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Half-value layer refers to the first half-value layer, where subsequent (i.e. second) half-value layers refer to the amount of specified material that will reduce the air kerma rate by one-half after material has been inserted into the beam that is equal to the sum of all previous half-value layers.

Quarter-value layer is the amount of specified material that reduces the air kerma rate (or exposure rate, exposure, air kerma, etc...) to one fourth of the value obtained without any test filters. The quarter-value layer is equal to the sum of the first and second half-value layers.

The homogeneity factor (HF) describes the polychromatic nature of the beam and is given by:

H F = 1 s t H V L 2 n d H V L {\displaystyle HF={\frac {1^{st}HVL}{2^{nd}HVL}}}

The HF for a narrow beam will always be less than or equal to one (it is only equal to one in the case of a monoenergetic beam). In case of a narrow polychromatic beam, the HF is less than one because of beam hardening.

HVL is related to Mean free path, however the mean free path is the average distance a unit of radiation can travel in the material before being absorbed, whereas HVL is the average amount of material needed to absorb 50% of all radiation (i.e., to reduce the intensity of the incident radiation by half).

In the case of sound waves, HVL is the distance that it takes for the intensity of a sound wave to be reduced to one-half of its original value. The HVL of sound waves is determined by both the medium through which it travels, and the frequency of the beam. A "thin" half-value layer (or a quick drop of -3 dB) results from a high frequency sound wave and a medium with a high rate of attenuation, such as bone. HVL is measured in units of length.[3]

A similar concept is the tenth-value layer or TVL. The TVL is the average amount of material needed to absorb 90% of all radiation, i.e., to reduce it to a tenth of the original intensity. 1 TVL is greater than or equal to log2(10) or approximately 3.32 HVLs, with equality achieved for a monoenergetic beam.

Here are example approximate half-value layers for a variety of materials against a source of gamma rays (Iridium-192):[4]

Counselors need to make sure that students understand the key distinction between the two plans:

Approximately 450 colleges have early decision or early action plans, and some have both. Some colleges offer a nonbinding option called single-choice early action, under which applicants may not apply ED or EA to any other college.

ED plans have come under fire as unfair to students from families with low incomes, since they do not have the opportunity to compare financial aid offers. This may give an unfair advantage to applicants from families who have more financial resources.

A student should apply early to an ED or EA plan if the student:

For a student who has a definite first-choice college, applying early has many benefits:

Many students believe applying early means competing with fewer applicants and increasing their chances for acceptance. This is not always true. Colleges vary in the proportion of the class admitted early and in the percentage of early applicants they admit.

Higher admission rates for ED applicants may correlate to stronger profiles among candidates choosing ED. Students should ask the admission office whether their institution's admission standards differ between ED and regular applicants, and then assess whether applying early makes sense given their own profile.

The Common Application and some colleges' application forms require the student applying under early decision, as well as the parent and counselor, to sign an ED agreement form spelling out the plan's conditions.

Make it clear in your school handbook and at college planning events that your policy for early-decision applications is to send the student's final transcript to one college only: anything else is unethical.

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