You are probably not getting much x-radiation from your Crookes tubes.
X-rays are produced when electrons are accelerated by skimming close to the
nuclei of atoms in the target material. The efficiency of production and
the energy of the X-rays is proportional to the square of the atomic number
Z, the number of protons in the nucleus. That's why targets in high-power
tubes are made of tungsten--it has the best combination of high atomic
number and high melting point. Silicon is the highest Z nuclei of ordinary
glass (14) while the Z of tungsten is 74. The difference of Zs squared is
about 21. The chart in the Wikipedia article gives an idea of the voltage
dependence of X-ray production.
The front glass in a TV CRT is made of highly leaded glass to shield against
X-rays. The electron acceleration voltage is about 20 to 35 kV. That's why
the old tube sets were so heavy.
A vacuum tube largely produces X-rays from elections hitting the anode,
which has a higher Z than the glass. The high voltage rectifier in the old
tube sets was always encased in a metal X-ray shield.
You should be able to make a crude X-ray detector by enclosing a penny and a
piece of film in a light-tight envelope and seeing if there is a shadow of
the penny when the film is developed. Better results would be obtained with
a film badge from a company that does medical or safety radiation work.
A cool thing would be to get a piece of lead glass to put between you and
the tube.
---Carl
-----Original Message-----
From: David Rieben
Sent: Saturday, September 20, 2014 1:48 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Homemade Crookes Tube?
On Saturday, September 20, 2014 3:46 PM, David Rieben <drieben@xxxxxxx>
wrote:
Stefan, Dave(s),
Repeating my previous "disclaimer" - I am no authority on this subject
matter BUT ;^)
I really don't think that just a few thousand volts applied to a hard vacuum
is going to
pose a significant ionizing radiation hazard. Remember the old CRT computer
monitors
or better yet, the 25" or 27" CRT-type color TVs? Those "big screened"
colored TV
picture tubes required 25 to 30 kV to operate properly and I do seem to
recall as a
teenager (1970s) the concern for "some" x-ray exposure if one stood with
their face
pressed against the TV screen "for hours" while it was on ;^) Of course,
thanks to the
inverse-squared law, they were considered completely safe if viewed from a
"proper
distance". ;^)
BTW, Cobalt 60 DOES appear to be somewhat of pro in this matter and he also
seems
to agree with me that such relatively low voltages are NOT going to produce
significant
radiation hazards with hard vacuums.
Yet another David (Rieben)
On Saturday, September 20, 2014 8:35 AM, Dave Halliday <dh@xxxxxxxxxxxxxx>
wrote:
when I was experimenting with large transmitting tubes
(graphite anodes) at
only arround 6kV anode voltage, there was a bright blue
glowing visible
arround the complete inner surface of the glas bulb.
I'm sure, this "fluorescence" was generated by x-rays,
hitting the glas.
Although not sure, which amount of x-rays really was able to
go through the
glas and hitting me, this was the main reason, I immediately
canceled all my
further experiments with such tubes, because I was afraid of
x-rays, even at
such low voltages. So I think the dangerous level is far
below 30kV, as you
stated.
I even have some QB5/1750 Tetrodes, which are only 5kV anode
voltage or so,
and this tubes have "Danger X-Ray radiation" warning's on it.
Regards,
Stefan
Dangerous x-rays require a minimum voltage of
30 kV with some
> tubes operating with as much as 200 kV. Tungsten is the
typical target and
> a very hot high current filament is necessary to produce
powerful 10 keV
> and higher x-rays. The super hot filament is required to
emit enough
> electrons to generate the x-rays. What you have is
> not much different than using a fluorescent lamp to arc to
the secondary
> output.
>
> A Bishop
> Principal Device Physics Tech
>
> Sent from my iPad
>
>> On Sep 18, 2014, at 7:05 PM, David Rieben <drieben@xxxxxxx> wrote:
>>
>> Paul,
>> I am certainly no authority
on this matter but from what I
have gleaned,
>> you are not going to produce "hard" x-rays in this manner.
Besides the
>> hard vacuum, this would also require a white-hot filament (for an
>> electron source) for the cathode, a heavy metal target
(usually tungsten)
>> for the anode, and well over ~50 kVDC potential between
the said cathode
>> and anode to really produce the ionizing, body penetrating
hard x-rays
>> (even dental x-ray tubes typically run about 70 kV(DC)).
That being said,
>> if you are powering your tube with say 10 to 20 kVDC, you
may produce
>> some low energy "soft" x-rays, but I don't think it's
going to be a
>> significantly ionizing radiation source. I'm not even sure
if you can
>> produce any kind
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx