Start signal

An alternative to the TAC and PHA/ADC is the time-to-digital eonverter (TDC), a unit that eombines the fiinetions of the TAC and PHA/ADC. There are start and stop inputs and an output that provides a binary number direetly proportional to the time diflferenee between the stop and start signals. The TAC ean be direetly eonneeted to a MCA or PC with the appropriate digital interfaee. 

In TOP systems, particle energies are usually determined by SBDs in addition to particle velocities being obtained with a TOP set-up which primarily measures the time needed by a particle to pass the distance between two thin foils 0.5-1 m apart [3.170, 3.171]. The first foil delivers a start signal, the second a stop signal. The stop signal can also be obtained from the SBD, but usually foils provide better timing signals. 

A time-of-flight (TOF) mass analyzer separates ions according to the time difference between a start signal and the pulse generated when an ion hits the detector, that is, the time of flight. 

For most polypeptide chains initiation begins with one of the three initiation codons, most commonly the methionine codon AUG. When properly placed in an mRNA chain, GUG may also serve as a bacterial initiation codon. In such cases, it codes for methionine rather than for valine. Occasionally UUG, AUU, ACG, and perhaps other codons can initiate translation 288/289 This is less frequent in eukaryotes than in bacteria. The sequence of bases preceding the initiation codon must also be important for recognition of the "start" signal. 

The residence time was determined for our neutron counter by measuring the time intervals between beta start signals and neutron stop signals. With a residence half-time of 11 ms and a coincidence resolving time of 40 ms. 92 of the true coincidence events were included. The fraction of true events not detected does not influence the present results because we normalize the Pn measurements to a known Pn value measured under identical conditions. The coincidence rate was measured by a simple overlap coincidence module where the beta pulse Input was stretched to 40 ms by a gate and delay generator. To measure the accidental coincidence rate, the same beta pulse was sent to a second coincidence module and overlapped with neutron pulses which had been delayed 45 ms. After correcting each coincidence rate for deadtime effects, the difference was the true coincidence rate. 

Start signals included 80-300 keV electrons and stop signals were 20-40 keV electrons. 

Figure 4.47 Typical electronic circuit for the measurement of electron-electron coincidences with two spectrometers (SP1, SP2) placed at the positions 0 , and 5, , respectively. The pre- and main amplifiers are together represented by a triangle. The delay retards the signal from SP1, thus providing a STOP of the time-to-digital converter (TDC) if this time measuring device has been initiated by a START signal from a time-correlated event registered in SP2. The output of the TDC, i.e., the number of time-correlated events as function of the correlation time is stored in a histogramming memory (HIS. MEM.) which then is read out by a computer (COMP.).

This time difference, called the correlation time, is stored in a histogramming memory, and after the necessary processing time (dead time rdead) the TDC is reset and waits for the next START signal. The histogramming memory collects all individual coincidences by sorting them according to their correlation times. In this way, a time spectrum is obtained which can be transferred to a computer. 

In the calculation for the probability of START signals accepted by the coincidence... 

Hence, the probability of START signals within dead is given by... 

P(accepted START signals) = 1 — P(lost START signals)... 

The first four codons are not involved in protein synthesis. Protein synthesis starts with AUG and stops with UAA. The peptide coded by the codons between the stop and start signals is... 

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