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Related Concept Videos

SN1 Reaction: Stereochemistry

S_N1 Reaction: Stereochemistry

This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...

SN1 Reaction: Kinetics

S_N1 Reaction: Kinetics

In an SN2 reaction, the reaction rate depends on both the type of nucleophile and the substrate. A hindered tertiary alkyl halide is practically inert to the SN2 mechanism despite using a strong nucleophile.
However, Sir Christopher Ingold and Edward D. Hughes, who studied the kinetics of various nucleophilic substitution reactions, noticed that a tertiary alkyl halide does undergo a nucleophilic substitution reaction in the presence of a weak nucleophile. While studying the substitution...

SN1 Reaction: Mechanism

S_N1 Reaction: Mechanism

Kinetic studies of ionization of a tertiary halide in a protic solvent suggest that only the substrate participates in the rate-determining step (slow step). The nucleophile is involved only after the slowest step. The SN1 reaction takes place in a multiple-step mechanism.
Firstly, the haloalkane ionizes to generate a carbocation intermediate and a halide ion. This heterolytic cleavage is highly endothermic with large activation energy. The ionization of the substrate, facilitated by a...

Acidity of 1-Alkynes

Acidity of 1-Alkynes

The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.

Predicting Products: SN1 vs. SN2

Predicting Products: S_N1 vs. S_N2

Nucleophilic substitution reactions of alkyl halides can proceed via an SN1 or an SN2 mechanism. While in SN2 reactions, the nucleophile attacks the substrate simultaneously as the leaving group departs, in SN1 reactions, the substrate first dissociates to give the carbocation intermediate. Various factors such as the structure of the substrate, the strength of the nucleophile, and the nature of the solvent promote one mechanism over the other.
With increased substitution on the alkyl halide,...

X-ray Imaging

X-ray Imaging

German physicist Wilhelm Röntgen (1845–1923) was experimenting with electrical current when he discovered that a mysterious and invisible "ray" would pass through his flesh but leave an outline of his bones on a screen coated with a metal compound. In 1895, Röntgen made the first durable record of the internal parts of a living human: an "X-ray" image (as it came to be called) of his wife’s hand. Scientists worldwide quickly began their own experiments with...

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Retrospective Cardiac Gating with A Prototype Small-Animal X-ray Computed Tomograph

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Synchronous gating in dilation x-ray detector without 1:1 image ratio.

Houzhi Cai, Wenyong Fu, Dong Wang

|May 5, 2019

Summary

This study reports a novel x-ray detector achieving 14 ps resolution using pulse-dilation technology. The system enhances temporal resolution for advanced x-ray imaging applications.

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Area of Science:

Physics
X-ray detection technology
High-speed imaging

Background:

Traditional x-ray detectors face limitations in temporal resolution.
Achieving picosecond-level temporal resolution is crucial for advanced scientific applications.
Pulse-dilation technology offers a potential solution for enhancing detector speed.

Purpose of the Study:

To report on an x-ray detector utilizing pulse-dilation technology.
To achieve high temporal resolution in x-ray detection.
To investigate synchronous gating effects in the developed detector.

Main Methods:

Utilized a pulse-dilation device to expand electron pulses from a photo-cathode.
Employed a magnetic lens imaging system to direct dilated pulses onto a microchannel plate (MCP).
Detected dilated electron pulses using a gated MCP, achieving 14 ps resolution.

Main Results:

A temporal resolution of 14 picoseconds (ps) was successfully achieved.
Investigated synchronous gating in a detector without a 1:1 image ratio, observing synchronized and unsynchronized areas.
Identified methods to mitigate synchronization effects, including magnetic lens design, MCP gating pulse width, and imaging ratio.

Conclusions:

The developed pulse-dilation x-ray detector demonstrates high temporal resolution.
Understanding and mitigating synchronous gating effects are critical for optimal detector performance.
Design considerations such as magnetic lens properties, MCP gating pulse width, and imaging ratio are key to avoiding synchronization issues.