What you should know about Radiation

Radiation Emergencies
Measuring Radiation

When scientists measure radiation, they use different terms depending on whether they are discussing radiation coming from a radioactive source, the radiation dose absorbed by a person, or the risk that a person will suffer health effects (biological risk) from exposure to radiation. This fact sheet explains some of the terminology used to discuss radiation measurement.

Units of Measure

Most scientists in the international community measure radiation using the System Internationale (SI), a uniform system of weights and measures that evolved from the metric system. In the United States, however, the conventional system of measurement is still widely used.

Different units of measure are used depending on what aspect of radiation is being measured. For example, the amount of radiation being given off, or emitted, by a radioactive material is measured using the conventional unit curie (Ci), named for the famed scientist Marie Curie, or the SI unit becquerel (Bq). The radiation dose absorbed by a person (that is, the amount of energy deposited in human tissue by radiation) is measured using the conventional unit rad or the SI unit gray (Gy). The biological risk of exposure to radiation is measured using the conventional unit rem or the SI unit sievert (Sv).

Measuring Emitted Radiation

When the amount of radiation being emitted or given off is discussed, the unit of measure used is the conventional unit Ci or the SI unit Bq.

A radioactive atom gives off or emits radioactivity because the nucleus has too many particles, too much energy, or too much mass to be stable. The nucleus breaks down, or disintegrates, in an attempt to reach a nonradioactive (stable) state. As the nucleus disintegrates, energy is released in the form of radiation.

The Ci or Bq is used to express the number of disintegrations of radioactive atoms in a radioactive material over a period of time. For example, one Ci is equal to 37 billion (37 X 10⁹) disintegrations per second. The Ci is being replaced by the Bq. Since one Bq is equal to one disintegration per second, one Ci is equal to 37 billion (37 X 10⁹) Bq.

Ci or Bq may be used to refer to the amount of radioactive materials released into the environment. For example, during the Chernobyl power plant accident that took place in the former Soviet Union, an estimated total of 81 million Ci of radioactive cesium (a type of radioactive material) was released.

Measuring Radiation Dose

When a person is exposed to radiation, energy is deposited in the tissues of the body. The amount of energy deposited per unit of weight of human tissue is called the absorbed dose. Absorbed dose is measured using the conventional rad or the SI Gy.

The rad, which stands for radiation absorbed dose, was the conventional unit of measurement, but it has been replaced by the Gy. One Gy is equal to 100 rad.

Measuring Biological Risk

A person's biological risk (that is, the risk that a person will suffer health effects from an exposure to radiation) is measured using the conventional unit rem or the SI unit Sv.

To determine a person's biological risk, scientists have assigned a number to each type of ionizing radiation (alpha and beta particles, gamma rays, and x-rays) depending on that type's ability to transfer energy to the cells of the body. This number is known as the Quality Factor (Q).

When a person is exposed to radiation, scientists can multiply the dose in rad by the quality factor for the type of radiation present and estimate a person's biological risk in rems. Thus, risk in rem = rad X Q.

The rem has been replaced by the Sv. One Sv is equal to 100 rem.

Abbreviations for Radiation Measurements

When the amounts of radiation being measured are less than 1, prefixes are attached to the unit of measure as a type of shorthand. This is called scientific notation and is used in many scientific fields, not just for measuring radiation. The table below shows the prefixes for radiation measurement and their associated numeric notations.

Prefix Equal to Which is this much Abbreviation Example

atto- 1 X 10^-18.000000000000000001 a aCi

femto- 1 X 10^-15.000000000000001 f fCi

pico- 1 X 10^-12.000000000001 p pCi

nano- 1 X 10^-9 .000000001 n nCi

micro- 1 X 10^-6.000001 m m Ci

milli- 1 X 10^-3.001 m mCi

centi- 1 x 10^-2.01 c cGy

When the amount to be measured is 1000 (that is, 1 X 10³) or higher, prefixes are attached to the unit of measure to shorten very large numbers (also scientific notation). The table below shows the prefixes used in radiation measurement and their associated numeric notations.

Prefix Equal to Which is this much Abbreviation Example

kilo- 1 X 10³1000 k kCi

mega- 1 X 10⁶1,000,000 M MCi

giga- 1 X 10⁹100,000,000 G GBq

tera- 1 X 10¹²100,000,000,000 T TBq

peta- 1 X 10¹⁵ 100,000,000,000,000 P PBq

exa- 1 x 10¹⁸100,000,000,000,000,000 E EBq

Common Radiation Exposures

People are exposed to radiation daily from different sources, such as naturally occurring radioactive materials in the soil and cosmic rays from outer space (of which we receive more when we fly in an airplane). Some common ways that people are exposed to radiation and the associated doses are shown in the table below.

Source of exposure Dose in rem Dose in sievert (Sv)

Exposure to cosmic rays during a roundtrip airplane flight from New York to Los Angeles 3 mrem 0.03 mSv

One dental x-ray 4?15 mrem 0.04?0.15 mSv

One chest x-ray 10 mrem 0.1 mSv

One mammogram 70 mrem 0.7 mSv

One year of exposure to natural radiation (from soil, cosmic rays, etc.) 300 mrem 3 mSv

For more information

For more information about radiation measurement, you may visit the website for the Health Physics Society, or the Environmental Protection Agency's "Radiation Topics".

For more information about radiation, see CDC's Radiation Emergencies website. You may also call the CDC public response hotline at 800-CDC-INFO or 888-232-6348 (TTY).

The Centers for Disease Control and Prevention (CDC) protects people's health and safety by preventing and controlling diseases and injuries; enhances health decisions by providing credible information on critical health issues; and promotes healthy living through strong partnerships with local, national, and international organizations.

The Centers for Disease Control and Prevention (CDC) protects people's health and safety by preventing and controlling diseases and injuries; enhances health decisions by providing credible information on critical health issues; and promotes healthy living through strong partnerships with local, national, and international organizations.

What's iPhone’s Retina Display?

It's essentially a high-end screen with four times as many pixels in the same screen real estate.



AS you can see the difference between the lower-resolution displays and the new Retina Display.

...It's a screen...

A fancy screen, with a 326dpi resolution. Jobs said "there's a magic number around 300dpi, if you hold something about 10-12 inches away from your eye, it's the limit of the human retina to distinguish pixels." Here's what Apple has to say about the display:

All the breakthrough technology in iPhone 4 is situated between two glossy panels of aluminosilicate glass - the same type of glass used in the windshields of helicopters and high-speed trains. Chemically strengthened to be 20 times stiffer and 30 times harder than plastic, the glass is ultradurable and more scratch resistant than ever.

...and it's close to the iPad's display quality...

It's got 78 per cent of the pixels of the iPad, but in a far smaller screen-size. That should mean colors will pop, and video, browsing, app-playing—whatever else you do with your iPhone—will be clearer than ever. This will be especially beneficial with fonts, whether they be in emails or when reading in the browser.

...but it's better than your iPhone 3GS' screen...

An 800:1 contrast ratio doesn't say much—you know how misleading contrast ratios can be—but it sounds fantastic for a phone. It boils down to just how they measure it, but we do know that it's 4x better than the 3GS' contrast ratio. In the photo above,

...that's sharper, but mostly marketing hype.

We know you. We know you wouldn't bow down to marketing pressure. So while fancy display terms such as "OLED" and "CRT" (snicker) are thrown around, bear in mind that this is just Apple's branding on what's essentially a super-sharp display.

HDTV Ready vs. HDTV

Sometimes we get so wrapped up in the latest, cutting edge high-definition technologies, we inadvertently forget those who are new to the HD world. This article about HD Ready labeling in Europe reminded me that we should define the difference between "HD-Ready" and "HDTV".

If you're new to all of this: don't worry, this is one of the easiest, if not critical, definitions you need to understand when purchasing a new set. In fact, I think this information is relevant to one of the first decisions you need to make: where will you be getting your high-definition content from?

Currently, you can get your high-def programming from cable, satellite or what we call "OTA" or over-the-air. If you plan to receive an HD signal from cable or satellite, you only need an HD monitor, i.e., an "HD Ready" set. This label indicates that the set is capable of displaying a high-definition picture that is provided from some tuning device or set-box that is external to the set itself. Basically, this is high resolution monitor.

As we previously mentioned, you can receive HD via OTA signals as well. You have to provide a digital tuner box and an antenna, but the benefit is that there are no monthly fees. You will likely only receive your local channels but again, the benefit is that the signal is free.

An HDTV set is a "step up" because it includes a built in digital tuner, which is known as an ATSC tuner. There is no additional tuner box necessary as the set is capable of both receiving and displaying your high-def programming. This set will typically cost more to offset the additional digital tuning components. Again, you will need to supply an antenna for free OTA programming, but you don't need to purchase an external tuner.

Similar to an HD Ready set an HDTV set can also receive high-def programming from cable or satellite. All of this really boils down to one key difference: does the set have a built in digital tuner or not? If it does, it's an HDTV; if it doesn't, it's HD Ready.

If you have further questions please don't hesitate to contact me.

Cheers!!