Build Your Own Geiger-Müller Counter

Build your own Geiger-Müller counter Introduction: Geiger Counters and Radiation Detection F r t is p ec y i e i din a e y asic ei e e c ne e c n e is a nda en a ype adia i n de ec ne e s in d ced in 1928 c nsis in a as ed es ndin a i a e ie e e ec ic e d e een e e a and e i e is s c a i ni in adia i n inciden n e e i ca se e as ea d n and a a p se c en e een e i e and e a e e e e e y is s n in Figure 1. e ci c i e c ne y i e i din is desi ned e ai y si p e y i e e ipped nde s and i y e end the course i ea n a adia i n eas e en in 22 09 and y i ea n ea n c ea p ysics and adia i n in 22 02 cathode (thin-wall cylinder) anode (wire) insulating seal fill gas HV supply + – Background Basics of Radiation e e r ec Fas e ec t can e e any a ne ic adia i n ns p si e isi e i in ays a es γ ays ns β pa ic es ea y c a ed pa ic es α pa ic es ssi n a F ee ne ic en s ns n ec ne n c ea en inee in e i p i a i y e c nce ned i adia i n a es s any a a ie y n c ea eac i ns inc din e sp n ane s decay a n c e s e ssi n a n c e s and e si n n c ei n c e s nde es adi ac i e decay in de eac a es a es a e e e cess ene y s a y desi na ed as Q is ca ied a ay y e decay p d c s adia i n ic as s cien ene y p d ce i ni a i n in c n a e ia s is said e adia i n e e es d is is a 10 eV e e 1 eV 1 6 10−19 J is a c n enien ni ene y sed in n c ea en inee in and p ysics e t t a sa p e is e n e decays pe sec nd and is en e p essed in 226 1 Ci 3 7 1010 decays s y e ac i i y 1 a n e n c e s sp i s an a p a pa ic e c nsis in p ns and ne ns i e a e i n ce s ea y c a ed pa ic es s c as a p as e Figure 1: Basic geometry of a Geiger-Müller tube. 241 237 Am Np α Co − ν (b) β− decay of free neutron, Q = 0.78 MeV (a) α decay of 241Am, used in many smoke detectors, Q = 5.6 MeV 60 p n Figure 2: Illustrations of the various decay types. 60 Ni − ν 1.17 MeV γ 1.33 MeV γ (c) β− decay of 60Co (Q = 2.82 MeV) followed by two γ decays of the excited 60Ni. Figure 3: Examples of alphasensitive tubes: (a) is a “pancake” tube with a large-area mica window and an anode made of concentric rings. (b) shows the mica window of an end-window tube. The tip of the anode wire can be seen in the middle of the window. (a) “pancake” tube (photo credit: Wikimedia) © Wikipedia user N. Manytchkine. License CC BYSA. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/. (b) end-window tube (photo credit: ORAU) Courtesy of ORAU Foundation. Used with permission. interact with matter primarily through the electric forces between their positive charge and the negative electrons in the absorbing matter. As a high-energy alpha passes through matter, it slows down through many such interactions with the light electrons. Because any given interaction is very small, the particle trajectory tends to be mostly straight. The range of alpha particles is fairly short – in air, a 2 MeV alpha has a mean range of only about 1 cm! Hence, detectors for alpha radiation must have a very thin wall – often mica with a thickness of around 0.001′′ is used, see Figure 3 for examples. The SBM-20 tubes we will be using have steel walls that are too thick to be used for alpha radiation. There are two forms of beta decay. In β− decay, a neutron in the nucleus essentially splits into a proton, an electron and a neutrino, thereby increasing the atomic number of the decaying nucleus. In β+ decay, a proton in the nucleus essentially splits into a neutron, a positron (anti-electron) and a neutrino, thereby decreasing the atomic number of the nucleus. Electrons also interact primarily with the electrons in the absorbing material. Because the masses are equal in this case, any given interaction can cause a large change in the trajectory – thus electron tracks meander more than those of alpha particles. Fast electrons also lose energy by emitting light in a process known as bremsstrahlung. Electrons typically have much longer ranges than alpha particles, and hence can be seen by the SBM-20 GM tubes we will be using. Following a radioactive decay, a nucleus is often left in an excited state. When this state decays, a very high energy photon is emitted in what is called gamma decay. Photons can lose their energy through several mechanisms: photoelectric NSE: Build your own Geiger-Müller counter 2 absorption by the electrons of the absorber, Compton scattering from the electrons of the absorber and (at high energies) pair production in the electric field of the nuclei in the absorber. Gamma rays also have long ranges and hence can be detected by the SBM-20 GM tubes we will be using. You can tell the difference between beta and gamma radiation based on how easy it is to block with a high-Z absorber such as lead – gamma will penetrate the barrier more readily than beta. Principles of Soldering Safety Note: Solder contains lead – be sure to wash your hands after handling solder, and do not solder near food or drink! Safety Note: Molten solder can be flung towards you if a component slips. Always wear eye protection when soldering! Electronics are assembled using solder, alloys designed to have a low melting point, high electrical conductivity and good mechanical properties. The solder we will be using is 60% tin and 40% lead. The following tips will help you to make a secure joint every time: • Before you apply solder to a joint, make sure the joint is mechanically stable. For example, when soldering to a circuit board, bend the leads outwards once they are through the hole. • Clean the iron’s tip on a moist sponge or brass sponge before and after making each joint. • Put a small amount of solder on the tip of the iron right before touching it to the part – this will help to conduct heat to the part. • Make sure the tip of the iron contacts both parts to be connected – you need them both hot to make a good joint! • Feed the solder into the joint from opposite where the iron touches – if the solder doesn’t melt, then you probably haven’t gotten the joint hot enough. • Don’t overheat parts – a few seconds should do it. If you can’t get the solder to melt as described above within about ten seconds or less, take the iron away, clean the tip and put more solder at the tip of the iron to help conduct heat better. • A good solder joint should be shiny and “wetted” to the surface, not dull and “balled up” on the surface – see Figure 4. beaded up (a) good NSE: Build your own Geiger-Müller counter × Figure 4: Cross-section of good versus bad solder joint. solder is dull (b) bad 3 Resistors Resistors are one of the fundamental building blocks of electronics. As implied by the name, they resist the flow of current. Given a current I , a resistor of value R develops a potential difference V = IR across it (Ohm’s law). The value of a resistor is indicated by the color code shown in Figure 5. 4 7 ×100 ±5% color black brown red orange yellow green blue violet grey white gold silver band 1 0 1 2 3 4 5 6 7 8 9 band 2 0 1 2 3 4 5 6 7 8 9 (band 3) 0 1 2 3 4 5 6 7 8 9 1 0 0 ×10k ±1% = 4.7 kΩ ± 5% multiplier 1Ω 10 Ω 100 Ω 1 kΩ 10 kΩ 100 kΩ 1 MΩ 10 MΩ (100 MΩ) tolerance 0.1 Ω 0.01 Ω ±5% ±10% ±1% ±2% Figure 5: Resistor color code chart. The value of the resistor is indicated by two or three value bands, a multiplier band and a tolerance band. Note that 1% and 2% resistors are ambiguous as brown and red are valid as both tolerance and value bands – always doublecheck the value of precision resistors! ±0.5% ±0.25% ±0.1% ±0.05% = 1.00 MΩ ± 1% Capacitors Capacitors are another fundamental building block of electronics. A capacitor stores charge: given a capacitor of capacitance C charged to a potential V, the charge stored is Q = CV. The value of the capacitors we will be using is indicated by a three-digit number code that works similarly to the resistor color code described above: the first two digits are the first two digits of the value and the third digit is the power of ten multiplier to get the capacitance in picofarads (1 pF = 10−12 F). For example, the code “224” means 22 × 104 pF = 220 nF. Inductors Inductors are the third fundamental building block of electronics. An inductor responds to changes in current: given an inductor of inductance L with time-varying current I flowing through it, the voltage is V = LdI/dt. The value of the inductor we will be using is indicated by a three-digit number code that works very much like the capacitor code described above: the first two digits are the first two digits of the value and the third digit is the power of ten multiplier to get the inductance in microhenries (1 µH = 10−6 H). For example, the code “102” means 10 × 102 µH = 1 mH. NSE: Build your own Geiger-Müller counter 4 Assembling the Geiger-Müller Counter The circuit diagram is shown in Figure 6. The parts needed are shown in Figure 7 and parts lists broken down by subassembly are given at each step. You will be assembling the circuit on a printed circuit board (PCB), the top of which is shown in Figure 8. In the subsequent steps, components involved in the active step will be shown in full color and components that have already been installed but are unimportant for the step at hand will be shown as black silhouettes. Tip: Always disconnect the battery when done testing – you don’t want to wreck your circuit by shorting it out! Tip: When testing, always make sure the board is supported such that there is nothing shorting any connections out on it – this includes your hands as well as any debris on your workbench. The two safest approaches are to grip the edges of the board with the work holder such that the board is held off the workbench or to put standoffs in the mounting holes at the corners of the board. Tip: With components that start out with short leads that you can’t bend to hold the part in place, it sometimes helps to use a small piece of tape to hold the part in place while soldering. Safety Note: When using wire cutters, wire fragments can be flung with enough force to injure your eyes. Keep your eye protection on while trimming leads. Another useful tip is to hold your hand over the area where you are cutting to catch the leads as you trim them. Troubleshooting Tips If a circuit doesn’t work as expected, check the following: 1. Make sure the correct components have been placed in the correct spots. 2. Check the orientation of diodes, transistors and chips with respect to the silkscreen outline. 3. Make sure that adjacent pads didn’t become shorted together (“bridged”) during soldering. 4. Make sure you are making the measurement properly: always hook the ground of your scope and the “COM” lead of your meter to the “GND” pin or one of the grounded mounting holes in the corners of the board. NSE: Build your own Geiger-Müller counter 5 NSE: Build your own Geiger-Müller counter 4 3 2 1 7_RT 4_FB 5_SCP 3_COMP 6_DTC 2_VCC 1_OUT 8_GND - 8 5 6 7 + 3 VI VO 1 1 2 3 4 5 6 2 3 1 4 3 2 1 7_DIS 4_RES 5_CTRL 3_OUT 6_THR 2_TRIG 1_GND 8_VCC 6 5 5 6 7 8 7 9 10 8 13 12 14 Figure 6: Full schematic for the GM counter. 6 VCC GND 4 11 2 1 2 3 Figure 7: Parts of the GM counter. Figure 8: The board has two layers – in (a) the top layer is shown as red and the bottom layer is shown as blue. Notice that most of the bottom layer is a solid area of copper – this is the ground plane, and is what is defined as zero volts for the circuit. (a) top view with traces NSE: Build your own Geiger-Müller counter (b) top view with pads and silkscreen only 7 Step 1: Build the power supply Note: The 78L05 linear regulator must be installed in the correct orientation in this step. Be sure you have it right before soldering it in! Note: Don’t get the 78L05 linear regulator mixed up with the transistors, which are in the same style of package! The power supply provides the 5 V that the rest of the circuit runs on. Parts Used Name Description/Value Diagram 9V battery snap – – IC1 slider switch 78L05 C1, C2 10 µF filtering capacitor 1. Hook up the leads from the battery snap as shown in Figure 9, taking care that the red wire goes on the side marked with a “+.” Be sure to follow Figure 9 closely or the wires to the battery will soon break! 2. Solder the battery wires from the top of the board, trim any excess length. 106 106 flat side faces to right 106 3. Insert the parts as shown in the diagram. Make sure the flat side of the 78L05 linear regulator faces towards the two large capacitors. 4. Flip the board over, grip the edges with the work holder (if using one). 5. Solder all of the wires you just inserted and trim the excess lead length protruding from the board. Figure 9: Thread the battery leads in from the top of the board and up again through the back, leaving a loop to take up strain. (a) top of board NSE: Build your own Geiger-Müller counter (b) bottom of board 8 Step 2: Test the power supply verify 5V here connect “COM” here 1. Support the board such that none of the connections are shorted by hands, lead clippings, etc. 2. Check that the power switch is in the “off ” position (i.e., away from the text “ON” printed on the board). 3. Connect the battery and turn on the power switch. 4. Using the digital multimeter (DMM), check that the voltage at the pad indicated with the red circle in the figure is at about 5 V with respect to any of the grounded mounting holes in the corners. 5. Turn the power switch off. 6. Disconnect the battery. Step 3: Build the inverter Note: There are two parts that must be installed in the correct orientation in this step. Be sure you have them right before soldering them in! The inverter inverts the pulse from the GM tube. 470 flat side faces towards bottom Parts Used Name Description/Value R14 R15 R16 C5 Q3 IO1 5.1 kΩ 1/2 W resistor 10 kΩ resistor 1 kΩ resistor 47 pF capacitor 2N3904 NPN transistor 6-pin header Diagram 1. Insert the parts as shown. Make sure the FLAT side of the transistor faces DOWN. 2. The short end of the 6-pin header goes into the board, with the plastic part on top, see Figure 10. 3. Flip the board over, solder all of the components in and trim the excess leads. Figure 10: Correct header installation: long gold pins and plastic on TOP of board. NSE: Build your own Geiger-Müller counter 9 6-pin output header description The 6-pin header you just installed provides easy access to the important signals in the circuit, summarized in the table below: IO header pin descriptions Label Description HV GM INV PS PWM GND 1000:1 voltage divider on HV power supply GM tube I-to-V output Inverter output Pulse stretcher output Power converter switching waveform Ground Note: Whenever you are using the oscilloscope, always connect the shield (black lead when using a BNC-to-minigrabber cable) of each channel to the “GND” pin. Be careful to not short to adjacent pins! Step 4: Test the inverter use a wire to connect the nodes circled in red 1. Connect channel A of the scope to “GM.” 2. Connect channel B of the scope to “INV.” 3. Don’t forget the ground connections to “GND” for both channels! 4. Check that the power switch is off and that the board is supported away from any materials that might short it out. 5. Connect the battery, turn the power switch on. 6. Use a wire to short the two pads circled in red together, being careful to not touch adjacent pads. 7. On the scope, observe the signal from your wire on channel A and the output from the inverter on channel B, you should see signals like Figure 11. 8. Turn the power switch off and disconnect the battery. hook B to “INV” hook A to “GM” ground both channels to “GND” NSE: Build your own Geiger-Müller counter 10 Figure 11: Waveforms for the inverter test. The simulated GM pulse is channel A (green), the inverter output is channel B (blue). The inverter takes the 5 V pulse signal and converts it to zero. With the wire removed, the traces will be reversed – the inverter will output 5 V on channel B while the pulse on channel A is zero. Figure 12: Chip sockets allow chips to be removed without needing to remove any solder. When installing a chip, it is very important to make sure the notch on the top of the chip is in the right spot and that the chip is fully seated in the socket. (Note that what is shown is a slightly different board than what you are building.) notches aligned with silkscreen outline, chips not installed yet (a) while soldering: sockets only notches aligned with silkscreen outline/notch on socket (b) side of fully-inserted chip NSE: Build your own Geiger-Müller counter (c) for testing: chips inserted 11 Step 5: Build the pulse stretcher Note: There are two components that must be installed in the correct orientation in this step. Be sure you have them right before soldering them in! The pulse stretcher turns the very short pulse from the GM tube into a 1.5 ms pulse. Parts Used Description/Value IC3 R17 R19 C6 C7 LED1 LMC555 timer (and DIP-8 socket) 6.2 kΩ resistor 75 Ω resistor 220 nF capacitor 10 nF capacitor small blue LED notch at top Diagram LMC555 Name longer lead on this side 1. We will be using chip sockets to make it easier to replace chips if something goes wrong: see Figure 12. Do not insert the chips themselves until the last step. 2. Insert the small LED with the longer lead in the hole nearest to where LED2 will go. 3. Insert the parts as shown in the diagram, bending leads as necessary to hold them in place. Make sure to line up the notch at the top of the chip socket with the notch indicated on the silkscreen outline. 4. Flip the board over, solder all of the components in and trim the excess leads. 5. Insert the 555 chip into its socket with the notch as shown on the silkscreen outline. 224 103 NSE: Build your own Geiger-Müller counter 12 Step 6: Test the pulse stretcher use a wire to connect the nodes circled in red 1 C nnec c anne e sc pe 2 C nnec c anne e sc pe 3 n 4 C ec e e c anne s a e p e is any a e ia s a C nnec 6 7 nd c nnec i ns se a e e e a ey ie s ein ca e n and a i s e i a d is a ay ep e s ic n pads ci c ed in ed c ad acen pads e n n e sc pe se e e si na c anne and e p se s e c e y ie n n c anne y c d ap e i e ie y en e p se s e c e s dp a1 s n s a e p se appin is as is a d d hook B to “PS” hook A to “GM” ground both channels to “GND” e ise e s e c e e i e is in c n ac s n e n as en e p se s e c e is n d i p s 8 i ep d see si na s i e Fi e s ic e n as e e 13 and disc nnec e a ey Figure 13: Waveforms for the pulse stretcher test. The simulated GM pulse is channel A (green), the pulse stretcher is channel B (blue). Channel B can be up to 5 V, in which case the two traces would be right on top of each other. NSE: Build your own Geiger-Müller counter 13 Step 7: Build the filter and red LED pump Note: The chip and LED must be installed in the correct orientation in this step. Be sure you have them right before soldering them in! The filter collects the GM tube pulses to control the color of LED2: the higher the count rate, the more red LED2 will get. The red pump drives this proportional current through the red LED. 106 Parts Used Name Description/Value IC4 MCP6004 quad op-amp (and DIP-14 socket) LED2 R20 R21 R22 R23, R24, R25, R26 1 kΩ resistor C8 10 µF capacitor Diagram MCP6004 large red-green LED 100 kΩ resistor 36 kΩ resistor 2.4 kΩ resistor 106 1. Set the MCP6004 chip aside until the last step. 2. Insert the other parts into the board as shown. notch to left flat side to bottom 3. Make sure the notch at the top of the chip socket is oriented as indicated on the silkscreen outline. 4. The FLAT side of the large LED should face towards LED1. (The flat is only present on the thin band at the base of the plastic housing.) 5. Flip the board over, solder all of the components in and trim the excess leads. 6. Insert the MCP6004 chip into its socket with its notch towards the edge of the board. Step 8: Test the red LED pump 1. Check that the power switch is off and that the board is supported away from any materials that might short it out. 2. Connect the battery, turn the power switch on. 3. Repeat the procedure from the previous tests: hold a wire at the right lead of R16 and tap it on the lower, circular pad of the SBM-20. 4. You should see the red LED light up as you hold the wire in place, then gradually fade away when you remove the wire. 5. Turn the power switch off and disconnect the battery. NSE: Build your own Geiger-Müller counter 14 Step 9: Build the green LED difference amplifier and pump The green difference amplifier and pump drive current through the green LED inversely proportional to the count rate. Parts Used Name Description/Value R27, R28, R29, R30, R31, R32, R33, R34 1 kΩ resistor Diagram 1. Insert the parts into the board as shown. 2. Flip the board over, solder all of the components in and trim the excess leads. Step 10: Test the green pump 1. Check that the power switch is off and that the board is supported away from any materials that might short it out. 2. Connect the battery, turn the power switch on. 3. Repeat the procedure from the previous tests: hold a wire at the right lead of R16 and tap it on the lower, circular pad of the SBM-20. 4. The LED should go from green to red as you hold the wire in place, then fade back to green when you remove the wire. 5. Turn the power switch off and disconnect the battery. NSE: Build your own Geiger-Müller counter 15 Step 11: Build the piezo driver Note: The transistor and piezo must be installed in the correct orientation in this step. Be sure you have them right before soldering them in! The piezo driver uses the full battery voltage to produce more volume from the high-impedance piezoelectric speaker. flat side faces to right plus sign to right Parts Used Name Description/Value Diagram + – R18 Q4 + piezo speaker 1 kΩ resistor 2N3904 NPN transistor 1. Insert the components as shown, making sure to put the transistor in with the flat side facing TOWARDS the piezo as indicated on the silkscreen outline. 2. Also make sure the plus sign on the piezo is in the hole closest to the edge of the board. 3. Flip the board over, solder all of the components in and trim the leads. Step 12: Test the speaker 1. Check that the power switch is off and that the board is supported away from any materials that might short it out. 2. Connect the battery, turn the power switch on. 3. Repeat the procedure from the previous two tests: hold a wire at the right lead of R16 and tap it on the lower, circular pad of the SBM-20. 4. If you tap briefly enough you should hear a sharp click sound. The slower you are, the more of a tone you will hear. If you hold the wire in place, the tone will be continuous. 5. Turn the power switch off and disconnect the battery. NSE: Build your own Geiger-Müller counter 16 Step 13: Build the HV divider/bleeder Note: The 3-pin header must be installed in the correct orientation in this step. Be sure you have it right before soldering it in! The resistors installed in this step serve both to help drain the high voltage power supply’s capacitor after the device is turned off and to form a 1000:1 voltage divider that helps you to safely measure the output of the power supply with conventional test equipment. Because of the high impedances involved, it is necessary to use different resistors for different input impedances of test equipment. The two possible states of jumper JP1 select the type of equipment used: 10 MΩ for a DMM, 1 MΩ for a standard oscilloscope. Parts Used Name Description/Value R9, R10 R11 R12 1 GΩ resistor 1 MΩ resistor 1.1 MΩ resistor JP1 3-pin header Diagram 2-pin jumper 1. Set the 2-pin jumper aside until the last step. 2. The short end of the 3-pin header goes into the board, with the plastic part on top, just like the 6-pin header. 3. Insert the other parts as shown. 4. Flip the board over and grip it with the work holder, solder all of the components in and trim the excess leads. 5. Install the 2-pin jumper in the “10M” position. NSE: Build your own Geiger-Müller counter 17 Step 14: Build the boost converter Safety Note: From this step on, there will be parts of the board at 400 V when turned on. The available current is small, but is still enough to give you a nasty shock (and probably destroy all of your chips in the process). Be very careful to not touch the board whenever the battery is connected! Note: There are five components that must be installed in the correct orientation in this step. Be sure you have them right before soldering them in! The boost converter steps the 9 V battery voltage up to the 400 V needed for the GM tube. The TL5001 is a general-purpose power converter controller that provides active control of the output voltage. 105 Parts Used 153 102 BA brass dot to left flat sides to left 472 notch to left silver band to right Name Description/Value IC2 TL5001 chip (and DIP-8 socket) D2 R1, R2 R3 R4 BA159 diode 10 kΩ resistor 10 MΩ resistor 22 kΩ resistor R5 R6 R7 R8 C3 C4 CHV Q1, Q2 20 kΩ potentiometer 4.7 kΩ resistor 4.7 MΩ resistor 6.8 MΩ resistor 15 nF capacitor 1 µF capacitor 4.7 nF HV capacitor FQN1N50C MOSFET L1 1 mH inductor Diagram 102 1. Set the TL5001 chip aside until the last step. 2. Insert the other parts into the board as shown. 3. Make sure the silver band on the diode is towards the HV capacitor as indicated on the silkscreen outline/in the diagram. 4. Make sure the notch at the top of the chip socket is oriented as indicated on the silkscreen outline. 5. Make sure the brass screw on top of the potentiometer ends up away from the transistors as shown on the silkscreen outline/in the diagram. 6. Make sure the flat sides of the transistors face AWAY from the inductor. 7. Flip the board over, solder all of the components in and trim the excess leads. 8. Insert the TL5001 chip into its socket with the notch to the left. NSE: Build your own Geiger-Müller counter 18 Step 15: Note locations of high voltage Step 16: Tune the boost converter 1 C ec a e p e s i c is and a e a d is s pp ed a ay any a e ia s a i s i 2 C nnec y s C 3 a es e e e ead a e V pin C nnec pin e pe is se a 10 4 Make sure no one is touching the board and that there are no debris that could create a short in contact with the board. C nnec 6 make sure jumper is at “10M” measure HV supply on “HV” be sure to connect meter “COM” to “GND” 7 8 NSE: Build your own Geiger-Müller counter e a ey n ep e s ic n e p as ic ad s in n e ass p en i e e n c c ise n i e e e eads ess an 300 V Be careful to not touch the high voltage portions of the board while doing this! sin en n e n e eads 400 V esp nds 400 V n ep e s ic c n e c c ise n i e e e di ide is 1000 1 s is c p e V s pp y and disc nnec e a ey 19 Step 17: Install the GM tube resistor and clips Parts Used Name Description/Value Diagram – R13 fuse clips for GM tube 5.1 MΩ (×2) 1. Solder wires to the fuse clips: (a) Cut sections of red and black wire each about 6” (15 cm) long and strip about 1/8” (3 mm) of insulation from each end of these wires. (b) Use pliers to form one end of each wire into a “J” shape. (c) Hook the “J” into the hole in the tab on the clip, then crush the hook so it will stay still when soldering. Refer to Figure 14. (d) Solder the wires to the solder tab. 2. At this point, you may wish to use the GM tube and case as a guide to trim your wires to a more appropriate length – but don’t cut too much off! 3. Insert the loose end of the red wire into the hole marked with a “+,” insert the black wire into the other hole in the block labelled “SBM-20.” 4. Insert the 5.1 MΩ resistor as shown. Figure 14: Wire ready to be soldered to clip. 5. Flip the board over, solder all of the components in and trim the excess leads. Step 18: Install into the case 1. Make sure the battery is disconnected, the power switch is off and that you have removed all debris from the board. battery clip: #2-56 flat head screws into hex nuts circuit board: #4-40 self-tapping pan head screws GM tube: #4-40 flat head to attach hex standoffs to case, plastic #4-40 pan head to attach clips to standoffs 2. Use #2-56 × 3/16” screws with nuts inside the case to secure the battery clip, then put a piece of electrical tape over the nuts to protect the battery. 3. Use #4-40 × 3/16” self-threading screws to attach the board to the standoffs with the board oriented so the battery snap is close to the clip you just installed. 4. Use #4-40 × 1/4” screws to attach the two hexagonal standoffs inside the case. 5. Screw the fuse clips to the standoffs using plastic #4-40 × 3/16” screws. You might need to bend the clip out a bit using pliers, just don’t bend it too far! 6. Bend the tabs on the fuse clips so they are nearly vertical. 7. Insert the GM tube into the clips with the end marked with “+” at the clip with the red wire. Be gentle when pushing down on the tube – it is very fragile! Push down on the plastic right next to the clips to avoid crushing the tube. NSE: Build your own Geiger-Müller counter 20 Step 19: Test the GM tube 1. Mak ur th ow r switch s off h s f ll n on the bo r . l A of our co to th “GM” in. 3. Hook h l B of our co to th “PS” in. t th b tter , tur th ower sw t h on. o 5. At ty l b kgrou th fir t co 6. Obs rv the g Figur 15. l v l , you houl he r or less. l on the o 7. Tur th ow r switch off. You r fi l mbly. to bri 2. Hook h 4. hoo that no lick om r to ow r y for hoo to gro nd both channels to Figure 15: a eforms for the com lete co nter hannel green is the act al lse from the t be channel bl e is the lse stretcher o can ad st the ime i nob to get a closer loo at the lse from the t be NSE: Build your own Geiger-Müller counter Step 20: Final assembly 1. Make sure the power switch is off. 2. Insert the battery into the clip and connect it. 3. Make sure the battery is not touching the clip holding the GM tube! 4. If the posts in the top run into the tabs on the clips holding the GM tube, remove the GM tube to avoid damaging it, then bend the tabs to be closer to vertical. Once the top fits, carefully put the tube back in, following the same precautions as before. 5. Put the top on with the rectangular notch over the power switch and secure it with the four self-threading screws. Figure 16: Completed GM counter. This project was designed by Mark Chilenski (graduate student, Course 22). Numerous productive conversations were had with Prof. Anne White, Prof. Mike Short, Matthew D’Asaro (graduate student, Course 6) and John Walk (graduate student, Course 22) in the design of this project. NSE: Build your own Geiger-Müller counter 22