Particle Detectors

[kevin cooper]

Hello, everyone. I just signed on to this site because of my interest in astronomy and physics. My comprehension of both involves limited math; I simply want to gain a basic understanding of the universe I inhabit. My current interest involves the double slit experiment, specifically how the particle detectors work. At the risk of sounding like an idiot, and without ever actually seeing any of this equipment, I had envisioned the detector acting something like a police radar gun with a field or beam that the particle(atom) would interrupt as it passes, thus being DETECTED. It is my understanding that the atom behaves like a wave until it is observed, at which point it acts like a particle. Does the detector cause the atom to collapse, or shrink into a point of energy that is otherwise spread out?

[Strange]

The detector depends on the type of particles used. It could be photographic film or a phosphorescent screen (like an old-fashioned CRT dispaly) with a camera to record each flash, or probably something like the sensor in a digital camera. I expect there are other possibilities (I don't know what they use when the experiment is done with larger molecules).

 

 

It is my understanding that the atom behaves like a wave until it is observed, at which point it acts like a particle.

 

It always behaves like both. Or neither. It has some wavelike properties (frequency, wavelength, polarization, etc) and some particle like properties (being quantised, spin, etc). If you measure a wavelike property then the particle will appear wavelike (by definition) if you measure a particle-like property, then it will appear particle like.

[Sensei]

The simplest particle detector is Cloud Chamber.

 

Here you have instruction how to build one:

http://www.ultimate-theory.com/en/2014/6/8/how-to-build-cloud-chamber-particle-detector

 

It costs something between 20-50 usd.

 

Example result (radioactive isotope is decaying and highly accelerated new particles are ionizing medium they are passing)

 

Setup to double slit is even cheaper.

I have two 0.03 mm and 0.05 mm.

Bought for $6.

Of course you also need laser.

 

Couple photos taken by me with 405 nm wavelength laser:

Edited October 29, 2014 by Sensei

[Strange]

Related to that, worth checking out swansont's latest blog post: http://blogs.scienceforums.net/swansont/archives/15427

 

But I'm not sure a cloud chamber is appropriate for this application.

[Sensei]

Related to that, worth checking out swansont's latest blog post: http://blogs.scienceforums.net/swansont/archives/15427

 

But I'm not sure a cloud chamber is appropriate for this application.

 

In Swansont's blog there is information about muons.

Muons and positrons were discovered by Carl David Anderson in Cloud Chamber in 1932 & 1936.

http://en.wikipedia.org/wiki/Carl_David_Anderson

 

I just wanted OP to be aware of such device as Cloud Chamber.

Edited October 29, 2014 by Sensei

[Strange]

I just wanted OP to be aware of such device as Cloud Chamber.

 

Indeed. They are one of my favourite answers to idiotswho ask, "have you ever seen a sub-atomic particle?"

[derek w]

Google "Brian cox cloud chamber".

Watch his video.

Edited October 29, 2014 by derek w

[Sensei]

 

Indeed. They are one of my favourite answers to idiotswho ask, "have you ever seen a sub-atomic particle?"

 

Sorry, Strange, but they're not "idiots" but "victims of bad school education".

I would start learning kids from showing them particles on their own eyes..

 

Devices such as Cloud Chamber, electron gun, discharge tubes, high voltage generators, lasers, polarization filters, diffraction granting, single/double slits, are cheap and quickly allow showing quantum world..

There is no reason why they're not in every primary school.

[Strange]

Sorry, Strange, but they're not "idiots" but "victims of bad school education".

 

The ones I have had dealings with have been idiots with no interest at all in learning. Especially not science, which they seem to think of as some sort of inhuman, soul-destroying practice.

[Enthalpy]

[...] It is my understanding that the atom behaves like a wave until it is observed, at which point it acts like a particle. Does the detector cause the atom to collapse, or shrink into a point of energy that is otherwise spread out?

 

This understanding would suffice more or less for a double-slit experiment using a detector. It does not for other behaviours of quantum objects. You might have a browse at orbital pictures of "pentacene" made by "atomic force microscope": the same pair of electrons is observed all the way to image the shape of the wavefunction, which then isn't just a statistics over many point events. That's why one shouldn't restrict his interest to the double slit.

 

The object is a wave and a particle all the time. It's a wave in that it diffracts, and so on, as was previouslt known from light. It's a particle in that some properties (charge, energy, angular momentum) don't spread over the wave's extension. If one pixel of a detector gets a photon, the energy is all there and nowhere else. Also, to compute the extension and shape of the wave, we take the complete charge, mass etc. of the particle at each possible position.

 

When a photon gets absorbed in a CCD camera, it still behaves as a wave. It keeps an extension, for instance one thousand atoms, because the absorbing electron had such an extension before and has it after. Rather, the photon has adapted its extension to the electron that happened to absorb it; in that sense, it's a particle if you wish: it didn't need to reduce its extension to a point for it, but adapted its extension while keeping the other attributes, especially the energy transferred to the electron.

 

Wave and particle at the same time is abstract, but the good aspect is that you don't have to tell when it would be one or the other.

 

Interferences were produced with photons, then electrons, and more recently with atoms and molecules. These heavier objects use to have a delocalization that is smaller than their mechanical dimensions, so it takes difficult conditions to observe interferences - typically produce fringes wide enough.

[kevin cooper]

 

This understanding would suffice more or less for a double-slit experiment using a detector. It does not for other behaviours of quantum objects. You might have a browse at orbital pictures of "pentacene" made by "atomic force microscope": the same pair of electrons is observed all the way to image the shape of the wavefunction, which then isn't just a statistics over many point events. That's why one shouldn't restrict his interest to the double slit.

 

The object is a wave and a particle all the time. It's a wave in that it diffracts, and so on, as was previouslt known from light. It's a particle in that some properties (charge, energy, angular momentum) don't spread over the wave's extension. If one pixel of a detector gets a photon, the energy is all there and nowhere else. Also, to compute the extension and shape of the wave, we take the complete charge, mass etc. of the particle at each possible position.

 

When a photon gets absorbed in a CCD camera, it still behaves as a wave. It keeps an extension, for instance one thousand atoms, because the absorbing electron had such an extension before and has it after. Rather, the photon has adapted its extension to the electron that happened to absorb it; in that sense, it's a particle if you wish: it didn't need to reduce its extension to a point for it, but adapted its extension while keeping the other attributes, especially the energy transferred to the electron.

 

Wave and particle at the same time is abstract, but the good aspect is that you don't have to tell when it would be one or the other.

 

Interferences were produced with photons, then electrons, and more recently with atoms and molecules. These heavier objects use to have a delocalization that is smaller than their mechanical dimensions, so it takes difficult conditions to observe interferences - typically produce fringes wide enough.

Thank you so much for your response. According to my understanding of what you described, a subatomic particle like a photon can behave like a wave and a particle because it possesses properties of both, and simply observing the particle does not determine weather it has wave-like or particle-like characteristics because we can't detect all the properties at once. This has me thinking about the probabilistic nature of quantum mechanics; why is it that if a particle can exist in a number of different states or positions at once does it have to BE in all of them at once? I'm still trying to understand Schrodinger's Cat. Just because we can't observe what state something is in(or weather it's dead or alive)doesn't mean it's not one or the other.

[imatfaal]

Thank you so much for your response. According to my understanding of what you described, a subatomic particle like a photon can behave like a wave and a particle because it possesses properties of both, and simply observing the particle does not determine weather it has wave-like or particle-like characteristics because we can't detect all the properties at once.

 

Rather than thinking of a classical particle or a classical wave - one needs to try and get ones head around the concept of a quantum mechanical particle that just does not mesh with our preconceptions. Rather than a claissical particle which can exhibit wave-like properties (or vice versa) we must think of it as a quantum object which when classically observed can give rise to measurements that are analogous to classical waves.

 

 

This has me thinking about the probabilistic nature of quantum mechanics; why is it that if a particle can exist in a number of different states or positions at once does it have to BE in all of them at once? I'm still trying to understand Schrodinger's Cat. Just because we can't observe what state something is in(or weather it's dead or alive)doesn't mean it's not one or the other.

 

Actually your last sentence is completely the opposite of what we observe. There is a need for us to use the concept and (as much as you ever need to in science) believe that quantum mechanical objects exist in a state of super-position - you can have state 1 or state 0 or the superposition of them. Far and away the simplest explanation for what we observe is that a superposition of states is what happens - it answers all the questions, provides some of the most accurate predictions known to man, and requires the fewest external assumptions; all other explanations which do not have superposition necessitate horrible extras (non-linearity, hidden variables and wildly hypothetical physics etc).

[Enthalpy]

The wave versus particle debate is (1) outdated (2) philosophy, not physics (3) counter-productive. The useful way of understanding is whether objects interfere, can concentrate some attributes in a smaller volume than previously in some situations, and so on.

 

The superposition of states is routinely observed.

[swansont]

why is it that if a particle can exist in a number of different states or positions at once does it have to BE in all of them at once? I'm still trying to understand Schrodinger's Cat. Just because we can't observe what state something is in(or weather it's dead or alive)doesn't mean it's not one or the other.

 

 

You can do experiments whose results only make sense if the particle actually is in both states, rather than one or the other.

[kevin cooper]

 

Rather than thinking of a classical particle or a classical wave - one needs to try and get ones head around the concept of a quantum mechanical particle that just does not mesh with our preconceptions. Rather than a claissical particle which can exhibit wave-like properties (or vice versa) we must think of it as a quantum object which when classically observed can give rise to measurements that are analogous to classical waves.

 

 

 

Actually your last sentence is completely the opposite of what we observe. There is a need for us to use the concept and (as much as you ever need to in science) believe that quantum mechanical objects exist in a state of super-position - you can have state 1 or state 0 or the superposition of them. Far and away the simplest explanation for what we observe is that a superposition of states is what happens - it answers all the questions, provides some of the most accurate predictions known to man, and requires the fewest external assumptions; all other explanations which do not have superposition necessitate horrible extras (non-linearity, hidden variables and wildly hypothetical physics etc).

To be honest, I wouldn't mind if you could describe the different states a particle can be in. I'm actually in way over my head here; this is what I know about atoms-the nuclei are made up of protons and neutrons(except for hydrogen)which are composed of quarks, which are bound together by the fundamental strong atomic force. Protons and neutrons are bound together by the residual strong nuclear force, and electrons are bound to atomic nuclei by electromagnetism. The force particles are gluons, pions and photons respectively. When an atom interacts with the weak atomic force(bosons) it gains a positive or negative charge, becoming an ion. Anyway, I hope you can help me understand some of the more advanced ideas that I'm trying to understand, you obviously know what you're talking about and are able to break it down into layman's terms. Thanks!