Category: Electronics Workbench

Electrical & Electronic gadgets

Victoreen 710-104 Ionization Chamber: Gamma Rays!

Given this hairball circuit:

Current Amp – Dual Darlington – Schematic

Feeding the output voltage into the ‘scope, with AC coupling to strip off the DC bias, produces this:

Darlington 12k load – multiple

Those cute little spikes seem to be gamma ray ionization events: they are always positive-going, there are no similar negative-going pulses, they occur irregularly at a few per second with occasional clusters, and generally seem about like random radioactive events. The picture shows a particularly busy interval; mostly, nothing happens and the baseline voltage wobbles around in a low frequency rumble.

For what it’s worth, the shielding around the circuit completely eliminates not only 60 Hz interference, but everything else, too: astonishingly good results from a fairly simple layout.

Taking a closer look at one pulse:

Darl 12k – single detail

(Vigorous handwaving begins)

The tallest spikes are typically 20 mV above the baseline, corresponding to peak output current of 20 mV / 12 kΩ = 1.5 µA and a chamber current of 1.5 µA / 100×10⁶ = 15 fA.

They’re generally 5 ms wide, which is orders of magnitude longer than the actual ion generation time, but the area under that spike should be more-or-less proportional to the area under the actual impulse.

If you grant that and agree those pulses look mostly triangular, their integral is:

1/2 x 15 fA x 5 ms = 40 fA·ms = 40 aC

That’s “a” for “atto” =10^-18 = a billionth of a billionth = hardly anything at all.

Indeed, seeing as how one coulomb contains 6.2×10¹⁸ electron charges, that pulse represents 250 ion pairs, at least assuming a zero-current baseline.

Gamma rays arrive with various energies, produce ionization trails of various lengths, and don’t necessarily traverse the entire chamber, so the pulses have various heights & widths; you can see smaller pulses sticking up out of the grass in the first scope shot. Assuming all those average out to five “big” pulses every second, the chamber collector electrode passes 200 aC/s into the transistor base → 200 aA → 0.20 fA. At 1 fA per 100 µR/h, that’s 20 µR/h of gamma background.

Frankly, I don’t believe any of that to within an order of magnitude, but given that a free-air monitor counting alpha + beta + gamma background in NYC seems to be averaging 10-ish µR/h, it’s not entirely out of line.

Working from the other end of the scale, a bit of searching shows that 1 R produces 2.08×10⁹ ion pairs in 1 cm³ of dry air at STP. The ionization chamber dimensions give the can’s volume:

π x 4.5² x 3.5 = 220 cm³

So assuming a somewhat unreasonably large pure-gamma dose of 10 µR/h in that volume will produce:

10x10^-6 x 2.08x10⁹ x 220 = 4600x10³ ion pairs/h = 1300 ion pairs/s

That’s about five “big pulses” per second, under the stack of assumptions thus far, and seems absurdly close.

An old NIST report on Calibration of X-Ray and Gamma-Ray Measuring Instruments says that 1 R/s (that’s per second, not per hour) produces a current of 300 pA/cm³ in an “ideal ionization chamber”. Scaling that down to 10 µR/h and up to the chamber volume gives an average current of 180 aA. That’s absurdly close, too.

Note bene: Because 1 C = 6.241×10¹⁸ ion pairs, 2.08×10⁹ ion pairs is 333×10^-12 C and, if you do that in one second, you get 333 pA of current from your ideal 1 cm³ ionization chamber. Those two approaches should be equally close.

(Vigorous handwaving ends)

Again, I don’t trust any of the values to within an order of magnitude and surely made a major blunder in running some of the numbers, but the results seem encouraging.

The coaxial cable’s capacitance could explain why the pulses look like triangles: the capacitance integrates a rectangular current pulse into a voltage ramp. The cable measures 200 pF and the scope input adds 13 pF, but let’s call it 200 pF across the 12 kΩ emitter resistor. Raising the voltage across that capacitance by 20 mV in 2 ms requires a current of:

200x10^-12 x (20 mV / 2 ms) = 2 nA

Dividing that by 100×10⁶ gives a chamber current pulse of 20×10^-18 = 20 aA: three orders of magnitude less than the original guesstimate. That suggests the (handwaved) 15 fA chamber current, amplified by the absurd gain of two stacked Darlingtons, easily drives the cable capacitance. Something else causes the ramp.

The chamber itself has 10 pF capacitance, but it’s not clear to me how (or if) that enters into the proceedings. The entire collection of ions appears in mid-air, as if by magic, whereupon the +24 V chamber bias voltage draws them (well, the positive ones, anyway) to the transistor base without appreciable voltage change.

Perhaps the triangle represents the actual arrival of the ions: a few at first from the near side of the trail, a big bunch from the main trail, stragglers from the far side, then tapering off back to the baseline.

That’s definitely more than anyone should infer from a glitch produced by a pair of transistors…

2015-07-21
Victoreen 710-104 Ionization Chamber: Circuitry

The Victoreen 710-104 ionization chamber specs say it produces 1 pA in a 100 mR/h gamma environment, which suggests the actual current will be much, much lower in the Basement Laboratory. In fact, I’m hoping to spot individual gamma rays, rather than the overall radiation background current, which will involve counting groups of electrons as they march by.

The simplest possible electrometer amplifier, an MPSA14 NPN Darlington, produced 25 nA of current that, assuming a gain of 10 k, corresponds to an unrealistically high 2.5 pA of chamber current and is, realistically, entirely leakage current.

Adding an MPSA65 PNP Darlington boosts the overall gain to maybe (10 k)² = 100 x 10⁶:

Current Amp – Dual Darlington – Schematic

Granted, there’s not much to like about that circuit (“Any sufficiently sensitive instrument is indistinguishable from a thermometer”) and stuffing that much gain into a pair of inverters is basically crazy talk, but it looks like this:

Electrometer amp – circuitry

The blue trimpot in the foreground drives the base of a duplicate pair of transistors in a misguided attempt to make a differential amp that would balance out some thermal effects. Turned out to be not worth the effort, due to the adjustment’s fiddly nature, but also not worth unsoldering the parts.

The black lump covers two RG-174 coaxial cables that run off to the oscilloscope; they already had BNC connectors on the end and were small enough for the job.

Some DC measurements:

The output idles at 6.5 VDC → 550 μA of Q101 collector current → 6 pA of Q102 base current. Yeah, right.

Grounding the base of Q102 → 5.9 V output → 500 µA → 50 nA leakage into Q102’s collector. Maybe.

Shorting Q101’s base to its emitter produces 350 mV at the output → 30 µA of output current. Huh.

After restoring the status quo ante, the output idled at 10.2 V. See previous comment about thermometers, modulo soldering transistor leads.

So, given the predictably absurd temperature sensitivity of this whole lashup, it’s reasonable to say the entire DC output current comes from leakage, which also agrees with the fact that I’m not dying of gamma exposure. In point of fact, an ancient CDV-715 Radiological Survey Meter with a similar ionization chamber (which, at this late date, passes its “circuit test” function and seems to be perfectly happy) reports exactly zero background on its most sensitive 500 mR/h scale.

Whew!

2015-07-20
LED Sign Display Driver Glitch

Spotted this while walking out for supper after a day at the CNC Workshop:

Motel LED sign glitch – 1

Then it got worse:

Motel LED sign glitch – 2

The lower left block remained fixed throughout the glitzy scrolling / rolling / blinking updates in the sign’s repertoire:

Motel LED sign glitch – 3

The affected block is 64 pixels tall and, at a guess, 128 pixels wide. Looks like a module enable failure…

2015-07-19
Victoreen 710-104 Ionization Chamber: Shield Enclosure

Lining the shield support box with copper foil tape turned out to be surprisingly easy:

Electrometer amp – shield – end view

The flat surface is two overlapping strips of 2 inch wide copper tape. I traced the exterior of the support box on the tape, cut neatly along the lines, slit the corners, bent the edges upward, peeled off the backing paper, stuck the tape into the box, pressed the edges into the corners, and didn’t cut myself once.

Applying 1 inch wide tape to the wall went just as smoothly, after I realized that I should cut it into strips just slightly longer than the hexagon’s sides.

The tape along the rim is adhesive copper mesh that’s springy enough to make contact all around the edge. I cut the 1 inch wide tape in half, which was just barely wide enough to reach::

Electrometer amp – shield – mesh soldering

Although you’re supposed to join the entire length of each seam for best RF-proofing, I tacked the corners and the middle of the long edge, then hoped for the best. The copper mesh seems to be plated on plastic threads that requires a fast hand to solder without melting, but I’m getting better at it. The adhesive is said to be conductive, but I loves me some good solder blob action.

The resistance from the flat bottom to the side panels and the fabric on the edge started out at a few ohms before soldering and dropped to 0.0 Ω after soldering, so I’ll call it a success. Didn’t even melt the outside of the PETG box, but I admit I didn’t take it apart to see what the copper-to-PETG surface looks like.

Covering the foil on the sides with 1 inch Kapton tape completed the decoration. I didn’t bother to cover the flat surface, because none of the circuitry should reach that far, and didn’t worry about covering the fabric tape for similar reasons. As madbodger pointed out, this violates the no-plastic-on-the-inside rule, but I’m still hoping for better results than having the entire plastic structure with all its charges on the inside.

A strip of horribly clashing orange plastic tape (which might be splicing tape for reel-to-reel recording tape) covers the outside edges of the fabric, prevents fraying, and gives the black electrical tape that holds the box down a solid grip:

Electrometer amp – shield – exterior

Yeah, like you’d notice mismatched colors around here.

Using black tape as an anchor seemed easier and better than messing with nesting pins & sockets. The copper fabric tape makes good contact with the rim of the PCB all the way around the perimeter and the black tape holds it firmly in place.

Early reports suggest the shield works pretty well…

2015-07-16

Victoreen 710-104 Ionization Chamber: Shield Support

Although I’d thought of a Mu-metal shield, copper foil tape should be easier and safer to shape into a simple shield. The general idea is to line the interior with copper tape, solder the joints together, cover with Kapton tape to reduce the likelihood of shorts, then stick it in place with some connector pin-and-socket combinations. Putting the tape on the outside would be much easier, but that would surround the circuitry with a layer of plastic that probably carries enough charge to throw things off.

Anyhow, the hexagonal circuit board model now sports a hexagonal cap to support the shield:

Victoreen 710-104 Ionization Chamber Fittings - Show with shield — Victoreen 710-104 Ionization Chamber Fittings – Show with shield

The ad-hoc openings fit various switches, wires, & twiddlepots:

Victoreen 710-104 Ionization Chamber Fittings - Shield — Victoreen 710-104 Ionization Chamber Fittings – Shield

Ya gotta start somewhere.

The OpenSCAD source code:

// Victoreen 710-104 Ionization Chamber Fittings
// Ed Nisley KE4ZNU July 2015

Layout = "Show";
					// Show - assembled parts
					// Build - print can parts + shield
					// BuildShield - print just the shield
					// CanCap - PCB insulator for 6-32 mounting studs
					// CanBase - surrounding foot for ionization chamber
					// CanLid - generic surround for either end of chamber
					// PCB - template for cutting PCB sheet
					// PCBBase - holder for PCB atop CanCap
					// Shield - electrostatic shield shell

//- Extrusion parameters must match reality!
//  Print with 2 shells and 3 solid layers

ThreadThick = 0.25;
ThreadWidth = 0.40;

HoleWindage = 0.2;

Protrusion = 0.1;			// make holes end cleanly

AlignPinOD = 1.75;			// assembly alignment pins = filament dia

inch = 25.4;

function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

//- Screw sizes

Tap4_40 = 0.089 * inch;
Clear4_40 = 0.110 * inch;
Head4_40 = 0.211 * inch;
Head4_40Thick = 0.065 * inch;
Nut4_40Dia = 0.228 * inch;
Nut4_40Thick = 0.086 * inch;
Washer4_40OD = 0.270 * inch;
Washer4_40ID = 0.123 * inch;

//----------------------
// Dimensions

OD = 0;											// name the subscripts
LENGTH = 1;

Chamber = [91.0 + HoleWindage,38];				// Victoreen ionization chamber dimensions

Stud = [										// stud welded to ionization chamber lid
	[6.5,IntegerMultiple(0.8,ThreadThick)],		// flat head -- generous clearance
	[4.0,9.5],									// 6-32 screw -- ditto
];
NumStuds = 3;
StudSides = 6;									// for hole around stud

BCD = 2.75 * inch;								// mounting stud bolt circle diameter

PlateThick = 3.0;								// layer atop and below chamber ends
RimHeight = 4.0;								// extending up along chamber perimeter
WallHeight = RimHeight + PlateThick;
WallThick = 5.0;								// thick enough to be sturdy & printable
CapSides = 8*6;									// must be multiple of 4 & 3 to make symmetries work out right

PCBFlatsOD = 85.0;								// hex dia across flats + clearance
PCBClearance = ThreadWidth;						// clearance on each flat
PCBThick = 1.1;
PCBActual = [PCBFlatsOD/cos(30),PCBThick];
PCBCutter = [(PCBFlatsOD + 2*PCBClearance)/cos(30),PCBThick - ThreadThick];		// OD = tip-to-tip dia with clearance

echo(str("Actual PCB across flats: ",PCBFlatsOD));
echo(str(" ... tip-to-tip dia: ",PCBActual[OD]));
echo(str(" ... thickness: ",PCBActual[LENGTH]));

HolderHeight = 11.0 + PCBCutter[LENGTH];		// thick enough for PCB to clear studs
HolderShelf = 2.0;								// shelf under PCB edge
PinAngle = 15;									// alignment pin angle on either side of holder screw

echo(str("PCB holder across flats: ",PCBCutter[OD]*cos(30)));
echo(str(" ... height: ",HolderHeight));

ShieldInset = 1.0;								// shield inset from actual PCB flat
ShieldWall = 2.0;								// wall thickness
Shield = [(PCBFlatsOD - 2*ShieldInset)/ cos(30),35.0];		// electrostatic shield shell shape

//----------------------
// Useful routines

module PolyCyl(Dia,Height,ForceSides=0) {			// based on nophead's polyholes

  Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);

  FixDia = Dia / cos(180/Sides);

  cylinder(r=(FixDia + HoleWindage)/2,
           h=Height,
	   $fn=Sides);
}

//- Locating pin hole with glue recess
//  Default length is two pin diameters on each side of the split

module LocatingPin(Dia=AlignPinOD,Len=0.0) {

	PinLen = (Len != 0.0) ? Len : (4*Dia);

	translate([0,0,-ThreadThick])
		PolyCyl((Dia + 2*ThreadWidth),2*ThreadThick,4);

	translate([0,0,-2*ThreadThick])
		PolyCyl((Dia + 1*ThreadWidth),4*ThreadThick,4);

	translate([0,0,-Len/2])
		PolyCyl(Dia,Len,4);

}

module ShowPegGrid(Space = 10.0,Size = 1.0) {

  RangeX = floor(100 / Space);
  RangeY = floor(125 / Space);

	for (x=[-RangeX:RangeX])
	  for (y=[-RangeY:RangeY])
		translate([x*Space,y*Space,Size/2])
		  %cube(Size,center=true);
}

//-----

module CanLid() {

	difference() {
		cylinder(d=Chamber[OD] + 2*WallThick,h=WallHeight,$fn=CapSides);
		translate([0,0,PlateThick])
			PolyCyl(Chamber[OD],Chamber[1],CapSides);
	}

}

module CanCap() {

	difference() {
		CanLid();

		translate([0,0,-Protrusion])											// central cutout
			rotate(180/6)
				cylinder(d=BCD,h=Chamber[LENGTH],$fn=6);						//  ... reasonable size

		for (i=[0:(NumStuds - 1)])												// stud clearance holes
			rotate(i*360/NumStuds)
				translate([BCD/2,0,0])
					rotate(180/StudSides) {
						translate([0,0,(PlateThick - (Stud[0][LENGTH] + 2*ThreadThick))])
							PolyCyl(Stud[0][OD],2*Stud[0][LENGTH],StudSides);
						translate([0,0,-Protrusion])
							PolyCyl(Stud[1][OD],2*Stud[1][LENGTH],StudSides);
					}

		for (i=[0:(NumStuds - 1)], j=[-1,1])									// PCB holder alignment pins
			rotate(i*360/NumStuds + j*PinAngle + 60)
				translate([Chamber[OD]/2,0,0])
					rotate(180/4 - j*PinAngle)
						LocatingPin(Len=2*PlateThick - 2*ThreadThick);
	}

}

module CanBase() {

	difference() {
		CanLid();
		translate([0,0,-Protrusion])
			PolyCyl(Chamber[OD] - 2*5.0,Chamber[1],CapSides);
	}
}

module PCBTemplate() {

	difference() {
		cylinder(d=PCBActual[OD],h=max(PCBActual[LENGTH],3.0),$fn=6);		// actual PCB size, overly thick
		translate([0,0,-Protrusion])
			cylinder(d=10,h=10*PCBActual[LENGTH],$fn=12);
	}
}

module PCBBase() {

	difference() {
		cylinder(d=Chamber[OD] + 2*WallThick,h=HolderHeight,$fn=CapSides);		// outer rim

		rotate(30) {
			translate([0,0,-Protrusion])										// central hex
				cylinder(d=(PCBActual[OD] - HolderShelf/cos(30)),h=2*HolderHeight,$fn=6);

			translate([0,0,HolderHeight - PCBCutter[LENGTH]])					// hex PCB recess
				cylinder(d=PCBCutter[OD],h=HolderHeight,$fn=6);

			for (i=[0:NumStuds - 1])											// PCB retaining screws
				rotate(i*120 + 30)
					translate([(PCBCutter[OD]*cos(30)/2 + Clear4_40/2 + ThreadWidth),0,-Protrusion])
						rotate(180/6)
							PolyCyl(Tap4_40,2*HolderHeight,6);

			for (i=[0:(NumStuds - 1)], j=[-1,1])								// PCB holder alignment pins
				rotate(i*360/NumStuds + j*PinAngle + 30)
					translate([Chamber[OD]/2,0,0])
						rotate(180/4 - j*PinAngle)
							LocatingPin(Len=PlateThick);
		}

		for (i=[0:NumStuds - 1])												// segment isolation
			rotate(i*120 - 30)
				translate([0,0,-Protrusion]) {
					linear_extrude(height=2*HolderHeight)
						polygon([[0,0],[Chamber[OD],0],[Chamber[OD]*cos(60),Chamber[OD]*sin(60)]]);
				}
	}
}

//-- Electrostatic shield
//		the cutouts are completely ad-hoc

module ShieldShell() {

CutHeight = 7.0;

	difference() {
		cylinder(d=Shield[OD],h=Shield[LENGTH],$fn=6);
		translate([0,0,-ShieldWall])
			cylinder(d=(Shield[OD] - 2*ShieldWall/cos(30)),h=Shield[LENGTH],$fn=6);

		translate([Shield[OD]/4 - 20/2,Shield[OD]/2,(CutHeight - Protrusion)/2])
			rotate(90)
				cube([Shield[OD],20,CutHeight + Protrusion],center=true);

		translate([-Shield[OD]/4 + 5/2,Shield[OD]/2,(CutHeight - Protrusion)/2])
			rotate(90)
				cube([Shield[OD],5,CutHeight + Protrusion],center=true);

		translate([-Shield[OD]/2,0,(CutHeight - Protrusion)/2])
				cube([Shield[OD],5,CutHeight + Protrusion],center=true);

	}

}

//----------------------
// Build it

ShowPegGrid();

if (Layout == "CanLid") {
	CanLid();
}

if (Layout == "CanCap") {
	CanCap();
}

if (Layout == "CanBase") {
	CanBase();
}

if (Layout == "PCBBase") {
	PCBBase();
}

if (Layout == "PCB") {
	PCBTemplate();
}

if (Layout == "Shield") {
	ShieldShell();
}

if (Layout == "Show") {
	CanBase();
	color("Orange",0.5)
		translate([0,0,PlateThick + Protrusion])
			cylinder(d=Chamber[OD],h=Chamber[LENGTH],$fn=CapSides);
	translate([0,0,(2*PlateThick + Chamber[LENGTH] + 2*Protrusion)])
		rotate([180,0,0])
			CanCap();
	translate([0,0,(2*PlateThick + Chamber[LENGTH] + 5.0)])
		PCBBase();
	color("Green",0.5)
		translate([0,0,(2*PlateThick + Chamber[LENGTH] + 7.0 + HolderHeight)])
			rotate(30)
				PCBTemplate();
	translate([0,0,(2*PlateThick + Chamber[LENGTH] + 15.0 + HolderHeight)])
		rotate(30)
			ShieldShell();}

if (Layout == "Build") {

	translate([-0.50*Chamber[OD],-0.60*Chamber[OD],0])
		CanCap();

	translate([0.55*Chamber[OD],-0.60*Chamber[OD],0])
		rotate(30)
			translate([0,0,Shield[LENGTH]])
				rotate([0,180,0])
					ShieldShell();

	translate([-0.25*Chamber[OD],0.60*Chamber[OD],0])
		CanBase();
	translate([0.25*Chamber[OD],0.60*Chamber[OD],0])
		PCBBase();
}

if (Layout == "BuildShield") {

	translate([0,0,Shield[LENGTH]])
		rotate([0,180,0])
				ShieldShell();

}

2015-07-13

HP 7475A Plotter: LED Lighting

If white LED strips had existed in the early 1980s, the engineers responsible for the HP 7475A plotter would surely have done this:

HP 7475A Plotter – LED paper illumination

Not, that’s not stretched vertically: I bought a ream of B-size paper (11×17 inches) just for plotter demos.

Although the power supply does have a +12 V output, it comes from a TO220 transistor without a heatsink. The +5 V supply uses a robust TO3 transistor on a huge quad heatsink that can surely dissipate another watt or two without getting any sweatier.

I powered the LEDs from a dirt-cheap boost converter that provides a convenient brightness adjustment; it’s set to 10.5 V and that’s plenty bright enough. The converter attaches to pair of wires soldered across VR1, which is probably a crowbar that blows F3 (not shown) in the event the regulator fails hot:

HP 7475A – LED power tap – schematic

They don’t make power supplies like that any more.

The part locations (“O9” looks like a typo):

HP 7475A – LED power tap

The PCB has holes in exactly the right spot for a zip tie anchoring the wires exiting to the bottom:

HP 7475A Plotter – LED power tap – PCB top

This vertiginous view shows the inside of the case atop the chassis, with the boost converter affixed to the galvanized steel pan with foam tape and the LED wires stuck down with Gorilla Tape:

HP 7475A Plotter – LED strip and boost converter

Red silicone tape around a PCB-mount coax jack rounds out a true hack job.

Although I didn’t bring the plotter to the CNC Workshop, that venue’s dim light reminded me that you can never have enough light when you’re showing off your toys: the LED panels on the M2 and the LED light bars on the Model 158 sewing machine were the brightest spots to be seen.

2015-07-09
Electrometer Amp: Darlington NPN

I soldered up the simplest possible “electrometer amplifier” at Squidwrench, based on Charles Wenzel’s writeup:

Electrometer Amp – MPSA14 NPN Darlington

It’s an MPSA14 NPN Darlington transistor, with the base soldered directly to the Victoreen 710-104 ionization chamber collector pin. The flying leads connect to an ordinary digital voltmeter set to read voltage, rather than current, so that you see the voltage created by the transistor’s collector current through the meter’s input resistance.

The MPSA14 data sheet specifies DC current gain h_FE > 10 k for low collector currents, with a graph suggesting it might be somewhat larger. Alas, all those are for “ordinary” currents, not the countably finite number of electrons coming from an ionization chamber, but let’s assume 10 k is close.

I used a Radio Shack 22-805 DMM, set to auto-ranging DC volts. The specs say the input “impedance” is 10 MΩ for all voltage ranges, so let’s run with that, too.

With 24 V (actually 24.6 V) applied to both the chamber (through the red wire) and the DMM (through the yellow wire), it read 250 mV: a mere 25 nA through the 10 MΩ meter resistance.

Assuming a transistor gain of 10 k, that’s a chamber current of 2.5 pA.

The ionization chamber specs say it produces 5 pA at 0.5 röentgen/hour → 100 mR/h produces 1 pA.

No, I do not believe the Squidwrench Operating Table is bathed in gamma radiation at 250 mR/h.

I should wipe down the transistor to see if that reduces the external leakage, then try a few others, but obviously the signal will remain lost in the noise.

We replaced the DMM with an oscilloscope and 10 MΩ probe, which conclusively demonstrated that unshielded high-impedance circuits make excellent 60 Hz receivers.

2015-07-08