide brush, relax. Have a Coke and a smile. 

Every tree larger than an inch within 50 meters of the Site 1 source.

The Enterprise Radiation Forest’s cesium-137 source weighed about 14 ounces and produced ~10,000 Curies of radioactivity. For this fictional scenario, we’ll be using rems (roentgen equivalent man).

Two variables I’m skipping:

1.) Radiation doses are cumulative, so receiving 10 rems a day for 10 days means 100 rems. It’s not like your blood-alcohol level, which your body naturally processes and lowers. There’s no body mechanism for “processing” radiation. Once it’s there, it stays, mostly.

2.) This scenario does not include rems received as you approach or walk away from the cesium-137 source. Once you’re within a few hundred feet, those gamma rays beam into and through your body, growing in intensity and accumulating with every step. I left that variable out.


Imagine you’re traipsing through this lovely forest one fine afternoon and come upon this odd contraption: a six-foot square platform standing six feet in the air. The platform sits under three poles tied together with a steel band, like a teepee-tripod. It’s incredibly sturdy and incredibly ugly.

The platform’s legs are anchored deep into a dirt and concrete base. Rough wood slats serve as a ladder on one side and a thick sheet of plywood forms the floor. Squatting in the platform’s center is a gray, metal container and above this, a hollow metal cylinder. That round container, affectionately known as a lead pig, is a 2,000 pound sleeve of lead containing the cesium source…but not in this scenario.

From the Enterprise Radiation Forest Pre-irradiation ecological studies, 1974.

In this imaginary scenario you see a small cylinder of silvery-white metal dangling about five-feet above the lead pig. Every element of this structure—the legs, the tripod, the lead pig, and the cables—all orbit this cylinder. You’re not sure why, but let’s say you decide to hang around and check it out.

If you stay 20 seconds, you’ll receive a 1 rem dose, equivalent to an average CT scan.

If you stay 3 minutes, you’ll receive 10 rems, increasing your risk of cancer by searing your DNA a bit.

If you hunker down for a nice 30 minute lunch. You’ll be hit with a 100 rem dose. This damages your white blood cells in the short term, making you vulnerable to infection. A few hours later mild radiation sickness settles in: nausea, fatigue, diarrhea, and a weakened immune system.

If it’s been a pretty tough day, you might enjoy a 90-minute nap next to the platform. That’s 300 rems. Severe radiation sickness will hit you in your sleep, and you’ll likely wake up coughing blood. 35% of exposed individuals never recover from this dose.

Stay two hours and you’ll get 400 rems. This dose destroys your bone marrow and turns your intestines to bloody goo. You’ll have no immune system to fight off the subsequent peritonitis and sepsis. About 60% of victims don’t escape this dose.

Stay three hours or more (1000 rem+) and 95% of you would never leave again, unless someone in PPE gear drags your body out.

Exposure levels around the ERF source. From ‘The Enterprise, Wisconsin, Radiation Forest : Radioecological Studies’, 1977

The lesson is simple: you don’t ever get near an unshielded cesium-137 radiation source. 

Creating the Enterprise Radiation Forest

Finding an entirely new section of prime forest would be too costly and time-consuming, so the researchers suggested slicing off a 1440-acre chunk of land they already had under observation, a chunk that would be renamed the Enterprise Radiation Forest. Oneida County and the State of Wisconsin already owned the land, it was a short jog from the Institute in Rhinelander, and the land was free of homes or habitation. The forest’s flat topography would make shuttling around instruments and equipment that much easier. The US Forest Service leased the land from the county and state and got to work.

Some diagrams refer to Site 1 as “Site 3,” likely reflecting the experiment’s early end

There was virtually no construction needed to prepare the forest. An intimidating 8′ fabric-covered fence (topped by six rolls of barbed wire) surrounded the entire tract. The Institute also built a 20′ by 30′ building constructed of dense concrete blocks. From there, scientists could not only control the radiation source but remain protected from it.

The most time-consuming task didn’t require any construction at all. The cost came in the thousands of hours researchers spent plotting, cataloging, and documenting the 1440 acres of the Enterprise Radiation Forest. These men and women spent years recording the exact location, age, health of every tree larger than one-inch, then following that tree’s growth over several seasons. It wasn’t enough to know how flora and fauna responded to intense radiation in the short term, but how exposure would affect growth and reproduction months or years afterwards.

The control building and security fence.

The specific site contained 203 trees (greater than 1″ in diameter) of 13 different species. The forest itself was secondary growth, which had been originally logged in the early 1900s and again in the late 1940s. Trees had an average age of 24 years.

The forest’s fauna also remained. Although this might disturb some conservationists today, it’s not hard to see the importance in learning how small animals would respond and recover from such prolonged exposure*.

*To soothe our readers’ consciences, I’ll spoil the ending. The source exposure had little measurable effect on the Wisconsin animals. The only result attributed to the exposure was, in fact, a single birds’ nest which failed to hatch. 

Over a dozen of the forest’s largest fauna—white-tailed deer—were removed from the experimental forest. It had nothing to do with preference or affection, but the feeding habits of deer. In the timespan of the entire study, about four years, the few deer trapped in the Enterprise Radiation Forest would easily double or triple, and any forester knows a few dozen hungry deer will kill a hunk of forest as quickly as any radiation. Bambi is a voracious beast.

This experiment exposed the surrounding forest 20 hours a day, every day. Researchers used the other four hours for observation and measurement. By the experiment’s end, the cesium-137 source will have dangled naked and deadly above that Rhinelander forest for 166 days, or 2717 hours. Accumulated doses would range from 209,000 rems adjacent to the source, to 40 rems at more than 300 feet away.

TAU researchers tout cancer breakthrough with first 3D bioprint of active tumor | The Times of Israel
Characteristic blue glow of cesium chloride.

What did they discover in the radiation forest? 

Originally, the AEC designed the experiment for several growing seasons in several locations, but that didn’t happen. As with any fringe study, funding came up short, and tightened purse strings reduced the scope of the experiment to a single season. But the Institute squeezed as much as data as possible out of that single season, and while it would be possible to cover everything in a single article, I can cover some highlights.

r = rad, and 1 rad = 1 rem. 

#1. Total exposure over the six months (a total of 2,717 hours) ranged from 209,000r on the platform itself to 40r at 200 feet.

#2. Environmental factors like humidity, solar radiation, and temperature had almost no effect on radiation doses.

#3. Contrary to expectations, the “zone of devastation“—where virtually all plant life would be exterminated by radiation—only occurred directly adjacent to the cesium source, where exposure was 2000 rads/day*.

*all use of day in these radiation measurements is a 20-hour day, since researchers shielded the cesium source four hours daily to document effects.  

#4. The most resilient vegetation (not including trees) was found along the old logging road used to access the site. It is assumed these weeds had already gone through a culling process from decades of vehicle use, leaving only the strongest species.

#5. The most sensitive trees were basswood and balsa trees, two of the lightest hardwoods in the world. These suffered severe crown mortality (leaves) for years afterwards, although they received fairly low doses of 25r/day. The most resilient tree surprised almost everyone: the sugar maple, the very tree that provides your breakfast syrup. Several sugar maples survived within spitting distance of the cesium source, even after receiving doses of 430r/day.

#6. Researchers virtually gave up analyzing the data from small mammals. Chipmunks, squirrels, mice, and others were just too good at escaping the enclosure and many spent time under cover or underground. Without tracking all that data, no real conclusions could be drawn.

Results of a similar study at New York’s Brookhaven Research Laboratory

#7. The most practical discovery drawn from the Enterprise Radiation Forest was that forest canopy damage was a reliable tool in measuring surface radiation. This meant that pilots, flying overhead and at a safe distance, can assess radiation damage to a huge swath of forest in minutes by simply eyeballing it.

Discovering you can draw safe conclusions by eyeballing something from a safe distance is, in any military scenario, a beautiful thing and well worth a few million

Common questions about the Enterprise Radiation Forest.

Did any researchers suffer from radiation exposure? 

It’s been 50+ years and none of those involved have come forward with any illnesses. None have shown an increase in the likelihood of cancer. The Institute and AEC were extremely cautious during the study and while the cesium source was highly radioactive, it always remained contained when necessary, as it would in any radiotherapy machine. The control building itself was two miles from Site 1 and built from concrete blocks. It was as secure a structure one could hope for, short of an underground bunker.

Was it a pointless waste of taxpayer money? 


First off, it wasn’t very expensive. I’m guessing the entire experiment cost less than a single Apache helicopter.

Secondly, don’t be cheap. No one likes a skinflint.

Third, when the experiment began in 1968, the US was neck-deep in the Cold War. The Cuban Missile Crisis had come and gone, as had JFK, and now Lyndon Johnson was staring at a confrontation with the Soviets in Southeast Asia. Analyzing how an ecosystem reacts to a prolonged dose of radiation was a practical concern at the time, and even today.  Knowing how, or if, American agriculture can recover from such a nightmare is essential to human survival.

Is Site 1 still radioactive? 

No. This was 50 years ago. The radiation was intense, but very short-lived. You’d hardly recognize any of the Enterprise Radiation Forest today and none of it is radioactive. 

Did the government keep it secret? 


From Wausau Daily-Herald. May 13th, 1972. 

Want to Know More? 

Look no further than the well-organized reports bookmarking this study.

The first, the Enterprise Radiation Forest: Preirradiation [sic] Ecological Studies documents not only the thousands of hours spent cataloging the forest, but also studying how it grows as an ecosystem. This is available as a free download in a variety of formats.

The second volume, written 4 to 5 years after the first, summarizes the mountain of data collected after irradiating the Enterprise Experimental Forest for a growing season. This report, Enterprise Radiation Forest: Radioecological Studies, is available to read online at the Internet Archive.