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Atomic and Nuclear Physics

RadioactivityDetection of Radioactivity

LD

Physics

Leaflets

Recording theC hara c teris tics of a

Geiger-Mller (end-window)C ounte r Tube

Objects of the experiment

Rec ording the frequenc y of discha rge pulse s o f a G eiger-Mller counter tube a s a function of the co unter tube po tentia l

for a cons tant radiation so urce.

Determination of the Geiger threshold and the operating range.

Principles

Until today the Geiger-Mller counter tube, developed in 1928

by H. W. Geigerand his partner W. Mlleris an indispensable

means for detecting radioactivity. It even registers radiation

with minute ionizing pow er. It cons ists of a tube with a thin w ireiso la ted a nd sus pende d in its a xis a nd filled w ith inert ga s a t a

pres sure of a bo ut 100 mba r. The rad ia tion e nters from the frontside via a thin mica layer. Be tween the w all as ca thode a nd the

wire as anode a potential of several hundred volts works via ahigh-ohmic resistor. The voltag e is s elected to a void se lf-dis-

charge.

Incident radioactivity ionizes gas atoms in the counter tube.

The libe rated electrons move a t high velocity tow a rds the w ire,

the po sitive ions d rift a t relatively low veloc ity tow a rds the w a ll.In the vicinity of the wire the electrons a re s trong ly a cc elera ted

and through collisions produce new charge carriers or excitega s atoms to emit photons, which in turn can form add itional

cha rge ca rriers through photo-ionization. The cha rge ca rriersproduce a current impulse, which ends when all ions have

rea ched the ca thode . At the outer res isto r a s hort voltag e drop

oc curs, w hich is ap plied to a co unter via a co upling c onde nse r.

If the counter tube potential is low, the charge avalanches are

limited loc a lly. As the c ounter tube p otential rise s they s tart tospread until they finally are distributed over the entire counter

tube. All ionized particles, no matter what kind and of whatenergy, ca use a n ide ntica l volta ge impulse . The c ounting rate

for a cons tant radioactive so urce b ecomes independent of theco unter tube po tential. A counter tube w orking in this range is

called the Geiger-Muller counter tube. If the counter tube

voltag e is raised, a s elf-ac tuated g as discharge occ urs, w hich

easily can destroy the entire counter tube.

A quenching process must prevent secondary electrons from

being dissolved from the s urface of the ca thode, w hich c ould

restart the discharging process. In self-quenching countertubes a quenching gas (e.g. halogen) is mixed with the inert

ga s. It largely ab sorbs photons o n their wa y to the cathode a ndthus prevents electrons from being triggered photonically.

Furthermore the inert gas ions transmit their charge through

co llisions to mo lec ules of the extinguishing ga s ions . Thus o nlyions of the extinguishing g as rea ch the c athod e. Due to their

low ionizing power these are not able to activate secondary

electrons there. The disc harge proce ss is e xtinguished.

P6.4.1.4

0909-Sel

Fig. 1 Assemb ly diag ram of a Geiger-Mller counter tube

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Setup

Note:

The window of the end-w indow counter (1215m mica) ea s ily

can b e da maged mechanica lly. If the window is damaged theend-w indow c ounter tube is rendered unusable:

Do not touch the m ica window; store end-window counter onlywith p rotective cover in place. Remove protective cover for

making measurements only. Carefully remove and replace pro-

tective cover without tw isting and w ithout covering the air hole.

If the operating voltage is too high, the end-w indow c ounter

will be damaged by self-activated gas discharge:

Do not p ermanently exceed the maximum op erating voltage of

600 V.

The s et-up for the e xperiment is s how n in Fig. 2.

Mount la rge clip plug on co nnection rod. Fasten the e nd-window counter with the brackets, connec t

to inlet A of the digital counter and carefully remove pro-

tective c over.

Fasten Ra226 preparation with 4-mm plug at suitableheight o n the insulated s tand rod . Ca refully guide prepara-tion to a distance of a bout 1 mm from the e nd-window

counter.

Carrying out the experiment

S witch on the d igital counter or, if a vailab le, pus h button A.

S witch on loudspea ker, once press button Rate (ga te time:1 sec) and press button Sta rt Stop.

To reduc e the potential of the c ounter tube, turn the poten-tiometer A to the left until the acoustic signal disappearsand the display show s zero; read the voltag e of the counter

tube in the measuring display and record as Geiger thre-shold U0.

Apparatus

1 End-window counter for

-, -, - and X-rays . . . . . . . . . . . 559 01

1 R a226 preparation . . . . . . . . . . . . 559 430

1 Digital co unter . . . . . . . . . . . . . . . 575 48

1 Insulated sta nd rod, 25 cm . . . . . . . . 590 13

1 Large clip plug . . . . . . . . . . . . . . 591 21

1 Connection rod . . . . . . . . . . . . . . 532 162 Sa ddle bas es . . . . . . . . . . . . . . . 300 11

Safety notes

When radioactive preparations are handled, country

specific regulations must be observed, in Germany e.g.the Ra dia tion Prote ction Re gula tion (S trlS chV). The

radioactive substances used in the experiment are per-

mitted a cc ording to S trlS chV for scho ol instruction. Thefollowing safety regulations must be observed however,

since ionizing radiation is p rod uced :

P rotec t preparation from unauthorized use.

Check preparation for damage prior to use.

Store preparation in safety container for shielding.

To g ua rante e minimum time of exposurea nd minimumactivityremove preparation from the safety container

for performing the e xperiment only.

To en s ure maximum d istance, hold preparation at theupper lip of the metal container only.

Fig. 2 Recording the characteristics o f a Geiger-Mller counter

tube

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Reduce the counter tube voltag e by another ab out 100 Vand ag ain press the button Rate in order to raise the ga te

time to 10 sec.

Start the counting rate measurement with button StartS top; a fter the ga te time has pa ss ed, rea d the counting rate

R, and record together with the counter tube voltage U.

Pus h button Sta rt Stop to stop c ounting rate mea surement,raise co unter tube voltag e by 40 V a nd resta rt co unting rate

measurement.

To reco rd a dd itiona l meas urements raise co unter tube volt-a ge to a tota l of 640 V (cho os e s ma ll increments in the

range of the G eiger thres hold U0).

Widen d ista nces betw een preparation a nd c ounter tube to10 mm or 20 mm and rec ord ad ditiona l se ries of mea sure-

ments.

Measuring example

Working voltag e :U0 = 356 V

Evaluation and results

The G eiger thres hold is a pprox. 356 V. B etw ee n 380 V a nd

640 V the c ounter tube w orks a s a G eiger-Mller co unter tube.The cha racte ristics o f the counter tube s how a pla tea u.

The co unting rate on tha t pla tea u depend s for insta nce o n the

distance between preparation and counter tube, i .e. on the

activity at the location of the counter tube.

Ta b. 1: Co unting rate Ras a function of the counter tube

voltage Uat three different distances dbetw een preparationand counter tube

d= 1 mm d= 10 mm d= 20 mm

U

V

R

s 1R

s 1R

s 1

240 0.0 0.0 0.0

280 0.0 0.0 0.0

320 0.0 0.0 0.0

352 0.0 0.0 0.0

356 0.2 0.8 0.4

360 526.2 307.6 101.1

364 1012.4 627.0 149.1

368 1090.7 690.3 146.2

372 1156.7 691.6 145.0

380 1132.6 725.9 148.9

400 1220.4 727.7 145.4

440 1210.4 732.2 156.6

480 1223.1 743.2 152.6

520 1224.6 744.9 150.9

560 1205.1 737.3 153.0

600 1191.8 740.6 152.5640 1219.4 747.8 154.9

Fig . 3 Characteris t ics of the counter tube (counting rate Ra s a

function of the counter tube voltage U) a t three different

distances dbetween preparation and counter tube

LD DIDACTIC G mbH Leyboldstrasse 1 D-50354 Hrth P hone (02233) 604-0 Telefa x (02233) 604-222 E-mail: info@ld-didactic.de by LD DIDAC TIC G mb H P rinted in the Federa l Repub lic o f G erma ny

Technical alterations reserved

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