ACCORDING to Los Angeles Police Chief William Bratton, “Terrorism is all about getting them before they get us.” But if that’s so, you’ve got to question the wisdom of the Los Angeles Police Department’s efforts to spare the city from a dirty-bomb attack.

The LAPD has recently procured—using some of the $3 million in funding from the Department of Homeland Security—devices capable of detecting so-called dirty bombs, or radiological dispersion devices. One of the detectors will be deployed on board a helicopter and supposedly can detect a radiation signature from an altitude of 800 feet. Six other devices are handheld units for use by police officers on the ground (with, presumably, a detection range of only a few feet).

Assuming the helicopter-mounted device is 100 percent effective—and nothing ever is—and that it can detect small quantities of radioactive material within an 800-foot radius, its effective surface area of detection is about 2 million square feet, or less than one-tenth of a square mile.

If a dirty bomb could be anywhere inside the L.A. city limits (468 square miles), and the helicopter could be flying anywhere within that airspace, the probability that a single helicopter would detect the bomb is less than 0.02 percent. In other words, very poor odds.

Even worse are the odds for six police officers carrying handheld detectors with a range of only a few feet. Each would be responsible for covering nearly 80 square miles.

Moreover, there are many legitimate sources of radiation—by definition, a dirty bomb is made with explosives laced with radioactive material, not a nuclear device—that could set off detectors and result in countless false alarms.

According to the U.S. Nuclear Regulatory Commission, radioactive materials are routinely used at hospitals, research facilities and industrial and construction sites for such purposes as diagnosing and treating illnesses, sterilizing equipment and inspecting welding seams. And there are many naturally occurring sources of radiation, such as fertilizers, ceramics, bananas, kitty litter and smoke detectors. All of which could be carried in sufficient quantities by trucks traveling on the 6,800 miles of roadways, including 27 intertwining freeways, in Los Angeles to trigger detectors.

Beyond being able to detect the relatively small amount of radioactive material used in a dirty bomb, there is also the question of how real such a threat is. There are only two known cases—in Russia and Chechnya—of attempted terrorism using a radiological dispersion device.

In May 2002, Jose Padilla was arrested on suspicion that he was an al-Qaida terrorist intending to detonate a dirty bomb in the United States. But when he was finally indicted—after being held for three years as an enemy combatant—the dirty-bomb charges were dropped. (Padilla was convicted in August on charges of aiding terrorist organizations abroad.)

The truth is that although a dirty bomb is a relatively simple device compared to a nuclear weapon, it is also a high-tech weapon, which makes it more difficult for terrorists to construct and use. (Indeed, if it was easy and simple to build dirty bombs, one would think they would be as prolific as improvised explosive devices in Iraq.)

And despite rhetorical sensationalism about dirty bombs—announcing Jose Padilla’s arrest, former Attorney General John Ashcroft claimed that a dirty bomb “can cause mass death and injury”—they are hardly weapons of mass destruction. The actual physical damage caused by a dirty bomb would likely be no more than that of a conventional bomb using the same amount of explosives.

So the dirty-bomb threat to L.A. is probably overstated. But that should come as no surprise, since Chief Bratton doesn’t understand the larger terrorist threat.

According to Bratton, Los Angeles is at risk of terrorism—and presumably a dirty bomb attack—because it is “the symbolism of so much of what they hate. In their rush to get back into the 7th century again, 21st century Hollywood is not exactly where they want to be.”

The reality is that—as the 9-11 Commission concluded and as numerous polls conducted throughout the Islamic world show—“they” do not hate us for our freedoms, way of life, culture, accomplishments, or values. Rather, the growing tide of Muslim anti-American hatred—the basis for the radical Islamists to cultivate terrorism—is fueled more by what we do, i.e., U.S. policies, than who we are.

With more than 1 billion Muslims in the world, we cannot continue to ignore the reality of why so many of them possess a growing hatred of the United States—largely U.S. foreign policy. Otherwise, getting them before they get us will be as elusive as trying to find a dirty bomb in Los Angeles.