SUMMARY / RELATED TOPICS

Phosphor

A phosphor, most is a substance that exhibits the phenomenon of luminescence. This includes both phosphorescent materials, which show a slow decay in brightness, fluorescent materials, where the emission decay takes place over tens of nanoseconds. Phosphorescent materials are known for their use in radar screens and glow-in-the-dark materials, whereas fluorescent materials are common in cathode ray tube and plasma video display screens, fluorescent lights and white LEDs. Phosphors are transition-metal compounds or rare-earth compounds of various types; the most common uses of phosphors are in fluorescent lights. CRT phosphors were standardized beginning around World War II and designated by the letter "P" followed by a number. Phosphorus, the chemical element named for its light-emitting behavior, emits light due to chemiluminescence, not phosphorescence. In inorganic phosphors, these inhomogeneities in the crystal structure are created by addition of a trace amount of dopants, impurities called activators.

The wavelength emitted by the emission center is dependent on the atom itself and on the surrounding crystal structure. The scintillation process in inorganic materials is due to the electronic band structure found in the crystals. An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band; this leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap; the excitons are loosely bound electron–hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers. The latter rapidly de-excite by emitting scintillation light. In the case of inorganic scintillators, the activator impurities are chosen so that the emitted light is in the visible range or near-UV, where photomultipliers are effective; the holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons.

The delayed de-excitation of those metastable impurity states, slowed down by reliance on the low-probability forbidden mechanism, again results in light emission. Many phosphors tend to lose efficiency by several mechanisms; the activators can undergo change of valence, the crystal lattice degrades, atoms – the activators – diffuse through the material, the surface undergoes chemical reactions with the environment with consequent loss of efficiency or buildup of a layer absorbing either the exciting or the radiated energy, etc. The degradation of electroluminescent devices depends on frequency of driving current, the luminance level, temperature. Harder, high-melting, water-insoluble materials display lower tendency to lose luminescence under operation. Examples: BaMgAl10O17:Eu2+, a plasma-display phosphor, undergoes oxidation of the dopant during baking. Three mechanisms are involved. Thin coating of aluminium phosphate or lanthanum phosphate is effective in creating a barrier layer blocking access of oxygen to the BAM phosphor, for the cost of reduction of phosphor efficiency.

Addition of hydrogen, acting as a reducing agent, to argon in the plasma displays extends the lifetime of BAM:Eu2+ phosphor, by reducing the Eu atoms back to Eu. Y2O3:Eu phosphors under electron bombardment in presence of oxygen form a non-phosphorescent layer on the surface, where electron–hole pairs recombine nonradiatively via surface states. ZnS:Mn, used in AC thin-film electroluminescent devices degrades due to formation of deep-level traps, by reaction of water molecules with the dopant; the traps damage the crystal lattice. Phosphor aging leads to elevated threshold voltage. ZnS-based phosphors in CRTs and FEDs degrade by surface excitation, coulombic damage, build-up of electric charge, thermal quenching. Electron-stimulated reactions of the surface are directly correlated to loss of brightness; the electrons dissociate impurities in the environment, the reactive oxygen species attack the surface and form carbon monoxide and carbon dioxide with traces of carbon, nonradiative zinc oxide and zinc sulfate on the surface.

Sulfur can be removed as sulfur oxides. ZnS and CdS phosphors degrade by reduction of the metal ions by captured electrons; the M2+ ions are reduced to M+. The reduced metal can be observed as a visible darkening of the phosphor layer; the darkening is proportional to the phosphor's exposure to electrons and can be observed on some CRT screens that displayed the same image for prolonged periods. Europium-doped alkaline earth aluminates degrade by formation of color centers. Y2SiO5:Ce3+ degrades by loss of luminescent Ce3+ ions. Zn2SiO4:Mn degrades by desorption of oxygen under electron bombardment. Oxide phosphors can degrade in presence of fluoride ions, remaining fro

Stayman convention

Stayman is a bidding convention in the card game contract bridge. It is used by a partnership to find a 4-4 or 5-3 trump fit in a major suit after making a one notrump opening bid and it has been adapted for use after a 2NT opening, a 1NT overcall, many other natural notrump bids; the convention is named for Sam Stayman, who wrote the first published description in 1945, but its inventors were two other players: the British expert Jack Marx in 1939, who published it only in 1946, Stayman's regular partner George Rapée in 1944. A game contract bid and made in a major suit scores better than a game contract bid and made in a minor suit or in notrump; the success rate for a game contract in a major suit when a partnership has a combined holding of 26 points and eight cards in the major is about 80%, whereas a game contract in 3NT with 26 high card points has a success rate of only 60%, or 50% with 25 HCP. Accordingly, partnership priority is to find an eight card or better major suit fit when jointly holding sufficient values for a game contract.

5-3 and 6-2 fits are easy to find in basic methods as responder can bid 3♥ or 3♠ over 1NT, opener will not have a 5 card major to bid 1NT. However, finding 4-4 fits presents a problem; the 2 ♥ and 2 ♠ bids can not be used for this as they are weak a sign-off bid. After an opening bid or an overcall of 1NT, responder or advancer bids an artificial 2♣ to ask opener or overcaller if he holds a four- or five-card major suit; the artificial club bid promises four cards in at least one of the major suits and, in standard form, enough strength to continue bidding after partner's response. It promises distribution, not 4333. By invoking the Stayman convention, the responder takes control of the bidding since strength and distribution of the opener's hand is known within a limited range; the opener responds with the following rebids. 2♦ denies four or more cards in either major suit. 2♥ shows at least four hearts. 2♠ shows at least four spades. A notrump opener should have neither a suit longer than five cards nor more than one 5-card suit since an opening notrump bid shows a balanced hand.

A notrump bidder who has at least four cards in each major suit responds in hearts, as this can still allow a spade fit to be found. Variant methods are to bid the longer or stronger major, with a preference given to spades, or to use 2NT to show both majors. In the standard form of Stayman over 1NT, the responder has a number of options depending on his partner's answer: If the notrump opener names a major suit and the responder has four cards in that suit, the responder bids three of the notrump bidder's suit with 8-9 HCP or four of the notrump bidder's major suit with 10 or more HCP. If the notrump bidder bids a major suit in which the responder does not have at least four cards, the responder may bid 2NT with 8-9 HCP or 3NT with 10 or more HCP. However, if responder has 5 cards in the unnamed major, he may bid it at a convenient level in an attempt to find a 5-3 fit. If the notrump bidder bids 2♦, denying a four-card major, responder may bid his five-card major with a call of 2 of his major with 8-9 HCP, or with a call of 3 of his major with 10 HCP.

This allows notrump bidder to find game in a major with a 5-3 split. Otherwise, opener has the option to retreat to the appropriate notrump contract. Over these bids, the notrump bidder with a maximum hand, goes to game over an invitational bid and with four cards in each major suit, corrects to the unbid major suit. In the standard form of Stayman over 2NT, the responder has only two normal rebids. If the notrump bidder names a major suit and the responder has four cards in that suit, the responder bids four of the notrump bidder's suit. If the notrump bidder names a major suit in which the responder does not have at least four cards or bids diamonds to deny a major suit, the responder bids 3NT. If the notrump bidder has four cards in each major suit, the notrump bidder corrects to the unbid major suit. In either case, a responder who rebids notrump over a response in a major suit promises four cards of the other major suit. Thus, a notrump opener who holds at least four cards in each major suit should "correct" by bidding the other major suit at the lowest level.

Of course, once a fit is found, responder who has sufficient strength may bid 4♣ or 4NT, or cue bid aces, depending upon partnership agreement, to explore slam in any of the above sequences. Some partnerships admit responder's rebids of a major suit that the notrump bidder did not name. A bid of 4♣ over an opening bid of 3NT may be either Stayman or Gerber, depending upon the partnership agreement. If an adverse suit bid is inserted after a 1NT opening, Stayman may be employed via a double or a cue bid, depending on the strength of his hand; the cue bid, conventional, is artificial and means nothing other than invoking Stayman. For example, if South opens 1NT, West overcalls 2♦, North, if he ha

City and Liberty of Westminster

The City and Liberty of Westminster was a unit of local government in the county of Middlesex, England. It was located to the west of the City of London. Under the control of Westminster Abbey, the local authority for the area was the Westminster Court of Burgesses from 1585 to 1900; the area now forms the southern part of the City of Westminster in Greater London. Following the dissolution of Westminster Abbey, a court of burgesses was formed in 1585 to govern the Westminster area under the Abbey's control; the City and Liberties of Westminster were further defined by Letters Patent in 1604, the court of burgesses and liberty continued in existence until 1900, the creation of the Metropolitan Borough of Westminster. The court of burgesses was headed by the High Steward of Westminster Abbey, a prominent national politician, he appointed a high bailiff, who served for life, performed most of the functions exercised by a high sheriff of a county. The city and liberty were divided into twelve wards, each with a burgess and assistant burgess, this arrangement being adopted from the system used in the City of London.

Eight wards were located in the parish of St Margaret, three in St Martin in the Fields, one for St Clement Danes and the Strand area. The burgesses chose two head burgesses, one for the city and one for the liberty, who ranked next after the high bailiff. A high constable was appointed under whom was a force of constables; these were absorbed by the Metropolitan Police in 1829. Following the dissolution of the court of burgesses in 1900, a link has been retained to the old corporation, as the Lord Mayor of Westminster is ex officio Deputy High Steward of Westminster Abbey. Westminster returned two members to parliament. Although outside the Liberty of Westminster, eligible inhabitants of the Liberty of the Savoy, which included part of the parishes of St Clement Danes and St Mary le Strand voted with Westminster; the City and Liberty of Westminster was a franchise coroner's district until 1930, when it became part of the Central district of the County of London. The City of Westminster consisted of: The main part of the parish of St Margaret.

The extra-parochial Close of the Collegiate Church of St Peter around Westminster AbbeyThe Liberty of the City of Westminster consisted of: The detached part of the parish of St Margaret The parish of St Martin in the Fields. The boundary leaves the river about midway between Waterloo bridge and Hungerford market, with a little deviation follows the course of the Strand eastward to Temple Bar, being separated from the river in this part by what is termed the liberty of the duchy of Lancaster and by the western part of the Temple; the boundary turns northward from Temple Bar up Shire Lane, runs in an irregular line westward, keeping to the south of Lincoln’s Inn Fields till it reaches Drury Lane: it turns north-westward up Drury Lane to Castle Street, again turn westward and northward runs by Castle Street, West Street, Crown Street, Soho, to the eastern end of Oxford Street. The northern boundary runs in a direct line westward along Oxford Street and the north side of Hyde Park and Kensington Gardens, making a small detour in one place, so as to include St. George’s burying-ground, to the northern end of the Serpentine river.

From this point the western boundary follows the course of the Serpentine and of a stream which runs from its south-eastern extremity, now for the most part covered over, west of Kinnerton Street, Lowndes Street, Chesham Street, Westbourn Street, the Commercial Road, to the Thames just in front of Chelsea Hospital