Elektor Radiation Meter by lanyuehua


									TesT & MeasureMenT

Improved Radiation Meter
Counter for
alpha, beta and gamma radiation

By Burkhard Kainka (Germany)

all that’s required to measure
radiation is a simple PIn
photodiode and a suitable
preamplifier circuit. We present
here an optimised preamplifier
and a microcontroller-based
counter. The microcontroller
takes care of measuring time and
pulse rate, displaying the result
in counts per minute.

The device we describe can be used with            allowing a more detailed investigation of        a comparator, and which generates output
different sensors to measure gamma and             the characteristics of a sample. The optional    pulses that can be heard directly or pro-
alpha radiation. It is particularly suitable for   PC-based software displays the energy spec-      cessed further. The amplifier uses a BF245B
long-term measurements and for examin-             trum, permitting a very detailed analysis to     JFeT at its input followed by an opamp cir-
ing weakly radioactive samples. The pho-           be carried out.                                  cuit, providing an overall voltage gain of
todiode has a smaller sensitive area than a                                                         30 000. at the output it delivers pulses with
Geiger-Müller tube and so has a lower back-        Preamplifier                                     an amplitude of up to 200 mV with a pulse
ground count rate, which in turn means that        In the June 2011 issue of elektor we             width of around 0.5 ms, which can be ren-
the radiation from a small sample is easier        described experiments using a BPW34              dered audible without further processing or
to detect against the background.                  photodiode as a detector for gamma radia-        which can be used to drive a counter.
                                                   tion [1]. Only simple experiments could be       The circuit (see Figure 1) can use sev-
a further advantage of a semiconduc-               carried out as the pulses from the detector      eral photodiodes in parallel at its input.
tor sensor is that is offers the possibility       are very short. Here we look at an improved      On the one hand, this increases the pulse
of measuring the energy of each particle,          preamplifier design, which avoids the use of     rate; unfortunately, this advantage is on

  Elektor products and services
  •	Printed	circuit	board:	#	110538-1
  •	Kit	(components	and	circuit	board):	#	110538-71                        •	PDF	layout:	free	download	at	[2]
  •	FT232R	USB/Serial	Bridge:	#	110553-91                                  •	Software	and	firmware:	free	download	at	[2]	(file	#	110538-11)

20                                                                                                                                 11-2011    elektor
                                                                                                                                                                                                                                                TesT & MeasureMenT

                                   •	Measures	alpha,	beta	and	gamma	radiation                                                                                                   •	Threshold	configurable	in	software	from	the	PC
                                   •	Simple	construction	using	standard	components                                                                                              •	Two	different	types	of	sensor	can	be	used
                                   •	PC	connection	using	Elektor	FT232R	USB/Serial	bridge

                 the other hand offset by                                                       BF245B’s source resistor, more or less inde-                                                                         the gate of the JFeT being pulled to ground
                  the reduced amplitude                                                         pendent of the supply voltage. If a BF245C                                                                           through a 20 MΩ resistance.
                   of the sensor’s output                                                       is used, this voltage will be higher; in the
                  due to its increased total                                                    case of a BF245a, lower. This voltage level                                                                          The counter
                capacitance.                                                                    is a suitable operating point for the opamp.                                                                         The pulse counter is constructed using an
The JFeT input offers good signal-to-noise                                                      The capacitance of the photodiode drops                                                                              aTmega88 and a two-line LCD. The power
ratio and a high input impedance. a DC                                                          as the voltage across it increases, and so it                                                                        supply voltage of 9 V to 12 V is taken via D1
level of about 2 V or 3 V appears across the                                                    is operated at the full supply voltage, with                                                                         (for reverse polarity protection) to voltage

                                                                    R1                                    R2                                                                                    R11
                                                                       1M                                   1k                                                                                    100R
                                                                                                     C6                                                                                    C7
                                                                                T1                   100u                                                                                  100u

                                                 BPW34                                                                                       IC1 = LM358N

                                                                         BF245                              3          8
                                                                                                                                                                                                              3    K2
                                                                                                                                 1                             5
                                                                                                            2                           R6                                      7          COUNTER 2
                                                                                                                                                    C4             IC1.B
                                                                                                                                                               6                       R8

                                                              R14                                                                                                     4

                                                                                                                                                                                                   47p             GND

                                                              R3                       R4                                               R5                                             R7



                                                                                                                                       C2                                             C3

                                                                                                                                       100n                                           100n

                             D2             7805

               K8        1N4004
                     2                                        C9
                                                                                                                             7                 20
                                                                                                21                         VCC              AVCC
                                                              100n                                    AREF



               K1                                                                               28                                                                                                       1     2     3     4     5   6     7     8     9     10 11 12 13 14 L+ L-
                     3                                                                                PC5(ADC5/SCL/PCINT13)
                     2 COUNTER                                                                        PC4(ADC4/SDA/PCINT12)

                     1 GND
                                     C8                                                         25                                                                         2
                                                                                                      PC2(ADC2/PCINT10)                             PD0(RXD/PCINT16)
                                                                                                24                                                                         3

                                                                                                      PC1(ADC1/PCINT9)                               PD1(TXD/PCINT17)
                                     10u                                                                                     ATMEGA88
                                                                                                23                                                                         4
               K4                                                                                     PC0(ADC0/PCINT8)                              PD2(INT0/PCINT18)
                     2                                                                                                                                                     5
                     1                                                                          19                                                                         6
                                                                                                      PB5(SCK/PCINT5)                           PD4(T0/XCK/PCINT20)
                                           K6                                                   18                                                                         11
               OUT                                                                                    PB4(MISO/PCINT4)                         PD5(T1/OC0B/PCINT21)
                                             1     3    5                                       17                                                                         12
                                                                                                      PB3(MOSI/OC2A/PCINT3)                  PD6(AIN0/OC0A/PCINT22)
                              S1                                                                16                                                                         13
                                                                                                      PB2(SS/OC1B/PCINT2)                           PD7(AIN1/PCINT23)
                                             2     4    6                                       14
                             ZERO                ISP                        R10            R9                                                                                                                        R12
                                                                                                                 GND       XTAL1        XTAL2        AGND                                                                                                                                 K3    1



                                                                                                                   8         9                 10         22
                                           +5V                    K5                                                                                                                               TXD
                                                                                           LED1                                                                                                    GND
                                                                                                                                                                                                                                                                                   Backlight 2
                                                                                                                                                                                      Elektor BOB

                                                                                                                                                                                                                                                                               110538 - 11

                                                        Figure 1. Circuit diagram of the preamplifier and microcontroller board.

elektor   11-2011                                                                                                                                                                                                                                                                                   21
TesT & MeasureMenT

     Resistors                                                                   Semiconductors
     r1 = 1MΩ                                                                    D1 = BPW34
     r2 = 1kΩ                                                                    D2 = 1n4001
     r3,r14 = 10MΩ                                                               D3 = LeD, 5mm, green
     r4 = 4.7kΩ                                                                  IC1 = aTmega88Pa-Pu (atmel), programmed
     r5, r7 = 10kΩ                                                               IC2 = LM358n
     r6,r8 = 330kΩ                                                               IC3 = 78L05                                         sound starts to get annoying.
     r9,r10 = 470Ω                                                               T1 = BF245B
                                                                                                                                     The unprocessed signal is also taken to con-
     r11,r12 = 100Ω
     r13 = 5.6Ω                                                                  Miscellaneous                                       nector K4 via C8. This can be wired to a BnC
                                                                                 s1 = pushbutton, 1 make contact                     socket for to allow, for example, connection
     Capacitors                                                                  K1–K8 = pinheader, e.g. Te-Connectivity type        of an oscilloscope. alternatively, an audio
     C1,C2,C3,C9 = 100nF                                                           3-826926-6                                        amplifier can be connected, in which case
     C4,C5 = 47pF                                                                LCD1 = DeM16217, elektor shop # 030451-72
                                                                                                                                     particles of different energies will be distin-
     C6,C7 = 100µF 16V
     C8 = 10µF 16V                                                                                                                   guishable by their sounds.

                                                                                                                                     Printed circuit board
                                                                                                                                     The printed circuit board for the project
                                           110538-1                                         K6                                       (Figure 2) comes in two sections. The sen-
                                           V1.22                                                                                     sor board can be separated from the main
                                           (c) Elektor                                                                               part and connected using a three-core

                                                                                                                 R10                 cable. This allows the sensor to be mounted
                                                                                                                                     more easily in a light-tight enclosure.

                                                                                                                          D3         The LCD and pushbutton are mounted on
                   K8                                                                                                                the reverse of the board; all other compo-
                                                                                                                                     nents are mounted on the top side. sen-

                                                               I O
                                                                     R13                  R12
                                                                                                                               OUT   sor D1 can be mounted on whichever side
                                                                                                                                     of the board is more convenient, bearing
                                                                                                                                     in mind how the unit will subsequently be
                                                                                      K1            Backlight                        mounted in its enclosure.
                                                                                                1           K3
                                                                                                                                     The circuit is best tested first without the
                                                                                                                                     sensor diode fitted. The output of the
                                                                                                1           (c) Elektor     C3
                                                                                                                                     opamp circuit should be a mid-level DC


                               T1            R14                                          K2

                                    32 1                                                                         C5
                                                                                                                                     voltage, and the input should be so sensi-


                                                                                                                                     tive that even moving a finger near it will

                                                                                     R6                                              trigger the counter.

                                                                           C7                        110538-2    V1.21
                                                                                                                                     The sensor can now be mounted: Figure 3
                                                                                                                                     shows it mounted on the reverse of the
                                                                                                                                     board. The sensitive area of the device is
                         Figure 2. Printed circuit boards for the radiation meter.                                                   then under the lower right side of the board.
                                                                                                                                     It is important to ensure that the BPW34
                                                                                                                                     photodiode is fully shielded from incident
                                                                                                                                     light and that the circuit area around the
regulator IC2, a 78L05, which generates the                                      level and any incoming pulses that exceed           diode is carefully screened. During con-
5 V supply for the microcontroller and the                                       this level by at least a preset amount are          struction fit a small piece of black insulating
sensor circuit. There is an IsP connector on                                     counted. Button s1 starts a new measure-            tape under the photodiode: this will block
the board to make software updates easy                                          ment without resetting the stored quies-            light that might otherwise pass through the
and there is a further connector to which                                        cent level: this lets you first set the quies-      not-quite-opaque printed circuit board. Fig-
a serial interface can be attached to allow                                      cent level with the sensor not pointing at          ure 4 shows the component side of the pop-
transfer of data and settings to and from the                                    the sample of interest, and then start the          ulated prototype board.
PC (K7). The rXD and TXD signals on this                                         measurement proper with the sensor in               aluminium foil can be used to screen the
connector are at TTL levels and compatible                                       position.                                           area around the photodiode, on both sides
with the FT232r usB serial bridge breakout                                       any pulse that is counted also results in a         of the board. The foil should be connected
board (‘BOB’) published in the september                                         visible indication on the LeD and on output         to ground. This will give good shielding
2011 issue of Elektor.                                                           K5, where a small loudspeaker can be con-           against both light and interference that
The sensor amplifier is attached at connec-                                      nected for an audible indication. any loud-         could otherwise lead to false counts. Fur-
tor K1. Its output signal, which is super-                                       speaker with an impedance of between 8 Ω            ther pieces of insulating tape can be used
imposed on a quiescent DC level, is taken                                        and 32 Ω can be used, although it is a good         under the aluminium foil to prevent shorts
directly to the aDC0 analogue input of                                           idea to wire it in series with a volume con-        to the rest of the circuit. The foil should be
the microcontroller. at start-up the micro-                                      trol (a 1 kΩ potentiometer with a logarith-         fitted tightly over the photodiode, as other-
controller determines the quiescent DC                                           mic taper) in case after a while the ticking        wise it is possible accidentally to construct

22                                                                                                                                                                  11-2011   elektor
                                                                                                                 TesT & MeasureMenT

                     Figure 3. Components mounted                                            Figure 4. Component side of the boards.
                    on the reverse of the circuit boards.

a condenser microphone, with the counter            active sample. Place a sample, for example         through. However, we can replace the
responding to loud noises! The connection           of a suitable mineral, directly in front of the    BPW34 with a BPX61, with the glass win-
to the foil can be made, for example, using         sensor. any gamma radiation emitted by             dow removed. The photodiode itself is now
a bolt with two washers.                            the sample will give rise to pulses clearly vis-   completely exposed and alpha particles can
We can now test the operation of the board          ible above the background noise level. each        now be detected, resulting in signals with
with the photodiode in its light-tight enclo-       pulse whose amplitude exceeds a certain            an amplitude some ten times greater than
sure. a DC level of about 2 V to 3 V should         level will be counted. The threshold level         those produced by gamma rays. The same
appear at its output. This serves to check          can be adjusted later in the software. If you      sensor amplifier can be used, but it must

 Measuring radiation using a common-or-garden photodiode
that there is indeed no light falling on the        do not have a suitable sample to hand, all         now be separated from the counter board
photodiode: if the operating point of the           you need to do is wait: after a few minutes        and mounted in its own light-tight screened
circuit is too high, this means that some           at the most a cosmic ray will impinge on the       box. The sample to be examined must be
light is probably getting in. When every-           sensor and be counted (see Figure 5, Fig-          placed inside the box next to the sensor,
thing appears to be working satisfactorily,         ure 6 and Figure 7).                               as even one layer of aluminium foil is thick
it should be possible to use an oscilloscope                                                           enough to impede alpha particles.
to observe background noise with an ampli-          Alpha particles                                    a small abrasive disc mounted in a hobby
tude of around 5 mVpp.                              The BPW34 comes in a plastic package               drill can be used to remove the glass win-
The unit can now be tested with a radio-            which is too thick to allow alpha particles        dow from the BPX61. The job has to be done

 Figure. Measuring the background count                     Figure 6. an individual pulse.                Figure 7. Background noise and the
  rate: 0.33 pulses per minute is normal.                                                                            desired signal.

elektor   11-2011                                                                                                                               23
TesT & MeasureMenT

     Listing 1:
     Averaging readings and setting                    M = M + 1
     the trigger threshold                             Count = N
                                                       Count = Count / M
     Readeeprom L , 1                                  Locate 2 , 10
      If L = 255 Then L = 10                           Lcd Count
      U = 0                                            Lcd „      „
      For N = 1 To 1000                              End If
        D = Getadc(0)                                Locate 1 , 10
        U = U + D                                    Lcd N
      Next N                                         Locate 2 , 1
      U = U / 1000                                   Lcd M
      Um = U                                         Lcd “:”
      U0 = Um + L                                    Lcd S
      N = 0                                          Lcd “ “

     Listing 2:                                    Listing 4:
     Detecting a pulse                             Processing the received energy values
     Do                                            Private Sub Timer1_Timer()
          Max = U0                                   While INBUFFER() > 0
          Do                                           d = READBYTE()
             D = Getadc(0)                             bin(d) = bin(d) + 1
          Loop Until D > U0                          Wend                                                     Figure 8. screening.
          Portb.0 = 1                                For n = 1 To 255
          Portb.1 = 1                                  x1 = 2 * n
          If D > Max Then Max = D                      x2 = 2 * n + 2                            reasonably easy to understand. The output
          Do                                           y1 = 200 - bin(n)                         voltage from the preamplifier in the qui-
             D = Getadc(0)                             y2 = 200 - bin(n + 1)                     escent state is about 2 V, on top of which
             If D > Max Then Max = D                   If y1 > 255 Then y1 = 255                 we see the sensor pulses. In order to count
          Loop Until D < U0                            If y2 > 255 Then y2 = 255                 these pulses we need to use some kind of
          N = N + 1                                    Picture1.Line (x1, y1)-                   comparator, and the aTmega is fast enough
          Max = Max - Um                           (x2, y2)                                      to do this job in software. at start-up the
          If Max > 255 Then Max =                    Next n                                      unit computes an average background sig-
     255                                           End Sub                                       nal level by averaging 1000 readings (see
        Print Chr(max);                                                                          Listing 1). The averaged value u is then
        Portb.0 = 0                                                                              increased by a trigger threshold increment
        Portb.1 = 0                                Listing 5:                                    L to produce a threshold u0 which gives
      Loop                                         Setting the trigger threshold                 adequate margin over background noise
                                                   Private Sub Command2_Click()                  for reliable counting.
                                                     l = HScroll1.Value                          During the measurement itself (Listing 2)
     Listing 3:                                      SENDBYTE l                                  the output of the software comparator
     Timer processing and LCD output               End Sub                                       is also used to drive digital output ports
     Tim1_isr:                                                                                   PortB.0 and PortB.1. an LeD is connected to
       Timer1 = -7812                              Private Sub Command4_Click()                  pin B.0 to give a brief flash of light for every
       S = S + 1                                     l = 100 + HScroll1.Value                    pulse detected. an small loudspeaker can
       If S = 60 Then                                SENDBYTE l                                  be connected to B.1 with a resistor or a vol-
         S = 0                                     End Sub                                       ume control potentiometer in series.
                                                                                                 The maximum level achieved by each pulse
                                                                                                 is also measured and output as a single byte
                                                                                                 over the serial interface, to which a PC can
extremely carefully as any damage to the           peaks, up to 2 V, corresponding to alpha      be connected. This single-byte format is
diode itself or to its bonding wires spells        particles. smaller peaks can also be seen:    chosen so that no extra time is wasted in
doom for the device. a lidded tin makes a          the BPX61 is, like the BPW34, sensitive to    data transmission. as a consequence the
good screened enclosure (Figure 8). The            gamma rays too. Hence we can discriminate     maximum pulse height that can be reported
metal must be connected to signal ground           between the two types of radiation on the     is 1.25 V, which corresponds to a byte value
so that it provides electrical shielding as well   basis of the pulse amplitude.                 of 255. Pulses of greater amplitude than this
as screening from ambient light. The lid must                                                    are clipped to the maximum value.
of course be fitted before any readings can        Firmware                                      The display continuously shows the current
be taken.                                          The firmware is available for free download   counter value, updated once per second
The oscilloscope shows high-amplitude              at [2]. It is written in BasCOM-aVr and is    (Listing 3). accompanying this on the lower

24                                                                                                                               11-2011   elektor
                                                                                                                 TesT & MeasureMenT

 Figure 9. The alpha spectrum of a sample          Figure 10. Gamma radiation: pitchblende              Figure 11. Beta particle measurements
              of pitchblende.                          behind a piece of aluminium foil.                  from a potassium chloride sample.

line of the display is the total time elapsed      ure 9). The higher energy values are due           readings over a long period using the BPX61
since the beginning of the measurement,            to alpha particles: because of the relatively      and a small sample of potassium chloride.
shown in minutes and seconds. at the end           thick sample used these tend to lose a lot         Ordinary potassium chloride contains a
of each full minute the program calculates         of their energy on their way to the sensor,        small percentage of radioactive potas-
the count rate in pulses per minute.               and so no sharp energy lines are seen with         sium-40. some 90 % of the disintegrations
The sensitivity of the unit depends chiefly        this sample. When looking only at gamma            produce beta particles with energies of up
on the threshold setting for the compara-          rays using the BPW34 the higher energy             to 1.3 MeV; the other 10 % produce gamma
tor. This is initially set at ten a/D conversion   levels are empty; when alpha particles             quanta with energies of 1.5 MeV. The beta
steps above the quiescent level, but the           are detected they usually have an energy           spectrum shows a characteristic falling-off
value can be modified if desired using the         beyond the range of the display, which             in energy up to a clearly-visible maximum

                                 Measuring background radiation
serial interface. a threshold of three steps       accounts for the sharp peak seen at the            value, while the gamma spectrum takes
has proved a good compromise, giving reli-         maximum energy value.                              the form of a sharp line. The result is the
able pulse counting while still being above        Listing 4 shows the main timer routine. In         full spectrum shown in Figure 11, demon-
the background noise level. To change the          this routine all the bytes found in the input      strating that the photodiode is capable of
threshold simply send a single byte to the         buffer are read in and processed. For each         detecting alpha, beta and gamma radiation.
unit over the serial port. Values of up to 100     byte one element of the array ‘bin’ is incre-                                              (110538)
are used immediately as the new threshold          mented and then the results are displayed          Internet Links
value. If you wish to store the new threshold      graphically.
                                                                                                      [1] www.elektor.com/110372
as the default value in the microcontroller’s      The program also allows the trigger thresh-
eePrOM, add 100 to the value before send-          old to be set to any value from 2 to 100. The      [2] www.elektor.com/110538
ing it. In this case the new setting does          value can be used immediately or stored in
not take immediate effect. so, for exam-           eePrOM (Listing 5).
ple, sending the byte value 103 will set the       alpha particles can easily be filtered out, for
threshold after the next reset to three.           example by placing a piece of aluminium foil
                                                                                                        Kit of parts
                                                   in front of the ‘naked’ BPX61. Doing this            a kit of parts is available for this project,
PC software                                        considerably reduces the energy spectrum             containing the printed circuit board and
The Visual Basic program alphaGamma                obtained from a sample of pitchblende                all components including a pre-pro-
(available for free download at [2]) receives      (Figure 10).                                         grammed microcontroller. a suitable
incoming bytes over the serial port and            Beta particles also produce measurable sig-          display can also be purchased. Prices
counts them in 255 bins. after a while col-        nals. The pulse amplitude they produce is            and further information can be obtained
lecting these values it becomes apparent           generally similar to that which results from
                                                                                                        from the web pages accompanying this
which energy levels are particularly com-          gamma radiation, and it is therefore difficult
                                                                                                        article at [2] and in the ‘shop’ pages at
mon in the incident radiation. The energy          to distinguish between the two. To test for
                                                                                                        the back of this issue.
spectrum is plotted in a simple graph (Fig-        beta particle sensitivity it is possible to take

elektor   11-2011                                                                                                                                       25

To top