Geiger Counter with Adjustable High Voltage Power Supply

Circuits from the Lab? reference designs are engineered and tested for quick and easy system integration to help solve today's analog, mixed-signal, and RF design challenges. For more information and/or support, visit 0536.

Circuit Note

CN-0536

Devices Connected/Referenced LTC6906 Micropower, 10 kHz to 1 MHz Resistor Set

Oscillator LTC6994 TimerBlox: Delay Block/Debouncer

LTC1540 Nanopower Comparator with Reference LTC1441 Ultralow Power Single/Dual Comparator

Geiger Counter with Adjustable High Voltage Power Supply

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards CN-0536 Circuit Evaluation Board (EVAL-CN0536-ARDZ) Ultralow power, Cortex-M3 Arduino form factor compatible development board (EVAL-ADICUP3029)

Design and Integration Files Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

Radiation monitoring is an essential part of ensuring health and safety in and around nuclear energy production facilities, marine propulsion applications, contaminated environments, medical facilities, and other industrial settings where radioactive material may be present.

There are various passive and active methods of measuring instantaneous radiation intensity and total radiation dose. The simplicity, low cost, and reliability of the Geiger-Mueller counter make it a compelling choice as a primary radiation measurement device, or as a secondary measurement in conjunction with other methods.

The circuit shown in Figure 1 is a low power Geiger counter radiation detector in an Arduino shield form factor compatible with 3 V and 5 V platform boards. This circuit features an on-

board miniature Geiger-Mueller tube, with a bias power supply that is adjustable from 280 V to 500 V, allowing the circuit to be tuned for maximum sensitivity, or to be adapted to other Geiger-Mueller tubes.

Audible and light emitting diode (LED) event indicators provide a qualitative measure of radiation intensity, and the conditioned event pulse is routed to an Arduino interrupt pin for quantitative measurement and long-term datalogging.

The solution is adaptable to a local physical display (liquid crystal display (LCD), organic LED (OLED), and so on) or wired data connection using the universal asynchronous receiver and transceiver (UART) interface of the platform board. Wireless connectivity via Bluetooth or WiFi provides electrical isolation and simplifies remote monitoring and applications that aggregate data from multiple sensors.

The high voltage power supply is a unique architecture centered around a micropower comparator with built in reference. Hysteretic regulation reduces the quiescent current consumption to microamps, and a second power-good comparator detects fault conditions, making this circuit ideal for battery-powered, longterm monitoring applications.

Rev. 0

Circuits from the Lab? reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)

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Tel: 781.329.4700



Fax: 781.461.3113

?2021 Analog Devices, Inc. All rights reserved.

CN-0536

Circuit Note

GEIGER COUNTER CIRCUIT BLOCKS

IO_VREF

GEIGER-M?LLER TUBE ~65V TO 78V

HIGH VOLTAGE

C28 250pF

25V

R21 200k

+5V

INDUCTOR

Cn

?8 VOLTAGE MULTIPLIER

R19

C26

7.5M 10pF

630V Cn

+5V

IO_VREF

V+

SET

LTC6906

250kHz OSCILLATOR

250kHz

BOOST ENABLE

287k

IO_VREF

? LTC1540 GEIGER_DETECTB

+

LTC1540

VREF 1.182V

IO_VREF

V+ R27 2M 1.98V

R28 3M

? DETECT_OUTPUT

+ LTC1441B

1.025V

+ LTC1441A

? IO_VREF

HIGHV_PG

4-RADIATION LEVEL LED INDICATOR

GEIGER_DETECTB LTC6994

TIMERBLOX DELAY BLOCK

BUZZER

POWER LED

HIGHV_PG

MICROCONTROLLER

EVAL-ADICUP3029

IO_VREF

IO08 IO33

+5V

IO13

IO15

IO27

GPIO35 GPIO36 GPIO37 GPIO38

22186-001

Figure 1. EVAL-CN0536-ARDZ Block Diagram

CIRCUIT DESCRIPTION

Geiger-Mueller Tube Principles of Operation

The Geiger-Muller tube is an ionizing radiation detector that is sensitive to alpha, beta, gamma, and x-ray radiation. While it cannot discriminate between radiation types, its sensitivity, simplicity, and reliability make it a practical choice in a wide range of radiation monitoring applications, either alone or in conjunction with other sensors.

Geiger-Muller tubes are commonly biased to a voltage ranging from 250 V to 500 V with a near zero current drawn when no radiation events are taking place. Several typical Geiger-Muller tubes are shown in Figure 2.

When an ionized particle enters the tube, ion pairs consisting of a free electron and a positively charged gas molecule are created. Electrons accelerate toward the anode, creating additional ion pairs, causing an avalanche of additional electrons. Electrically, this effect produces a momentary low impedance path between the anode and the cathode that can be detected as a voltage pulse across an external resistance. The external resistance is also necessary to allow the anode to the cathode voltage to drop to a point where the avalanche stops (known as quenching).

The output signal can be measured at either the cathode (see Figure 3) or anode (see Figure 4).

IONIZATION RADIATION

+300V DC TO +500V DC

7.5M

PARTICLE PATH POSITIVE ELECTRODE

METAL CASING (NEGATIVE ELECTRODE)

ANODE TERMINAL

CATHODE TERMINAL

IO_VREF (+3.3V)

GEIGER PULSE

SIGNAL TO GEIGER PULSE COUNTER

+3.3V

0V

24k

Figure 3. Geiger-Mueller Tube with Pulse Signal at Cathode

22186-002 22186-003

Figure 2. Geiger-Muller Sensor Tubes

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Circuit Note

While taking the signal from the cathode avoids the need for a high voltage, level shifting capacitor, it is more sensitive to electrical noise because the cathode impedance to ground is relatively high. The CN-0536 takes the signal from the anode (see Figure 4) with the cathode grounded to minimize susceptibility to noise.

IONIZATION RADIATION

+300V DC TO +500V DC

IO_VREF (+3.3V)

PARTICLE PATH POSITIVE ELECTRODE

METAL CASING (NEGATIVE ELECTRODE)

7.5M

ANODE TERMINAL

CATHODE TERMINAL

SIGNAL TO GEIGER PULSE COUNTER

+3.3V 0V

GEIGER PULSE

Figure 4. Geiger-Mueller Tube with Pulse Signal at Anode

Geiger Pulse Detection

Figure 5 shows the LTspice? schematic of the Geiger-Mueller pulse level shifting and detection circuitry.

AVALANCHE REACTION SIMULATOR

mySW

+ ? S1

V3

PULSE (0 2 0 0.5m 0.5m 0.5 1 20) Rser = 0

R19 7.5M

V2

350

.model mySW SW(Ron = 1 Roff = 1MEG Vt = 0.5 Vh = ?0.4) G.M TUBE SENSOR GM_tube_anode

GM_tube_cathode

IO_VREF

IO_VREF

C26 10pF

C28

R21

250pF 200k

R27 2M

D11 BAS21HM

V1

1.98V

LTC1441

?

GEIGER_DETECT

DETECT_OUTPUT_TO_MCU

3.3

+

U1

R28 3M

D12 BAS21HM

.tran 5s

Figure 5. LTspice Schematic of the Geiger Pulse Shifting and Detection Circuitry

22186-005

22186-004 22186-006

CN-0536

When no radiation events are occurring, the anode is biased at 400 V. C28 and C26 form a capacitive voltage divider, whose output is biased at IO_VREF through R21. When an event occurs, the anode is momentarily pulled toward ground potential, producing a 400 V negative going step. When the output of the attenuator drops to less than the 1.98 V threshold set by R27 and R28, the output of the LTC1441 comparator drives high. The comparator output is routed to an interrupt pin on the platform board, such that software can respond to the event. Figure 6 shows the attenuator and comparator outputs for a typical radiation event.

Figure 6. Geiger-Mueller Output Pulse Detection and LTC1441 Output Pulse

High Voltage Supply The Geiger-Mueller bias is provided by the micropower, high voltage power supply shown in Figure 7. The circuit is a fixed frequency, fixed 50% duty cycle, nonsynchronous boost converter followed by a Cockroft-Walton multiplier, with hysteretic voltage mode control. An LTC6906 resistor-programmable oscillator is configured to produce a 250 kHz, 50% duty cycle square wave. An AND gate buffers this clock signal and functions as a gate driver for the boost power metal-oxide semiconductor field effect transistor (MOSFET). Because the LTC6906 draws its full 12.5 A supply current whenever power is applied, the converter is enabled and disabled by simply switching the V+ supply of the LTC6906 on and off. The AND gate ensures that the switching FET is either off or fully enhanced, even as the supply voltage of the LTC6906 is ramping up or down. Voltage feedback is taken from the output of the first multiplier stage, which greatly reduces power dissipation in the feedback divider at the expense of a slight reduction in load regulation. Figure 7 is the LTspice schematic simulation of the high voltage power supply that is included in the design support package.

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CN-0536

Circuit Note

IO_VREF V2 3.3

C1 1?F

SYS_5V V1

C2 10?F 5

U3 V+

C3 10?F

L1 47?H

SYS_5V Q2

IRFL4310

C4 1?F

U1 OUT

SW_NODE

C6

0.1?F D2

D1 BAS21VM

C8 1 0.1?F D3

C11 2 0.1?F D4

C14 3 0.1?F D5

C16 4 0.1?F D6

C19 5 0.1?F D7

C23 6 0.1?F D8

C25 7 0.1?F D9

HIGH_VOLTAGE

IO_VREF

8

R21 200k

C26 10pF

C28 250pF

BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM BAS21VM

C5 0.1?F

R3 30M

C7 0.1?F

C10 0.1?F

C12 0.1?F

C15 0.1?F

C17 0.1?F

C21 0.1?F

C24 0.1?F

C27 0.1?F

R19 7.5M

GEIGER_DETECTB

GND

GRD

DIV

SET

LTC6906

R1 287k

R4 30M

R5 30M

R20 7.5M

.tran 3600s

R2 221

R7 3M

SYS_5V

1

R = 2M PERCENT_SWIPE = 99 3

R9 2

R8 2.7M

U2 POT_METER

? VREF = 1.182V +

LTC1540

C9 47pF

G

R12 127k

IO_VREF

R13 453k

C13 47pF

C22 1?F

POT_METER

LTC1441

?

+

HIGHV_PG

BOOST ENABLE

Figure 7. High Voltage Power Supply LTspice Simulation

22186-007

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Circuit Note

High Voltage Generation (280 V to 500 V)

When power is first applied, the feedback voltage is at ground potential, driving the output of U1 high, which in turn powers up the LTC6906, and the boost stage begins switching at 350 kHz, 50% duty cycle. L1, Q2, D1, and C5 form a conventional nonsynchronous boost converter, and once the voltage at C5 rises to 64 V, U1 trips off. The output voltage slowly decays to 55 V, determined by the hysteresis of U2, and the cycle begins again. The initial turn-on transients are shown in Figure 8 and Figure 9.

CN-0536

Figure 10. Hysteretic Regulation Measured at C11 at a Time Scale of 20 ms

22186-010

22186-008

22186-011

Figure 8. Turn-On Transient at a Time Scale of 2 ms

Figure 11. Hysteretic Regulation Measured at C11 at a Time Scale of 200 ms

22186-009

22186-012

Figure 9. Turn-On Transient at a Time Scale of 100 ms

The output voltage is set to 400 V nominal and maintained between 390 V and 410 V. The duty cycle is low, approximately 0.1%, with a 200 ?s burst every 600 ms. This results in an average current consumption of 33 ?A. Probing the output of the multiplier is challenging because even the 10 M impedance of a typical scope probe can affect the operation of the circuit. However, probing earlier stages illustrates the principle. Figure 10 and Figure 11 show the voltage at C11 at the output of the second multiplier stage, while Figure 12 and Figure 13 show the pulse switching of the 100 V, N-channel, enhancement mode MOSFET Q2 driven to its gate terminal by the 350 kHz switching frequency using a combination of an AND gate and the LTC6906 frequency clock generator.

Figure 12. Boost Pulses at MOSFET Q2 Drain Terminal at a Time Scale of 200 ?s Figure 13. Boost Pulses at MOSFET Q2 Drain Terminal at a Time Scale of 5 ?s

22186-013

Rev. 0 | Page 5 of 8

CN-0536

Geiger Pulse Visual/Audible Indicators

The 18 ?s output pulse from the Geiger-Mueller event detector circuit is too short to reliably generate a visual indication (LED) or audible click from a buzzer. An LTC6994 programmable delay block/debouncer is configured with a master timebase (tMASTER) of 2.5 ?s, N divider value of 512, and a delay falling edge (POL) = 1. This results in a 1.28 ms pulse to an LED indicator and buzzer, producing an easily visible flash and audible click. Figure 14 shows the detailed schematic of the timer block delay.

IO_VREF

TLMS1000GS08 DS1 D10 BAS16HT1G

22186-014

DETECT_OUTPUT DGND DGND

LTC6994

C29

14TRMPBF

1?F

1 IN

U4 OUT

6

DGND

2 GND 3

SET

V+ 5 4

DIV

R15

R22 330 A

R6

127k A

C

127k

R14 453k

C

P + M1 N

A1-1223-TWT-5V-5-R

DGND

DGND

DGND

Figure 14. Timer Block LTC6994 Click Sound Generator

tDELAY = (NDIV ? RSET)/50 k ? 1 ?s

where: NDIV = 1, 8, 64, ..., 221. RSET is R6 (see Figure 14).

High Voltage Power Good

The second half of the LTC1441 comparator functions as a high voltage power-good indicator (see Figure 15).

The circuit uses the REF output (VREF) of the LTC1540 as a reference, which is the same reference that is used to set the regulation voltage. If the potentiometer wiper falls to less than the reference voltage, the comparator output asserts, indicating a loss of regulation.

R20 7.5M

R7 3M

R = 2M R9 POT_METER

PERCENT_SWIPE = 99

VREF = 1.182V

R8 2.7M

C9 47pF

SYS_5V U2

? +

LTC1540

IO_VREF

R12 127k

G

C22

1?F

R13 453k

C13 47pF

BOOST ENABLE

1.182V

LTC1141

?+ POT_METER U5

+?

HIGHV_PG

22186-015

Figure 15. High Voltage Power-Good Indicator

Circuit Note

COMMON VARIATIONS

The output pulse of the Geiger-Mueller tube can be taken from the cathode (outer shell) to avoid the need for a high voltage capacitor. However, taking the output pulse this way is more sensitive to electrical interference because the cathode acts as an antenna. For long-term, battery-powered applications, the LTC6994 and power LED can be omitted from the circuit to minimize quiescent current.

CIRCUIT EVALUATION AND TEST

The following section outlines the procedure for bringing up the CN-0536. For complete details on the hardware and software setups, see the CN0536 user guide. Equipment Needed The following equipment is needed: ? The EVAL-CN0536-ARDZ ? The EVAL-ADICUP3029 ? An microUSB cable ? A host computer with a serial terminal program, such as TeraTerm or PuTTY ? A safe radiation source, for example, the Flinn Scientific AP8795 1Ci Cobalt-60 gamma source or similar. ? The ADuCM3029_demo_cn0536_uart.hex software file System Block Diagram Figure 16 shows the system block diagram.

Figure 16. The EVAL-CN0536-ARDZ Combined with the EVAL-ADICUP3029

22186-016

Rev. 0 | Page 6 of 8

Circuit Note

Getting Started To setup the EVAL-CN0536-ARDZ and the associated software, use the following steps: 1. Connect the EVAL-CN0536-ARDZ on top of the EVAL-

ADICUP3029 platform board as shown in Figure 17.

CN-0536

6. Place the radioactive source near the Geiger-Mueller tube and observe the terminal.

22186-019

Figure 17. EVAL-CN0536-ARDZ and EVAL-ADICUP3029 Connection

2. Connect the EVAL-ADICUP3029 to the PC using the microUSB cable provided.

3. From the PC, drag and drop the prebuilt ADuCM3029_demo_cn0536_uart.hex file onto the DAPLINK drive.

4. Open the serial terminal program (for example, Tera Term or Putty)

5. Connect the EVAL-ADICUP3029 by using the COM port it was assigned to set the baud rate to 115200 (see Figure 18).

22186-017

Figure 19. EVAL-CN0536-ARDZ Connected to the EVAL-ADICUP3029 Next to the Uranium Ore Sample

22186-020

22186-018

Figure 18. Using Tera Term as Serial Monitor With Baud Rate Set to 115200

Figure 20. Geiger-Mueller Detection Pulse from an Uranium Ore Sample

Rev. 0 | Page 7 of 8

CN-0536

LEARN MORE

CN0536 Design Support Package: CN0536 User Guide CN0536 Software Reference Design

Circuit Note

Data Sheets and Evaluation Boards LTC6906 Data Sheet LTC6994 Data Sheet LTC1540 Data Sheet LTC1441 Data Sheet LT6906 Evaluation Kit (DC2073B-H) LTC6994 Evaluation Kit (DC1562B-K) LTC1540 Evaluation Kit (DC600A) REVISION HISTORY 4/2021--Revision 0: Initial Version

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?2021 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.

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