ODAKA, Japan >> Every year when winter finally loosens its grip on northern Japan, Tomoko Kobayashi begins what has become an annual rite for her and a small band of collaborators. They head out with measuring devices to keep tabs on an invisible threat that still pollutes the mountains and forests around their homes: radioactivity.

In her car, Kobayashi follows a route that she now knows by heart, making regular stops to probe the air with a survey meter that looks and acts like a Geiger counter. She uses it to detect gamma rays, a telltale sign of the radioactive particles that escaped when three reactors melted down at the Fukushima Daiichi nuclear plant in March 2011 after an undersea earthquake sent a towering tsunami crashing into the coastline.

She and a group of fellow residents of Odaka, a small community 10 miles north of the plant, spend days collecting readings at hundreds of points, which they use to create color-­coded maps of radioactivity levels emanating from reactor particles still scattered across the countryside. Kobayashi posts them on the wall of her small inn for guests to see, making up for a lack of government maps detailed enough to reveal potentially hazardous spots.

“The government wants to proclaim that the accident is over, but it isn’t,” said Kobayashi, 72, who reopened her inn seven years ago, after the evacuation order in Odaka was lifted. The inn has been in her family for four generations.

“I choose to live here, but is it safe? Can I pick these nuts or eat those fruit? The only way to know for sure is do the measuring ourselves,” she said.

Kobayashi is one of Fukushima’s citizen scientists, residents around the plant who responded to official cover-ups and silences by acquiring their own measuring devices and teaching themselves how to use them.

Almost 14 years after the meltdowns, the citizen scientists persist, fueled by smoldering distrust of authority.

They have created new communities with their networks of like-minded people. By filling gaps left by government inaction, they have grown proficient at measuring and mapping radiation, leading to what experts have called a democratization of expertise. This grassroots embrace of science is an enduring legacy of the Fukushima disaster and a path to self-empowerment.

“Around the world, we have seen a growing contempt for expertise, but these citizen scientists are going against that trend,” said Kyle Cleveland, a sociologist at Temple University in Tokyo. “They are using knowledge to understand their environment and claim legitimacy for their grievances.”

While the citizen scientists were often the only source of radiation numbers in the months after the meltdowns, these days they play watchdog, verifying the government’s figures and providing a level of detail that officials still won’t. After falling for several years, radiation outside the plant has plateaued at levels often still many times higher than before the accident.

Some groups have achieved considerable expertise in detecting these invisible particles. One is the Mothers’ Radiation Lab Fukushima — Tarachine, started by a group of mothers in the city of Iwaki, an hour’s drive south of the plant, to protect their children.

Begun in a single room with three donated measuring machines, Tarachine now occupies almost the entire floor of its building, with 13 salaried staff, a health clinic and a laboratory filled with equipment. Its self-taught technicians, most of them mothers, can measure even tough-to-detect types of radiation. They publish their findings on the group’s website.

When the nuclear power plant’s reactor buildings started to explode, the group’s founder, Kaori Suzuki, was a homemaker. Anxious for her daughter, Suzuki joined protests against the lack of official information before concluding that the best response was to learn to measure radiation herself. When other mothers joined, they chose the name Tarachine, a term from ancient Japanese poetry used to describe a strong mother figure.

Suzuki learned to use the machines by deciphering English-­language manuals. Once Tarachine’s doors opened, demand was overwhelming, as parents brought food from supermarkets and farmers handed over their own produce to be measured.

“Within one month, we had a three-month waiting list,” she recalled.

Worries about food declined as radiation levels dropped, but Suzuki, 59, has taken on other concerns. One is the decision by the Fukushima plant’s operator, Tokyo Electric Power Co., to begin releasing into the Pacific Ocean more than 1 million tons of water that has been treated but remains contaminated. Tarachine now sends out boats.

“We still have to keep verifying the company’s claims,” Suzuki said.

For Kobayashi, it was her own maps that reassured her about moving back. She said citizen scientists must stay on the lookout for new leaks, with the cleanup expected to take several more decades.

“The radiation is not gone,” she said, “nor is the need to protect ourselves.”