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How about using several photodiodes with each their own amplifiers
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LM
Apr 21, 2012, 8:24:25 AM4/21/12
to
And then summing the amplified signal when it is larger and not so
critical.
By the way, what are good components for this, photodiodes and op
amps, PMTs are too hard to use for me
critical.
By the way, what are good components for this, photodiodes and op
amps, PMTs are too hard to use for me
LM
Apr 21, 2012, 8:26:40 AM4/21/12
to
Phil Hobbs
Apr 21, 2012, 8:54:17 AM4/21/12
to
do, there's a noise current contribution of 2*pi*f*C_diode*e_N, where
C_diode is the total capacitance on the input node (amplifier, pads, and
photodiode together). TIAs, bootstraps, cascodes, everything.
(Reactive matching networks too, but they're a little more complicated.)
2. Reverse biased PIN diodes are easy to use, and can reduce the
capacitance by a factor of about 7 from zero bias. Reverse bias is
always a win if you're using silicon or short-wavelength
InGaAs--there'll be some voltage that optimizes the capacitance vs dark
current tradeoff for your situation, and it won't be zero volts. (I'm
running some 32-volt photodiodes at 80 volts, which improves them out of
all recognition, and they leak about 10 nA at room temperature.) You do
have to make sure that your bias supply is extremely quiet--ideally at
least 3x lower noise voltage spectral density than your input amplifier.
3. A good reverse-biased silicon PIN diode will have a capacitance of 40
to 100 pF/cm**2, depending on the thickness. Thinner ones are often
faster but don't respond quite as well out beyond 750 nm or so.
4. If you mean collecting more light overall, you're better off
collecting it with a lens or non-imaging concentrator and stuffing it
into a smaller photodiode, which will have lower capacitance (see #1 and
#2).
5. There are some fairly desperate situations where doing as you suggest
can make a lot of sense, but they're rare. One was an intracavity
darkfield laser particle detector where sample air was drawn through a
very small box, whose walls were tiled with CCDs, with the laser light
running at right angles to the air flow. It was designed by Bob
Knollenberg, and worked very well at the low count rates it was designed
for. (You probably aren't doing that, though.)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
LM
Apr 21, 2012, 9:47:41 AM4/21/12
to
On 21 huhti, 15:54, Phil Hobbs
Interesting, thanks
This was originally meant as a post to the thread "Light pulse and
photodiode " by George Herold. That's why my opinion about Google in
the second post, when it put this into a new thread instead of where
it belongs.
If George Herold does not mind, still a (silly?) question. I did not
mean those PDs would be paralleled, does total capacitance still add
if there are several separate photodiodes and amplifiers each built
like you say in #1.
Collecting light is probably easier too if there more than one diode.
This was originally meant as a post to the thread "Light pulse and
photodiode " by George Herold. That's why my opinion about Google in
the second post, when it put this into a new thread instead of where
it belongs.
If George Herold does not mind, still a (silly?) question. I did not
mean those PDs would be paralleled, does total capacitance still add
if there are several separate photodiodes and amplifiers each built
like you say in #1.
Collecting light is probably easier too if there more than one diode.
Phil Hobbs
Apr 21, 2012, 10:57:01 AM4/21/12
to
> > hobbs at electrooptical dot nethttp://electrooptical.net
>
> Interesting, thanks
>
> This was originally meant as a post to the thread "Light pulse and
> photodiode " by George Herold. That's why my opinion about Google in
> the second post, when it put this into a new thread instead of where
> it belongs.
>
> If George Herold does not mind, still a (silly?) question. I did not
> mean those PDs would be paralleled, does total capacitance still add
> if there are several separate photodiodes and amplifiers each built
> like you say in #1.
>
> Collecting light is probably easier too if there more than one diode.
To make better measurements, #4 is really the way forward. Lenses are
>
> Interesting, thanks
>
> This was originally meant as a post to the thread "Light pulse and
> photodiode " by George Herold. That's why my opinion about Google in
> the second post, when it put this into a new thread instead of where
> it belongs.
>
> If George Herold does not mind, still a (silly?) question. I did not
> mean those PDs would be paralleled, does total capacitance still add
> if there are several separate photodiodes and amplifiers each built
> like you say in #1.
>
> Collecting light is probably easier too if there more than one diode.
cheap and work amazingly.
George Herold
Apr 22, 2012, 12:23:31 PM4/22/12
to
On Apr 21, 8:54 am, Phil Hobbs
above application, it looks to me like you want a Big feedback R. And
that is going to dominate the noise.
Of course you always want the PD C to be as low as possible!
(maybe 1 cm in diameter) I was pictureing gluing a PD on each end. (A
conical light pipe made out of the same scintalator on each end would
be nice...) But would gluing a lens on each end help also? I lose
photons but reduce the PD C.
>
> 5. There are some fairly desperate situations where doing as you suggest
> can make a lot of sense, but they're rare. One was an intracavity
> darkfield laser particle detector where sample air was drawn through a
> very small box, whose walls were tiled with CCDs, with the laser light
> running at right angles to the air flow. It was designed by Bob
> Knollenberg, and worked very well at the low count rates it was designed
> for. (You probably aren't doing that, though.)
Doesn't the correlation trick help? Half the signal, but two pulses.
Or is it a 'wash' in the end.
I'll have to do some thinking.
George H.
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> LM wrote:
>
> > And then summing the amplified signal when it is larger and not so
> > critical.
>
> > By the way, what are good components for this, photodiodes and op
> > amps, PMTs are too hard to use for me
>
> 1. Capacitance is the worst enemy in PD amplifiers. No matter what you
> do, there's a noise current contribution of 2*pi*f*C_diode*e_N, where
> C_diode is the total capacitance on the input node (amplifier, pads, and
> photodiode together). TIAs, bootstraps, cascodes, everything.
> (Reactive matching networks too, but they're a little more complicated.)
Hmm, I'm going to have to push the numbers around some... But for the
> LM wrote:
>
> > And then summing the amplified signal when it is larger and not so
> > critical.
>
> > By the way, what are good components for this, photodiodes and op
> > amps, PMTs are too hard to use for me
>
> 1. Capacitance is the worst enemy in PD amplifiers. No matter what you
> do, there's a noise current contribution of 2*pi*f*C_diode*e_N, where
> C_diode is the total capacitance on the input node (amplifier, pads, and
> photodiode together). TIAs, bootstraps, cascodes, everything.
> (Reactive matching networks too, but they're a little more complicated.)
above application, it looks to me like you want a Big feedback R. And
that is going to dominate the noise.
Of course you always want the PD C to be as low as possible!
>
> 2. Reverse biased PIN diodes are easy to use, and can reduce the
> capacitance by a factor of about 7 from zero bias. Reverse bias is
> always a win if you're using silicon or short-wavelength
> InGaAs--there'll be some voltage that optimizes the capacitance vs dark
> current tradeoff for your situation, and it won't be zero volts. (I'm
> running some 32-volt photodiodes at 80 volts, which improves them out of
> all recognition, and they leak about 10 nA at room temperature.) You do
> have to make sure that your bias supply is extremely quiet--ideally at
> least 3x lower noise voltage spectral density than your input amplifier.
Clearly I'm not 'abusing' my photodiodes enough!
> 2. Reverse biased PIN diodes are easy to use, and can reduce the
> capacitance by a factor of about 7 from zero bias. Reverse bias is
> always a win if you're using silicon or short-wavelength
> InGaAs--there'll be some voltage that optimizes the capacitance vs dark
> current tradeoff for your situation, and it won't be zero volts. (I'm
> running some 32-volt photodiodes at 80 volts, which improves them out of
> all recognition, and they leak about 10 nA at room temperature.) You do
> have to make sure that your bias supply is extremely quiet--ideally at
> least 3x lower noise voltage spectral density than your input amplifier.
>
> 3. A good reverse-biased silicon PIN diode will have a capacitance of 40
> to 100 pF/cm**2, depending on the thickness. Thinner ones are often
> faster but don't respond quite as well out beyond 750 nm or so.
>
> 4. If you mean collecting more light overall, you're better off
> collecting it with a lens or non-imaging concentrator and stuffing it
> into a smaller photodiode, which will have lower capacitance (see #1 and
> #2).
Oh, that's interesting... Say I've got a rod of plastic scintalator
> 3. A good reverse-biased silicon PIN diode will have a capacitance of 40
> to 100 pF/cm**2, depending on the thickness. Thinner ones are often
> faster but don't respond quite as well out beyond 750 nm or so.
>
> 4. If you mean collecting more light overall, you're better off
> collecting it with a lens or non-imaging concentrator and stuffing it
> into a smaller photodiode, which will have lower capacitance (see #1 and
> #2).
(maybe 1 cm in diameter) I was pictureing gluing a PD on each end. (A
conical light pipe made out of the same scintalator on each end would
be nice...) But would gluing a lens on each end help also? I lose
photons but reduce the PD C.
>
> 5. There are some fairly desperate situations where doing as you suggest
> can make a lot of sense, but they're rare. One was an intracavity
> darkfield laser particle detector where sample air was drawn through a
> very small box, whose walls were tiled with CCDs, with the laser light
> running at right angles to the air flow. It was designed by Bob
> Knollenberg, and worked very well at the low count rates it was designed
> for. (You probably aren't doing that, though.)
Or is it a 'wash' in the end.
I'll have to do some thinking.
George H.
George Herold
Apr 22, 2012, 12:29:24 PM4/22/12
to
For the application I'm talking about with two PD's you get (about)
1/2 the light in each PD, so if you're just adding the signals then
it's clearly better to try and get all your photons into one
detector.
George H.
>
> Collecting light is probably easier too if there more than one diode.- Hide quoted text -
>
> - Show quoted text -
Phil Hobbs
Apr 22, 2012, 2:58:13 PM4/22/12
to
George Herold wrote:
>
> On Apr 21, 8:54 am, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote:
> > LM wrote:
> >
> > > And then summing the amplified signal when it is larger and not so
> > > critical.
> >
> > > By the way, what are good components for this, photodiodes and op
> > > amps, PMTs are too hard to use for me
> >
> > 1. Capacitance is the worst enemy in PD amplifiers. No matter what you
> > do, there's a noise current contribution of 2*pi*f*C_diode*e_N, where
> > C_diode is the total capacitance on the input node (amplifier, pads, and
> > photodiode together). TIAs, bootstraps, cascodes, everything.
> > (Reactive matching networks too, but they're a little more complicated.)
>
> Hmm, I'm going to have to push the numbers around some... But for the
> above application, it looks to me like you want a Big feedback R. And
> that is going to dominate the noise.
> Of course you always want the PD C to be as low as possible!
The R_f noise only dominates at low frequency. The amplifier voltage
>
> On Apr 21, 8:54 am, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote:
> > LM wrote:
> >
> > > And then summing the amplified signal when it is larger and not so
> > > critical.
> >
> > > By the way, what are good components for this, photodiodes and op
> > > amps, PMTs are too hard to use for me
> >
> > 1. Capacitance is the worst enemy in PD amplifiers. No matter what you
> > do, there's a noise current contribution of 2*pi*f*C_diode*e_N, where
> > C_diode is the total capacitance on the input node (amplifier, pads, and
> > photodiode together). TIAs, bootstraps, cascodes, everything.
> > (Reactive matching networks too, but they're a little more complicated.)
>
> Hmm, I'm going to have to push the numbers around some... But for the
> above application, it looks to me like you want a Big feedback R. And
> that is going to dominate the noise.
> Of course you always want the PD C to be as low as possible!
noise dominates above f_N, where the noise current from e_N equals that
from R_f and i_N of the amplifier:
2*pi*f_N*C_d*e_N = sqrt(4kT/R_f + i_Namp**2)
or f_Namp = sqrt(4kT/R_f + i_Namp**2)/(2*pi*C_d*e_N)
Above there, the amplifier voltage noise dominates. Of course, your amp
may roll off before that happens, but it gets worse for large R_f and
large C_d.
> >
> > 2. Reverse biased PIN diodes are easy to use, and can reduce the
> > capacitance by a factor of about 7 from zero bias. Reverse bias is
> > always a win if you're using silicon or short-wavelength
> > InGaAs--there'll be some voltage that optimizes the capacitance vs dark
> > current tradeoff for your situation, and it won't be zero volts. (I'm
> > running some 32-volt photodiodes at 80 volts, which improves them out of
> > all recognition, and they leak about 10 nA at room temperature.) You do
> > have to make sure that your bias supply is extremely quiet--ideally at
> > least 3x lower noise voltage spectral density than your input amplifier.
>
> Clearly I'm not 'abusing' my photodiodes enough!
> >
> > 3. A good reverse-biased silicon PIN diode will have a capacitance of 40
> > to 100 pF/cm**2, depending on the thickness. Thinner ones are often
> > faster but don't respond quite as well out beyond 750 nm or so.
> >
> > 4. If you mean collecting more light overall, you're better off
> > collecting it with a lens or non-imaging concentrator and stuffing it
> > into a smaller photodiode, which will have lower capacitance (see #1 and
> > #2).
>
> Oh, that's interesting... Say I've got a rod of plastic scintalator
> (maybe 1 cm in diameter) I was pictureing gluing a PD on each end. (A
> conical light pipe made out of the same scintalator on each end would
> be nice...) But would gluing a lens on each end help also? I lose
> photons but reduce the PD C.
n**2 in going from the rod end into air, so you're much better off
cementing a plastic-packaged photodiode onto the end of the
scintillator. You have to collect all the available photons.
>
> >
> > 5. There are some fairly desperate situations where doing as you suggest
> > can make a lot of sense, but they're rare. One was an intracavity
> > darkfield laser particle detector where sample air was drawn through a
> > very small box, whose walls were tiled with CCDs, with the laser light
> > running at right angles to the air flow. It was designed by Bob
> > Knollenberg, and worked very well at the low count rates it was designed
> > for. (You probably aren't doing that, though.)
>
> Doesn't the correlation trick help? Half the signal, but two pulses.
> Or is it a 'wash' in the end.
> I'll have to do some thinking.
George Herold
Apr 22, 2012, 4:14:45 PM4/22/12
to
On Apr 22, 2:58 pm, Phil Hobbs
Duh, how silly of me. OK please disregard the beginning of this
thread.. or it's sire, where I related the needed pulse signal to the
Johnson noise... I'll have to amend it.
(Adding in the capacitance 'directly' complicates matters)
1000 photons is going to be hard.
I think the best thing to do (for we bears of very little brain) is to
do some measurement. And then explain the results. I'll pulse a LED
on Monday.
OK thanks, I guess the light cone idea only helps when you've got a
collimated light beam to begin with.
George H.
>
> - Show quoted text -- Hide quoted text -
thread.. or it's sire, where I related the needed pulse signal to the
Johnson noise... I'll have to amend it.
(Adding in the capacitance 'directly' complicates matters)
1000 photons is going to be hard.
I think the best thing to do (for we bears of very little brain) is to
do some measurement. And then explain the results. I'll pulse a LED
on Monday.
collimated light beam to begin with.
George H.
>
>
> > > 5. There are some fairly desperate situations where doing as you suggest
> > > can make a lot of sense, but they're rare. One was an intracavity
> > > darkfield laser particle detector where sample air was drawn through a
> > > very small box, whose walls were tiled with CCDs, with the laser light
> > > running at right angles to the air flow. It was designed by Bob
> > > Knollenberg, and worked very well at the low count rates it was designed
> > > for. (You probably aren't doing that, though.)
>
> > Doesn't the correlation trick help? Half the signal, but two pulses.
> > Or is it a 'wash' in the end.
> > I'll have to do some thinking.
>
> Multiplying the outputs might be interesting.
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC
> Optics, Electro-optics, Photonics, Analog Electronics
>
> 160 North State Road #203
> Briarcliff Manor NY 10510
> 845-480-2058
>
> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>
> > > 5. There are some fairly desperate situations where doing as you suggest
> > > can make a lot of sense, but they're rare. One was an intracavity
> > > darkfield laser particle detector where sample air was drawn through a
> > > very small box, whose walls were tiled with CCDs, with the laser light
> > > running at right angles to the air flow. It was designed by Bob
> > > Knollenberg, and worked very well at the low count rates it was designed
> > > for. (You probably aren't doing that, though.)
>
> > Doesn't the correlation trick help? Half the signal, but two pulses.
> > Or is it a 'wash' in the end.
> > I'll have to do some thinking.
>
> Multiplying the outputs might be interesting.
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC
> Optics, Electro-optics, Photonics, Analog Electronics
>
> 160 North State Road #203
> Briarcliff Manor NY 10510
> 845-480-2058
>
>
> - Show quoted text -- Hide quoted text -
George Herold
Apr 23, 2012, 9:46:31 PM4/23/12
to
>
> > - Show quoted text -- Hide quoted text -
>
> > - Show quoted text -- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
I feel in need of another "dope slap' for forgetting about TIA /PD
> > - Show quoted text -- Hide quoted text -
>
> > - Show quoted text -- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
noise gain.
So at the end of the day today.
I pulsed a red LED through 50 ohms, And looked at the response with a
~16mm^2 PD, biased to 10V, C = 70pF (according to spec sheet) with a
OPA134 as TIA, 1 meg ohm feed back and no feedback cap (enough stray C
in circuit)
Here's the step response,
http://bayimg.com/BAoHOaaDd
channel 1 is the drive AC coupled. And chan. 2 is the PD response.
It covers most of the screen. There's this long tail that I'm calling
edge effects.
The fast response is about 0.6V or 0.6 uA about 1 uW.
Then here's a 20ns pulse into the led, (about 20E-15 Joules) (with
x100 Amp on PD)
http://bayimg.com/caOhDaaDd
That's ~60 -70 k photons.
Except the 20ns pulse was a bit fast for the led... a 200ns pulse gave
a pulse response about 30 times larger. So maybe 20 k photons.
I also measured the noise from the PD circuit. Vrms was about 0.2
mV. With the noise peak at 150kHz.
If I use my previous Q*R*f = V equation to predict the number of
photons I get about 8k, Which seems at least reasonable.
A faster opamp should help.. that AD fast FET.
George H.
George Herold
Apr 24, 2012, 11:52:33 AM4/24/12
to
>
> - Show quoted text -
Yesterday I wrote,
> - Show quoted text -
"A faster opamp should help.."
A faster opamp (with everything else equal) will give me more noise at
a rate of fc^3/2 and more signal at a rate of fc, so the signal to
noise goes down as I increase the bandwidth. (fc is some corner
frequency of the circuit.)
This seems a bit counter-intuitive, so maybe someone would check my
work.
So imagine some PD TIA compensated so the signal bandwidth is fc and
noise bandwidth peaks at same fc.
Here’s a Bode plot of noise gain with fc = 10
(Opamp GBW = 100)
Log Noise gain
^
|
10 |
| /\
| / \
1 |--- \
| \
+---------> frequency
1 10
Then I wave my magic opamp wand and increase the opamp GBW
by 100 (GBW = 10k). And I can still compensate the thing to have
equal 3dB point and gain peaks but now at fc=100
Log Noise gain
^
100 | /\
| / \
| / \
| / \
|--- \
|
+------------->
1 100
When you do the integrals for the total noise it goes as fc^3/2!
George H.
Jon Elson
Apr 25, 2012, 5:54:07 PM4/25/12
to
Phil Hobbs wrote:
> 2. Reverse biased PIN diodes are easy to use, and can reduce the
> capacitance by a factor of about 7 from zero bias. Reverse bias is
> always a win if you're using silicon or short-wavelength
> InGaAs--there'll be some voltage that optimizes the capacitance vs dark
> current tradeoff for your situation, and it won't be zero volts. (I'm
> running some 32-volt photodiodes at 80 volts, which improves them out of
> all recognition, and they leak about 10 nA at room temperature.) You do
> have to make sure that your bias supply is extremely quiet--ideally at
> least 3x lower noise voltage spectral density than your input amplifier.
>
Oh, gosh, YES! There are FEW cases where you'd want to run a
> 2. Reverse biased PIN diodes are easy to use, and can reduce the
> capacitance by a factor of about 7 from zero bias. Reverse bias is
> always a win if you're using silicon or short-wavelength
> InGaAs--there'll be some voltage that optimizes the capacitance vs dark
> current tradeoff for your situation, and it won't be zero volts. (I'm
> running some 32-volt photodiodes at 80 volts, which improves them out of
> all recognition, and they leak about 10 nA at room temperature.) You do
> have to make sure that your bias supply is extremely quiet--ideally at
> least 3x lower noise voltage spectral density than your input amplifier.
>
PD in photovoltaic mode for pulse sensitivity. That would be awful!
The non-biased capacitance increases the noise gain of most
circuits. Filtering the bias supply is pretty easy, just a couple
RC stages and physical separation from interfering traces.
Jon
Jon Elson
Apr 25, 2012, 5:58:52 PM4/25/12
to
LM wrote:
> If George Herold does not mind, still a (silly?) question. I did not
> mean those PDs would be paralleled, does total capacitance still add
> if there are several separate photodiodes and amplifiers each built
> like you say in #1.
The problem here is that with two PDs, only half the light reaches each
> If George Herold does not mind, still a (silly?) question. I did not
> mean those PDs would be paralleled, does total capacitance still add
> if there are several separate photodiodes and amplifiers each built
> like you say in #1.
PD. So, it seems a losing proposition in the general case.
>
> Collecting light is probably easier too if there more than one diode.
Now, I can see a case where you have a long, skinny scintillator, and then
> Collecting light is probably easier too if there more than one diode.
putting a PD on each end of the scintillator may capture light better
than trying to reflect it all to one detector at one end. Some PETT
scanners are built like this. I've seen some other physics detectors
that had 10 foot long plastic scintillators with a detector at each end.
Time of arrival difference tells the position along the scintillator where
the particle hit.
But, in the general case where the scintillator is relatively small, I
don;t think the multiple PDs will help.
Jon
Jon Elson
Apr 25, 2012, 6:01:57 PM4/25/12
to
Phil Hobbs wrote:
> To make better measurements, #4 is really the way forward. Lenses are
> cheap and work amazingly.
Lenses usually suffer from loss at the interfaces. Light guides
> To make better measurements, #4 is really the way forward. Lenses are
> cheap and work amazingly.
can be made with silicone optical grease or clear RTV to reduce
loss at the interfaces. If it is all done well, you absolutely
can't detect the interface between scintillator and light guide.
With the super dense scintillators that gets harder, of course,
and a glass light guide might work better than Plexiglas, although
way harder to fabricate.
Jon
Jon Elson
Apr 25, 2012, 6:03:06 PM4/25/12
to
George Herold wrote:
> For the application I'm talking about with two PD's you get (about)
> 1/2 the light in each PD, so if you're just adding the signals then
> it's clearly better to try and get all your photons into one
> detector.
Yes, that's my take, too, unless the scintillator is long and
> For the application I'm talking about with two PD's you get (about)
> 1/2 the light in each PD, so if you're just adding the signals then
> it's clearly better to try and get all your photons into one
> detector.
skinny.
Jon
Jon Elson
Apr 25, 2012, 6:07:48 PM4/25/12
to
George Herold wrote:
> Oh, that's interesting... Say I've got a rod of plastic scintalator
> (maybe 1 cm in diameter) I was pictureing gluing a PD on each end. (A
> conical light pipe made out of the same scintalator on each end would
> be nice...) But would gluing a lens on each end help also? I lose
> photons but reduce the PD C.
Well, instead of lenses, which generally would require a quadruple
> Oh, that's interesting... Say I've got a rod of plastic scintalator
> (maybe 1 cm in diameter) I was pictureing gluing a PD on each end. (A
> conical light pipe made out of the same scintalator on each end would
> be nice...) But would gluing a lens on each end help also? I lose
> photons but reduce the PD C.
interface (scint -> air -> lens -> air -> PD) just a better light
guide would be best. Silicone grease for the interfaces, and make the
small end of the cone whatever is best for the PD. You can make the
guide out of plain Acrylic (trade names Plexiglas or Lucite) as most
plastic scintillators are basically doped Acrylic. You can glue the
guide to the scint with Acrylic glue.
Jon
Jon Elson
Apr 25, 2012, 6:19:59 PM4/25/12
to
George Herold wrote:
> 1000 photons is going to be hard.
>
> I think the best thing to do (for we bears of very little brain) is to
> do some measurement. And then explain the results. I'll pulse a LED
> on Monday.
The (nuclear instrumentation) literature should have plenty of articles
> 1000 photons is going to be hard.
>
> I think the best thing to do (for we bears of very little brain) is to
> do some measurement. And then explain the results. I'll pulse a LED
> on Monday.
on charge-sensitive preamps that have worked well to this noise level and
below. We used all BJTs other than the input JFET device in our
charge-sensitive preamps. Don't be surprised by the noise, if the LED is
run at low enough
level, the photon statistics will start to show up easily. In other words,
some of the noise you can observe is REAL optical noise! If you use a
PMT and a fast scintillator, then the individual photon counting will fill
a scope screen with "fuzz". With a PD and charge-sensitive amp the
individual photons will mush together, but the statistical nature will
still be quite evident. The key observation is you will see much more noise
with the low LED illumination than with the LED off. If you get that,
you are ready to see if you get discrete pulses with a scintillator.
There are abundant sources of radiation available. Orange glazed dinner
plates were once sold with Uranium glaze, you get 5000 counts/minute from
a 1" chunk. And, that's just a GLAZE on the surface! Americium sources
in smoke detectors, gas lamp mantles have Thorium in them, even a red brick
is a bit radioactive. Sources are good as you can move them closer and
farther away to prove you are actually detecting something. Cosmic ray
showers will hit a scintillator several times a minute, generally creating
MONSTER pulses of tens or hundreds of particles in a burst. But, you can't
turn these on and off.
Jon
Jon Elson
Apr 25, 2012, 6:26:54 PM4/25/12
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George Herold wrote:
> Then here's a 20ns pulse into the led, (about 20E-15 Joules) (with
> x100 Amp on PD)
>
> http://bayimg.com/caOhDaaDd
>
> That's ~60 -70 k photons.
Well, so you see the kind of problems you are running into.
> Then here's a 20ns pulse into the led, (about 20E-15 Joules) (with
> x100 Amp on PD)
>
> http://bayimg.com/caOhDaaDd
>
> That's ~60 -70 k photons.
The pulse is just a bit bigger than the noise. We usually
get these things so that we have about
2-5 mV noise at the output of the TIA.
I doubt you are going to get there with any off-the-shelf
monolithic op-amp.
Jon
Phil Hobbs
Apr 25, 2012, 6:49:34 PM4/25/12
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from total internal reflection at the scintillator/air boundary, like
more than half and probably nearer 2/3.
Ordinary Fresnel reflection from an air/glass interface is a nit by
comparison. For that job, you have to glue a plastic packaged
photodiode (or even a well-passivated chip) right to the scintillator.
There's no way to reduce the phase space volume in that instance, except
to use a smaller scintillator, closer to the source. That's why people
use PMTs for that job--they're unbeatable for high sensitivity at large
area. (See Super Kamiokande, for example.)
But the OP wasn't living in that special world, so the point is sort of
irrelevant. Lenses are a huge win in almost all instrumental
photodetection situations where they can be applied.
George Herold
Apr 25, 2012, 8:32:58 PM4/25/12
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same, both work great with a collimated beam, but with a rod like
scintillator you'd throw away all the 'glancing' rays.
George H.
George Herold
Apr 25, 2012, 8:44:08 PM4/25/12
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finally getting my head around the issues. I really care about C and
the voltage noise.
I measured about 0.2 mVrms of noise but only 1 Meg ohm Rf
George H.
George Herold
Apr 25, 2012, 8:46:05 PM4/25/12
to
out in the shop.
Better to get a new mantle though, not as fragile.
George H.
Bret Cannon
Apr 26, 2012, 1:49:43 AM4/26/12
to
"George Herold" wrote in message
news:bd181884-8db7-4810...@u7g2000yqc.googlegroups.com...
some modern lantern mantles are doped with cerium rather than thorium and
are not radioactive above background. They tell of taking a Geiger counter
into the store to find the "good" mantels for their radioactivity is
everywhere demonstrations.
Bret Cannon
Jan Panteltje
Apr 26, 2012, 5:09:02 AM4/26/12
to
On a sunny day (Wed, 25 Apr 2012 17:46:05 -0700 (PDT)) it happened George
Herold <ghe...@teachspin.com> wrote in
<bd181884-8db7-4810...@u7g2000yqc.googlegroups.com>:
>Oh I forgot about Thorium in my lamp mantles! My coleman latern is
>out in the shop.
>Better to get a new mantle though, not as fragile.
>
>George H.
I have some Thorium containing welding rods,
much safer than mantles, as they are rock solid, and
legal everywhere too.
Ebay :-)
Herold <ghe...@teachspin.com> wrote in
<bd181884-8db7-4810...@u7g2000yqc.googlegroups.com>:
>Oh I forgot about Thorium in my lamp mantles! My coleman latern is
>out in the shop.
>Better to get a new mantle though, not as fragile.
>
>George H.
much safer than mantles, as they are rock solid, and
legal everywhere too.
Ebay :-)
Jon Elson
Apr 26, 2012, 3:03:24 PM4/26/12
to
George Herold wrote:
>
> From my perspective the problem with lenses or light cones is the
> same, both work great with a collimated beam, but with a rod like
> scintillator you'd throw away all the 'glancing' rays.
Well, a scintillator isn't collimated at all, the photons shoot
>
> From my perspective the problem with lenses or light cones is the
> same, both work great with a collimated beam, but with a rod like
> scintillator you'd throw away all the 'glancing' rays.
out from the interaction in all directions. But, a properly made
light guide should work like an optical fiber, all hits on the
side of the guide should be glancing, and therefor totally reflected
back in. The idea is to try to make sure as few photons hit the walls
of the guide more normal than Brewster's angle. But, that can be tough.
With a thin disc of scintillator, it works better, but with a big
block of scintillator you can shoot more photons more directly at the
walls of the light guide.
Anyway, for charged particle detection, thin scintillators are all
you need, just a couple mm thickness will stop anything that comes out
of a source. The huge blocks of scintillator are only needed for
detecting Gamma rays, which have much deeper penetration.
So, a charged-particle detector can best be built by putting a square
slab of scintillator directly in front of a square PD, with a little
silicone grease as a coupling material. If the energies are high, you
can put white Teflon tape around it, but the downside is the particles
have to punch through the Teflon to reach the scintillator.
For a first test, I'd just put the scintillator directly in front of
the PD, no light guide and no wrapping or paint.
Jon
Jon Elson
Apr 26, 2012, 3:07:22 PM4/26/12
to
George Herold wrote:
>
> Well I did a bit better than that ~ 20k photons. But I think I'm
> finally getting my head around the issues. I really care about C and
> the voltage noise.
>
> I measured about 0.2 mVrms of noise but only 1 Meg ohm Rf
Well, one of the interesting things is to test the amp without
>
> Well I did a bit better than that ~ 20k photons. But I think I'm
> finally getting my head around the issues. I really care about C and
> the voltage noise.
>
> I measured about 0.2 mVrms of noise but only 1 Meg ohm Rf
the detector, then with the detector with no bias and then with
reverse bias on the detector. The lowest noise will be with no
detector (detector capacitance increases noise gain, and also there
is thermal noise generated in the junction.) This is worst
with the detector un-biased, you can watch the noise drop
dramatically with even a few Volts of reverse bias.
Rf really only controls the time it takes to recover to baseline.
Jon
Jon Elson
Apr 26, 2012, 3:08:50 PM4/26/12
to
Bret Cannon wrote:
> I've been told by colleagues who do radioactive detection for a living
> that some modern lantern mantles are doped with cerium rather than thorium
> and
> are not radioactive above background. They tell of taking a Geiger
> counter into the store to find the "good" mantels for their radioactivity
> is everywhere demonstrations.
Right, I think the Thorium ones are not legal for sale anymore, or
> I've been told by colleagues who do radioactive detection for a living
> that some modern lantern mantles are doped with cerium rather than thorium
> and
> are not radioactive above background. They tell of taking a Geiger
> counter into the store to find the "good" mantels for their radioactivity
> is everywhere demonstrations.
are at least being phased out.
Jon
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