Gei g er T ube T heo r y , Dead T i me

22. S 902 – D I Y G eiger C ount ers P rof . M ic hael S hort

Quest i o ns t o St ar t

Courtesy of Mark Chilenski. Used with permission. 2

M o t i vat i o n

 U nders t and how ioniz at ion c ham bers , and s pec if ic ally G eiger t ubes , f unc t ion

 Learn t he m ec hanis m of “dead t im e” in det ec t ors , and how it lim it s t hem

 C harac t eriz e det ec t ors as paraly z able or not

 P redic t how dead t im e w ill af f ec t c ount ing out put and s t at is t ic s

3

Gei g er- M üller T ubes

http://www.imagesco.com/geiger/gmt -0 3.jpg

A n SB M -2 0 U tu b e , s h o w i n g th e w i re a n o d e

Courtesy of Im age s SI, Inc. Used wit h p ermission.

O u r SBM- 20

Geiger t ubes 4

O ur SBM - 20 Gei g er- M üller T ubes

W a l l t h ickn e ss: 50 µ m st e e l W a l l De n sit y: 8 g/c m 3 Assu m e 1 at m equa l ga s mi x ture

T u b e d i ag ram © so u rce u n k n o w n . A l l ri g h ts re se rv e d . T h i s con te n t i s excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq -f air -u se/ .

For m or e inf or m a t ion, s e e : h ttp : //www .gs t ube .c om /da t a /2 3 9 8 /

Io ni zat i o n Chamber s

– –

–

–

P ubl i c doma i n i ma ge . –

Geiger tube photo courtesy of Jeff Keyzer on F lickr. – +

–

–

–

–

Circuit diagram © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection (2007). All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .

T he Io ni zat i o n P lat eau ( L o w- V )

P ubl i c doma i n i ma ge .

– –

–

–

–

– +

–

–

Graph © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection –

(2007). All rights reserved. This content is excluded from our Creative Commons

–

–

–

– – 7

T he Io ni zat i o n P lat eau ( M ed - V )

– –

–

–

P ubl i c doma i n i ma ge . –

– +

–

–

Graph © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection –

(2007). All rights reserved. This content is excluded from our Creative Commons

–

–

–

– – 8

T he Io ni zat i o n P lat eau ( H i g h- V )

– –

–

–

P ubl i c doma i n i ma ge . –

– +

–

–

Graph © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection –

(2007). All rights reserved. This content is excluded from our Creative Commons

–

–

–

– – 9

T he Io ni zat i o n P lat eau ( H IGH - V )

– –

–

–

P ubl i c doma i n i ma ge . –

– +

–

–

Graph © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection –

(2007). All rights reserved. This content is excluded from our Creative Commons

–

–

–

– – 10

No w fo r t he M at h…

Let' s s ay a gamma fl ux of Φ ̇ enters a c hamber of area A .

E ac h mak es N i on pai r s , eac h w i th a c har ge of e and ener gy W ,

an d th en s tops i n the c ham be r:

+

W ̇ = Φ ̇ 𝐸 γ

The c har ge produc e d per parti c l e i s 𝑁𝑒 , and t he number of

parti c l es enteri ng per s ec ond i s Φ ̇ 𝐴 . That mak es the c urrent I :

𝐼 = Φ ̇ 𝐴 𝑁𝑒

Let' s s ay a

gamma fl ux of Φ ̇

enters a c hamber of area A .

E ac h mak e s N i on pai r s , eac h w i th a c har ge of e and ener gy W ,

and then s t ops i n the c ham ber:

+

W ̇ = Φ ̇ 𝐸 γ

N ow us e t he radi ati o n in t e n sit y:

No w fo r t he M at h…

𝐼 = Φ ̇ 𝐴 𝑁𝑒

𝐼

𝐴 𝑁𝑒

= Φ ̇

W ̇ = 𝐼

𝐴 𝑁𝑒

𝐸 γ

𝐼 W

=

𝐴 𝐸 γ 𝑒

𝐸 γ

𝐼 W

=

𝐴 𝑒

No w fo r t he M at h…

Let' s s ay a gamma fl ux of Φ ̇ enters a c hamber of area A .

E ac h mak es N i on pai r s , eac h w i th a c har ge of e and ener gy W ,

an d th en s tops i n the c ham ber:

+

W ̇ = Φ ̇ 𝐸 γ

The abs or pti o n rate of energy i n the c ham ber i s 𝐸 ̇ 𝑎 𝑏 𝑠 = W ̇ 𝐴

Thes e c an be us ed to meas ure gamma ray energ y , pr oporti o n al to the meas ure d c urrent!

O ur Use: A Si mp le Co unt er

– –

–

–

P ubl i c doma i n i ma ge . –

Geiger tube photo courtesy of Jeff Keyzer on F lickr. – +

–

–

–

–

Circuit diagram © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection (2007). All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .

O ur Use: A Si mp le Co unt er

P ubl i c doma i n i ma ge .

Geiger tube photo courtesy of Jeff Keyzer on F lickr.

Circuit diagram © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection (2007). All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .

W e u se cir cu it r y t o d e t e ct t h e ri s i ng edge of the pul s e from the Gei ger tube

S et s om e thres hol d to c al l the edge r ise n

Se n d t h is ( n o w d ig it a l) sig n a l t o our LE D s and the s peak er to mak e l i ght and nos e!

Circuit diagram © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection (2007). All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .

L et ’ s R e- Exami ne O ur Ci r cui t

Courtesy of Mark Chilenski. Used with permission.

That’ s our r ising edge tr igger !

16

T ran si st o r i s bot h a s wit c h a nd a n inv e r t e r

555 t i mer i s t r igge r e d t o c ha r ge it s cap aci t o r ( ch arg e b u cket ) unt il f ull

L o o k Clo ser : Ri si ng Ed g e T r i g g er

Courtesy of Mark Chilenski. Used with permission. 17

O ur M easur ed W avefo r ms

Ac t ua l S BM - 20 G ei ge r t ube w av e f or m

P ul se st r et cher ( s ound and l i ght ) w av ef or m

L i mi t i ng Cases: H i g h Si g nal

GM T ube

This is known as “ dead tim e,” a per iod when the detector is unr esponsi ve

1

2 3 4

T rue Ev ents

Which app li es to our cas es?

t

t

P ar alyzable Detector

t

Non-P ar alyzable

Detector

P ar aly zable vs. No n- P ar aly zable

GM T ube

P ar aly zable vs. No n- P ar aly zable

Incr ease count r ate

Circuit diagram © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection (2007). All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/ help/faq-fair-use/ .

+

Geiger tube photo courtesy of Jeff Keyzer on F lic kr.

P ar aly zable vs. No n- P ar aly zable

GM T ube

Geiger tube photo courtesy of Jeff Keyzer on F lickr.

+

Circuit diagram © Wiley-VCH, from J. Turner, Atoms, Radiation, and Radiation Protection (2007). All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/help/faq-fair-use/ .

T hi nki ng Ahead fo r t he L ab

 How will you char a cter i ze GM dead tim e ?

 How will you char a cter i ze pu l se str etcher dead tim e?

 Wher e on your cir cuit will you connect the com puter to m easur e counts?

 What other sour ces of dead tim e exist in the system ? Hint: T her e ar e som e!

25

MIT OpenCourseWare http://ocw.mit.edu

22. S902 Do-It-Yourself (DIY) Geiger Counters

January IAP 2015

For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .