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What is Transmission Electron Microscopy?
Transmission electron microscopy is a method of imaging thin specimens, where the thinness of the specimen can be of the order of nanometers. The specimen is mounted on a solid specimen holder, such as a copper grid. The specimen is then placed inside a vacuum chamber, and the desired amount of electron beam is accelerated to vacuum conditions, which is a pressure of about 1013 Torr. The electron beam passes through the specimen, the specimen holder, and finally through a large, magnifying lens. The lens focuses the scattered electrons onto a fluorescent screen, which is located a distance away from the lens. An image of the specimen is then seen on the screen.
Transmission electron microscopy is ideal for biological specimens since it can be used on any kind of specimen, including liquids, and because the contrast of biological specimens in transmission electron microscopy is usually better than that in any other method.
The electron beam must be focused to a diameter of less than one nanometer, but not less than a few nanometers. If the beam is too large, it passes through the specimen and then the specimen holder, which makes the image indistinct.
Transmission electron microscopy is also used in condensed matter physics, materials science, and the study of magnetic materials.
Disadvantages
Transmission electron microscopy is a relatively new technique. Many important discoveries have been made using the method, but many remain unknown because of problems with artifacts and image clarity.
Transmission electron microscopy is very expensive, although relatively inexpensive in comparison with the costs of similar equipment. A typical set of instruments will cost over $100,000. A high-end system such as the Zeiss Leo 906 can cost more than $1,000,000.
To see the advantages of transmission electron microscopy, let us compare it to two other imaging methods. The first is scanning electron microscopy, which is a way to see the surface of a specimen. Scanning electron microscopy is not as useful for studying the internal structures of a specimen.
The second method is x-ray diffraction, which is a way to see the internal structures of a 01e38acffe
Category:Microstructure of materials
Category:Nuclear physics
Category:Nuclear technologyQ:
Calculate length of total time to reach a particular point?
I have an NSMutableArray of NSDates, all set to 1 year in the past. I want to get the cumulative length of time between now and a particular point in the past.
I'm not sure how to do this in the best way, I can think of the following:
NSMutableArray *dates = [NSMutableArray array];
[dates addObject:[NSDate date]];
while (NSCalendar* myCalendar = [NSCalendar currentCalendar]) {
NSDateComponents *comps = [myCalendar components:NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit fromDate:self.dates[0] toDate:self.dates[0] options:0];
[self.dates addObject:[self.dates dateByAddingComponents:comps toDate:self.dates[0] options:0]];
}
NSLog(@"Total time to reach time past: %@", [self.dates lastObject] - [NSDate date]);
The above code works fine, but it seems like there should be a more elegant way.
A:
If you want the cumulative total time, you are going to have to loop through the array, and calculate the time elapsed for each date in the array, and add them up.
It might be more efficient to not do this calculation for each date, but rather do it for a large date range and just sum up the values for each date you are interested in.
Measurement of tumor necrosis factor alpha and interleukin-1 beta in the cerebrospinal fluid of multiple sclerosis patients: relation with the clinical course.
Recent findings have shown that immune responses also play an important role in the pathogenesis of multiple sclerosis (MS). Interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF alpha) are cytokines that have been proposed to play a critical role in the immune-mediated neurodegenerative process. In this study, we quantified the concentrations of TNF alpha and IL-1 beta in the cerebrospinal fluid (CSF) of
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