Positron injection

The sizes and concentration of the free-volume cells in a polyimide film can be measured by PALS. The positrons injected into polymeric material combine with electrons to form positroniums. The lifetime (nanoseconds) of the trapped positronium in the film is related to the free-volume radius (few angstroms) and the free-volume fraction in the polyimide can be calculated.136 This technique allows a calculation of the dielectric constant in good agreement with the experimental value.137 An interesting correlation was found between the lifetime of the positronium and the diffusion coefficient of gas in polyimide.138,139 High permeabilities are associated with high intensities and long lifetime for positron annihilation. 

The proposed scenario for antihydrogen production involves stopping a bunch of approximately 109 antiprotons in dense helium this is followed shortly afterwards by the injection of a bunch of approximately 10s positrons into the same medium. A detailed discussion was given by Ito et al. (1993) this included a treatment of the differences between the positrons and the positronium atoms which result from the positron injection, in terms of the known behaviour of these species in helium gas see Chapters 6 and 7. Under optimum conditions, the number of antihydrogen atoms formed could be as great as 103-104 per antiproton bunch. 

Ito, Widmann and Yamazaki (1993) also addressed the problem of the detection of antihydrogen under these conditions. They estimated that most of the antihydrogen will be formed tens of nanoseconds after the positron injection into the 37 atmospheres of helium gas, envisaged as the stopping medium, and will probably be destroyed within 1 ns after formation. Thus the antihydrogen signature will be a small spike on the... 

Figure 13.3 (left) Two-dimensional spectrum of annihilation radiation of positrons injected into a p-Si (100), with 8 fl-cm. The diagonal feature indicates the condition of E) + E2= 1.022 MeV (right) Normalized annihilation lines as a function of photon energy for Au nanoparticle layer (solid circles), MgO layer (open circles), and Au film (solid line) [6],... 

Positrons injected into matter generally decay via one of the three processes as para-Ps (p-Ps, singlet state, channel 1), positrons having not formed Ps (e" ", channel 2), and ortho-Vs (o-Ps, triplet state, channel 3). A naive spectral analysis therefore assumes that the lifetime spectrum consists of three exponential decay terms [see, e.g Mogensen, 1995] ... 

PALS measures the rate of annihilation of positrons injected into a material from a radioactive Na source. The positron lifetime is inversely proportional to the overlap of the positron density and the electron density of the surrounding medium. The positron within the polymer matrix will pick up an electron from... 

Positron Annihilation Spectroscopy (PALS) can investigate the free volume existing between polymer chains. The lifetime of particles (positrons) injected into a sample can thus provide information on the void structure existing in polymers and polymer blends. For immiscible polymers, free volume existing at the interface due to poor adhesion can be detected by PALS. In miscible polymers, densification due to favorable interactions may be capable of determination. This technique involves the injection of positrons into a polymeric system from a radioisotope capable of emitting positrons, such as Na. The positrons (positively charged electrons) combine with electrons to annihilate or to form a bound state called a positronium (Ps). If the spins of the positron and electron are antiparaUel, para-positroniums (pPS) with a lifetime of 0.125 ns are formed. If the spins of the positron and electrons are parallel, an orthopositronium (oPs) is formed with a lifetime of 1-5 ns. The oPs hfetime, Ts, is related to the free volume cavity in which the oPs is formed [388,389]. 

Another relatively recent technique, in its own way as strange as Mossbauer spectrometry, is positron annihilation spectrometry. Positrons are positive electrons (antimatter), spectacularly predicted by the theoretical physicist Dirac in the 1920s and discovered in cloud chambers some years later. Some currently available radioisotopes emit positrons, so these particles arc now routine tools. High-energy positrons are injected into a crystal and very quickly become thermalised by... 

Positron emission tomography (PET) is an imaging technique that relies on the emission of positrons from radionucleotides tagged to an injectable compound of interest. Each positron emitted by the radioisotope collides with an electron to emit two photons at 180° from each other. The photons are detected and the data processed so that the source of the photons can be identified and an image generated showing the anatomical localization of the compound of interest. 

Positron emission tomography (PET) makes use of a short-lived positron emitter such as fluorine-18 to image human tissue with a degree of detail not possible with x-rays. It has been used extensively to study brain function (see illustration) and in medical diagnosis. For example, when the hormone estrogen is labelled with fluorine-18 and injected into a cancer patient, the fluorine-bearing compound is preferentially absorbed by the tumor. The positrons given off by the fluorine atoms are quickly annihilated when they meet... 

This measures the distribution of a previously administered positron-emitting isotope. PET could be regarded as a form of in vivo autoradiography except that the radioligand is not [ H] but [ 0], [ N], ["C] or [ F], all of which have short half-lives (2, 10, 20 and 110 min respectively) and so the labelled ligand can only be prepared just before use. After intravenous injection the ligand can be located in the brain in a particular place... 

Positron emission tomography (PET) scan A scan that produces images of the body after the injection of a radioactive form of... 

PALS is based on the injection of positrons into investigated sample and measurement of their lifetimes before annihilation with the electrons in the sample. After entering the sample, positron thermalizes in very short time, approx. 10"12 s, and in process of diffusion it can either directly annihilate with an electron in the sample or form positronium (para-positronium, p-Ps or orto-positronium, o-Ps, with vacuum lifetimes of 125 ps and 142 ns, respectively) if available space permits. In the porous materials, such as zeolites or their gel precursors, ort/zo-positronium can be localized in the pore and have interactions with the electrons on the pore surface leading to annihilation in two gamma rays in pick-off process, with the lifetime which depends on the pore size. In the simple quantum mechanical model of spherical holes, developed by Tao and Eldrup [18,19], these pick-off lifetimes, up to approx. 10 ns, can be connected with the hole size by the relation ... 

Nuclear medicine is used chiefly in medical diagnosis. A radiopharmaceutical—a relatively harmless compound with a low dose of radiation— is swallowed or injected into the patient and tracked through the bloodstream by instruments such as a PET (positron emission tomography) camera. The nuclear physician can use the results to create a... 

The radioisotope nitrogen-13 has a relatively short half-life of about 10 minutes that produces a positron as it decays. This makes N-13 useful in PET (Positron Emission Tomography) scan technology, in which it is injected into the patient. The positive electrons (positrons) interact with the patient s negative electrons to produce an image similar to an X-ray. 

The artificial radioactive fluorine isotope F-18 emits positrons (positive electrons) that, when injected into the body, interact with regular negative electrons, and they annihilate... 

Like the monoamine hypothesis of depression, such a simple hypothesis was appeaUng but, perhaps predictably, a Uttle too simple to be true. Further research using a technique known as positron emission tomography (PET) showed the relationship between dopamine and schizophrenia is more complex. PET detects radioactive emissions of certain isotopes these isotopes are incorporated into a molecule and injected into a patient. The machine measures the radioactivity with detectors positioned aroimd the body. PET lets researchers study the distribution of certain molecules in Uving tissue since, imUke autoradiography, the tissue is not sliced and treated chemically. The amoimt of radioactivity must be small, however, to avoid harming the human subjects. 

When a particle and its antiparticle, such as an electron and a positron, or a proton and an antiproton, are used in head-on collision experiments, acceleration of the particles can be accomplished in one ring. This is because electrons and positrons, for example, behave m the same way in terms of their response to magnetic and electric fields. Thus, both particles can be injected into the same ring, one to follow an orbit in a clockwise direction the other in a counterclockwise direction. Upon injection of a cluster of each type of particle, collisions occur at two points diametrically opposed. This arrangement provides maximum utilization of the equipment. 

Traditionally, experimental values of Zeff have been derived from measurements of the lifetime spectra of positrons that are diffusing, and eventually annihilating, in a gas. The lifetime of each positron is measured separately, and these individual pieces of data are accumulated to form the lifetime spectrum. (The positron-trap technique, to be described in subsection 6.2.2, uses a different approach.) An alternative but equivalent procedure, which is adopted in electron diffusion studies and also in the theoretical treatment of positron diffusion, is to consider the injection of a swarm of positrons into the gas at a given time and then to investigate the time dependence of the speed distribution, as the positrons thermalize and annihilate, by solving the appropriate diffusion equation. The experimentally measured Zeg, termed Z ), is the average over the speed distribution of the positrons, y(v,t), where y(v,t) dv is the number density of positrons with speeds in the interval v to v + dv at time t after the swarm is injected into the gas. The time-dependent speed-averaged Zef[ is therefore... 

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