A proton and an electron, both at rest initially, combine to form a hydrogen atom in the ground state. A single photon is emit- ted in this process. What is its wavelength？ （)

Rubidium, potassium and carbon are three common elements used to date the history of Earth. The rates of radioactive decay of these elements are absolutely regular when averaged out over a period of time; nothing is known to change them. To be useful as clocks, the elements have to be fairly common in natural minerals, unstable but decay slowly over millions of years to form. recognizable "daughter" products which are preserved minerals.

For example, an atom of radioactive rubidium decays to form. an atom of strontium (another element) by converting a neutron in its nucleus to a proton and releasing an electron, generating energy in the process. The radiogenic daughter products of the decay-in this case strontium atoms--diffuse away and are lost above a certain very high temperature. So by measuring the exact proportions of rubidium and strontium atoms that are present in a mineral, researchers can work out how long it has been since the mineral cooled below that critical "blocking" temperature. The main problems with this dating method are the difficulty in finding minerals containing rubidium, the accuracy with which the proportions of rubidium and strontium are measured, and the fact that the method gives only the date when the mineral last cooled below the blocking temperature. Because the blocking temperature is very high, the method is used, mainly for recrystallized (igneous or metamorphic) rocks, not for sediments--rubidium-bearing minerals in sediments simply record the age of cooling of the rocks which were eroded to form. the sediments, not the age of deposition of the sediments themselves.

Potassium decays to form. (a gas) which is sometimes lost from its host mineral by escaping through pores. Although potassium-argon dating is therefore rather unreliable, it can sometimes be useful in dating sedimentary rocks because potassium is common in some minerals which form. in sediments at low temperatures. Assuming no argon has escaped, the potassium-argon date records the age of the sediments themselves.

Carbon dating is mainly used in archaeology. Most carbon atoms (carbon-12) are stable and do not change over time. However, cosmic radiation bombarding the upper atmospheres constantly interacting with nitrogen in the atmosphere to create an unstable form. of carbon, carbon-14.

What is the common feature of rubidium, potassium and carbon?

A．They can be made into clocks.

B．They are rich in content.

C．Their decay is slow but regular.

D．The products of their decay are the same.

Another name for an electron shell is______

  A) Quantum

  B) Principal energy level

  C) Energy state

  D) orbital

What aspect of rubidium decay is useful for dating?

A．The atom produced by the decay is above a certain point of temperature.

B．The atom produced by the decay is easy to be detected at a cool temperature.

C．The decay produced a. neutron and an electron.

D．The decay is sensitive to the changes in temperature.

Classical physics defines the vacuum as a state of absence: a vacuum is said to exist in a region of space if there is nothing in it. In the quantum field theories that describe the physics of elementary particles, the vacuum becomes somewhat more complicated. Even in empty space, particles can appear spontaneously as a result of fluctuations of the vacuum. For example, an electron and a positron, or antielectron, can be created out of the void(空间). Particles created in this way have only a fleeting existence; they are annihilated (消亡) almost as soon as they appear, and their presence can never be detected directly. They are called virtual particles in order to distinguish them from real parades, whose lifetimes are not constrained in the same way, and which can be detected. So it is still possible to define the vacuum as a space that has no real particles in it.

One might expect that the vacuum would always be the state of the lowest possible energy for a given region of space. If an area is initially empty and a real panicle is put into it, the total energy, it seems, should be raised by at least the energy equivalent of the mass of the added panicle. A surprising result of some recent theoretical investigations is that this assumption is not invariably true. There are conditions under which the introduction of a real particle of finite mass into an empty region of space can reduce the total energy, If the reduction in energy is great enough, an electron and a positron will be instantly created. Under these conditions the electron and positron are not a result of vacuum fluctuations but are real particles, which exist indefinitely and can be detected. In other words, under these conditions the vacuum is an unstable state and can decay(衰减) into a state of lower energy; that is one in which real particles are created.

The necessary condition for the decay of the vacuum is the presence of an intense electric field. As a result of the decay of the vacuum, the space permeated by such a field can be said to obtain an electric charge, and it can be called a charged vacuum. The particles that materialize in space make the charged vacuum likely to be found in only one place: in the immediate vicinity of a super heavy atomic nucleus(原子核), one with about twice as many pro tons as the heaviest natural nuclei known. A nucleus that large cannot be stable, but it might be possible to assemble one next to a vacuum for long enough to observe the decay of the vacuum. Experiments attempting to achieve this are now under way.

Which of the following titles best describes the passage as a whole?

A．The Vacuum: Its Fluctuations and Decay.

B．The Vacuum: Its Creation and Instability.

C．The Vacuum: a State of Absence.

D．Particles that Materialize in the Vacuum.

A、emitted

B、absorbed

C、both

D、no effect

It has always been a problem to decide whether "popular music" is music which represents the people or is simply music that the people like. The same problem of【C1】______exists with jazz. So many different【C2】______of music have been called jazz at one time or another that it is hard to say what【C3】______it is. Jazz has always been considered tobe black music【C4】______when I first【C5】______an interest in it twenty years ago, I used to hear white【C6】______ playing music that was like Louis Armstrong's in the 1920s. I found out afterwards that they learnt to do this by playing Armstrong's records over and over again until their style. was close enough to his【C7】______for them to imitate him.

Since then white singers like Bob Dylan have rediscovered【C8】______own folk tradition, instead of【C9】______from black roots. But the main【C10】______since 1960 have been social and technical. One is that young people have more【C11】______to spend on records at an earlier age than they used to, so Tin Pan Alley, the 'pop' music industry, aims at the teenage audience.【C12】______is that electronic equipment has developed to such an【C13】______that technicians are now capable of mixing sound to【C14】______recordings that are quite different from a live【C15】______.

But the real【C16】______with 'pop' musis is that Tin Pan Alley has always worked against its being a【C17】______music of the people. It takes everything original and natural out of it and【C18】______it with cheap commercial imitations.【C19】______the American folk singer, Woody Guthrie, said: "They've always【C20】______the second-rate songs. They've never wanted to play the good ones."

【C1】

A．definition

B．classification

C．imitation

D．discussion