New User Special Price Expires in

Let's log you in.

Sign in with Facebook


Don't have a StudySoup account? Create one here!


Create a StudySoup account

Be part of our community, it's free to join!

Sign up with Facebook


Create your account
By creating an account you agree to StudySoup's terms and conditions and privacy policy

Already have a StudySoup account? Login here


by: Garett Kovacek


Garett Kovacek
GPA 3.95

Horst Wahl

Almost Ready


These notes were just uploaded, and will be ready to view shortly.

Purchase these notes here, or revisit this page.

Either way, we'll remind you when they're ready :)

Preview These Notes for FREE

Get a free preview of these Notes, just enter your email below.

Unlock Preview
Unlock Preview

Preview these materials now for free

Why put in your email? Get access to more of this material and other relevant free materials for your school

View Preview

About this Document

Horst Wahl
Class Notes
25 ?




Popular in Course

Popular in Physics 2

This 6 page Class Notes was uploaded by Garett Kovacek on Thursday September 17, 2015. The Class Notes belongs to PHY 3802L at Florida State University taught by Horst Wahl in Fall. Since its upload, it has received 51 views. For similar materials see /class/205518/phy-3802l-florida-state-university in Physics 2 at Florida State University.




Report this Material


What is Karma?


Karma is the currency of StudySoup.

You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!

Date Created: 09/17/15
IM3 The GeigerMuller Counter 1 Purpose Some measurements in nuclear decay notions of statistics 2 Apparatus ScalerTimer The Nucleus model 550 GeigerMuller tube oscilloscope radioactive sources 3 Introduction A typical GeigerMuller GM Counter consists of a GM tube having a thin mica endwindow a high voltage supply for the tube a scaler to record the number of particles detected by the tube and a timer which will stop the action of the scaler at the end of a preset interval The sensitivity of the GM tube is such that any particle capable of ionizing a single atom of the lling gas of the tube will initiate an avalanche of ionization in the tube The collection of the ionization thus produced results in the formation of a pulse of voltage at the output of the tube The amplitude of this pulse on the order of a volt or so is suf cient to operate the scaler circuit with little further ampli cation However the pulse amplitude is independent of the kinetic energy of the particle detected and therefore can give little information as to the nature of the particle In spite of this fact the GM Counter is a versatile device in that it may be used for counting alpha particles beta particles and gamma rays with however varying degrees of ef ciency 4 Setup Set up equipment as shown in Fig 1 Scaler Timer and High Voltage Supply may well be contained in one package Oscilloscope Digital Voltmeter Scaler Timer High Voltage Supply l GMTube Source 5 Measurements to be performed 51 Characteristics of the GM counter Put a radioactive source below the GM tube Put the counter in a counting mode and raise the voltage until counts are observed Counts should be somewhere from 1000 to 20000 per minute Note the shape of the pulse and what happens as the voltage on the GM tube is increased Do not exceed 900 volts at this time Note that the digital voltmeter and the meter on the voltage supply may indicate different voltages What could cause this and what is the real voltage across the GM tube What is the minimum voltage pulse necessary to activate the counter After properly triggering the scope sketch a picture of the pulse shape How would you describe it Every GM tube has a characteristic response of counting rate versus voltage applied to the tube A curve representing the variation of counting rate with voltage is called a plateau curve because of its appearance The plateau curve of every tube that is to be used for the rst time should be drawn in order to determine the optimum operating voltage Find the plateau curve for your tube using the procedure outlined below A Check to see that the high voltage as indicated by the meter on the instrument is at its minimum value B Insert a radioactive source into one of the shelves of the counting chamber Counts should be several thousand in the plateau region see below C Turn on the count switch and slowly increase the high voltage until counts just begin to be recorded by the scaler The voltage at which counts just begin is called the quotstarting voltagequot of the tube D Beginning at the nearest 25 volt mark above the starting voltage take 3 or 5 minute counts every 25 volts until a voltage is reached where a rapid increase in counts is observed Reset scaler to zero before each count Tabulate counts per minute versus voltage E Plot the data of D A plateau should be observed in the curve The optimum operating voltage will be about the middle of the plateau usually some 150 to 200 volts above the knee of the curve Set the high voltage to this point and record The slope S of the plateau ofa GM tube serves as a gure of merit for the tube The slope is de ned to be the percent change in count rate per 100 volts change in applied voltage in the plateau region A slope of greater than 10 indicates that the tube should no longer be used for accurate work The slope may be computed using 2 R2 R1 104 S per 100 V 1 R2 R1 v2 v1 where V is the voltage at the high end of plateau R2 is the count rate at this voltage V1 is the voltage at the low end of the plateau and R1 the corresponding rate Do you understand this equation and can you explain it In order to compare obtain a similar plateau curve for an old tube 52 Resolving time of the GM counter There is an interval of time following the production of a pulse in the GM tube during which no other pulse can be recorded This interval is called the resolving time of the system If this time is known it can be used to make a correction to the observed count rate to yield the true count rate The procedure below can give a good estimate of the resolving time gt Obtain a resolving time source a split source from the instructor This source is split into two parts Remove one half of the source and set it aside Place the carrier containing one part on the second shelf of the counting chamber and make a trial count of 1 minute duration Get the maximum count rate you can Hopefully this should be more than 20000 counts per minute but if not use what you can get Make a 3minute count and record the counts per minute R1 Put the two parts of the source back together taking care not to disturb the position of the rst part Make a 3minute count of the combined parts and record as RC Remove the part initially counted and make a 3 minute count on the second part Record as R counts per minute Calculate the resolving time of the GM system using the relation 03 P190 7391 Convert the time thus found to microsecondscount and record To understand the origin of the equation see Melissinos The resolving time 17 may be used to correct an observed count rate using the expression R rlr c 3 where r Observed count rate R True count rate To see the effect of the resolving time directly use a very high rate source and view the pulses on the oscilloscope 53 Statistical treatment of counting data The emission of particles by radioactive nuclei is a completely random process When under identical conditions a series of N measurements is made of the number of particles detected per unit time it will be observed that the individual measurements will vary about some average or mean value The true mean In can be determined only by averaging an infinite number of measurements However for a finite number of observations the best approximation of the true mean is simply the arithmetic average 7 ms lNn1nzn3nN 4 The quot J of the J viati of 39 J39 39J 39 measurements from the true mean is usually expressed in terms of a Standard Deviation 039 The Standard Deviation of any probability distribution is defined to be the square root of the average value of the squares of the individual deviations In the case of the Poisson Distribution which describes all nuclear processes the Standard Deviation is simply the square root of the true mean 5 m 5 The best approximation to the Standard Deviation of a nite series of measurements is given by onn1 Z2nzZz nN 2N 6 For values of m gt 20 the Poisson Distribution is essentially the same as the Normal Distribution for which certain con dence levels have been established regarding the Standard Deviation These con dence levels are as follows About 68 of the number of observations made will fall within the limits of Z i 039 About 95 of the number of observations made will fall within the limits of Z i 2039 About 99 of the number of observations made will fall within the limits of Z i 3039 This means that if one additional measurement is made it should have a 68 chance of falling within 2 i 039 When circumstances permit the making of only a single observation the number of counts obtained n must be assumed to be the true average value with the realization that the Standard Deviation i J is not an absolute and invariable quantity The Standard Deviation of a gross counting rate Rg is GRg t Rgt t g 7 wheret is the duration of the counting t This expression is adequate for high sample counting rates However when the background counting rate is appreciable compared to the sample counting rate the net counting rate R and its Standard Deviation 0 are determined by RickRgioRRbioR RgRbi1 8 or 0R i amp E 9 tg 1 17 where R Net counting rate Rg Gross counting rate of sample tg Period used to determine Rg Rb Background counting rate tb Period used to determine Rb To test the statistical nature of nuclear decay the following experiment can be performed A Adjust the height of a source in the counting chamber to produce about 2000 counts per minute B Take a set of 10 counts of 30 seconds duration C Compute the arithmetic mean D Compute the standard deviation 039S of the mean for a Poisson distribution of mean 2 F Compute the individual deviations from the mean ni Z and record in a table Do they sum very nearly to zero G Square the ni Z sum the square and apply Equation 6 to obtain the standard deviation 039quot Compare 039n with 039s G Count the number of measurements whose values lie within 2 i 039quot Compare your data to statistical predictions You may want to analyze previous data using some of these methods 54 Background Measurements EXtraneous radiation called background radiation is always present Gamma rays emitted by certain radioisotopes in the ground the air and various building materials as well as cosmic radiation from outer space can all provide counts in a detector in addition to those from a sample being measured This background counting rate should always be subtracted from a sample counting rate in order to obtain the rate from the sample alone Obtain a background counting rate using a 5minute sample time 55 Half life determination of an unknown radioisotope The activity number of disintegrations per second of a radioisotope is expressed as At A0 e39M 10 where At Activity at the end of an interval t A0 Activity at the beginning of the interval t e Base of natural logarithm 7 Decay constant characteristic of the radioisotope The half life Ty of a radioisotope is de ned to be that interval during which the activity decreases to onehalf its value at the beginning of the internal In terms of halflife the time dependence of the activity is At A0 e39039693UTV2 where Ty 0693 11 The counting rate of a sample of a radioisotope may be considered to be directly proportional to the activity at the moment of measurement provided that the counting interval is short compared to the halflife Reasonably short halflives can be determined by measuring activity at regular intervals The logarithm of the activity of the unknown when plotted as a function of elapsed time should yield points falling in a straight line Why Obtain your unknown sample from the instructor Examine the activity as a function of time and nd the halflife This is relatively short halflife isotope so adjust your sampling time accordingly 6 Report This discussion of the GM Counter is to serve to lead you through the experiment Your report need not follow the pattern of the writeup but should be a clear and complete discussion of the GM Counter and its characteristics as determined from experimental data


Buy Material

Are you sure you want to buy this material for

25 Karma

Buy Material

BOOM! Enjoy Your Free Notes!

We've added these Notes to your profile, click here to view them now.


You're already Subscribed!

Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'

Why people love StudySoup

Bentley McCaw University of Florida

"I was shooting for a perfect 4.0 GPA this semester. Having StudySoup as a study aid was critical to helping me achieve my goal...and I nailed it!"

Jennifer McGill UCSF Med School

"Selling my MCAT study guides and notes has been a great source of side revenue while I'm in school. Some months I'm making over $500! Plus, it makes me happy knowing that I'm helping future med students with their MCAT."

Steve Martinelli UC Los Angeles

"There's no way I would have passed my Organic Chemistry class this semester without the notes and study guides I got from StudySoup."


"Their 'Elite Notetakers' are making over $1,200/month in sales by creating high quality content that helps their classmates in a time of need."

Become an Elite Notetaker and start selling your notes online!

Refund Policy


All subscriptions to StudySoup are paid in full at the time of subscribing. To change your credit card information or to cancel your subscription, go to "Edit Settings". All credit card information will be available there. If you should decide to cancel your subscription, it will continue to be valid until the next payment period, as all payments for the current period were made in advance. For special circumstances, please email


StudySoup has more than 1 million course-specific study resources to help students study smarter. If you’re having trouble finding what you’re looking for, our customer support team can help you find what you need! Feel free to contact them here:

Recurring Subscriptions: If you have canceled your recurring subscription on the day of renewal and have not downloaded any documents, you may request a refund by submitting an email to

Satisfaction Guarantee: If you’re not satisfied with your subscription, you can contact us for further help. Contact must be made within 3 business days of your subscription purchase and your refund request will be subject for review.

Please Note: Refunds can never be provided more than 30 days after the initial purchase date regardless of your activity on the site.