______________________________________________
6. Data Acquisition INTRODUCTION
Nuclear Particles
Computer-Assisted Data-Acquisition System
This article describes how to use a computer to acquire real-time data from physical events that take place in a physics laboratory. For illustration purposes, two experiments will be performed with a computer that has an attached nuclear sensor. The nuclear sensor will collect sample data from radioactivity relating to three different sources of radioactive substances spontaneously decaying by emitting one or more nuclear particles or photons. The radioactivity can be mathematically represented by differential equations since the amount of radioactive decay is proportional to itself. The differential equations are solved by separating the variables and then integrating. These equations will be used to solve problems concerning the radioactive samples after collecting data from Experiment 1.
Legend:
N0 =number of radioactive nuclei present at to = 0
N = number of radioactive nuclei left after decaying t seconds.
T1/2 = time it takes for a radioactive substance to decay to half its previous amount.
l = decay constant characterizing the particular radioactive decay.
R = activity
|
Introduction to Computer-Assisted Data-Acquisition System |
The G-M sensor is a Geiger tube that contains a low-pressure gas, a center thin wire, and a thin mica window through which the radiation can pass. The insulated center wire is connected to the positive terminal and the tube wall to the negative terminal of the power supply. When a charged particle, such as a helium nucleus, enters the tube, it collides with the atoms of the gas, knocking off electrons. The atoms become positively ionized and move toward the tube wall. The electrons are attracted to the center wire where more electrons are freed thereby producing an electric pulse that lasts for a very short time. Each ionization results in a current pulse that is amplified and sent to the computer. The number of pulses received per second by the computer is recorded in a table as counts/sec.
The nuclear sensor, which is a Geiger-Müller tube with built-in power supply and amplifier, is used as the collecting sensor. An interface connects the sensor to a computer, which stores the data in a table. The computer also graphs the data and performs statistical computations to calculate the mean and standard deviation for each trial run.
|
Introduction to Computer-Assisted Data-Acquisition System |
The experiments provide the data that will be taken from three different sources of radioisotopes emitting the following types of radiation:
Ro = 1 m Ci, from the manufacturer.
The units that are used to measure the activities of radioactive isotopes and the dose of ionization radiation are the curie, rad, rem., and roentgen. These are not SI units but are temporarily used until the SI units, becquerel, gray, and sievert become familiar.
(Bq): 1 Bq = 1 decay per second. The more common unit is the curie (Ci), which is
the activity of 1 gm of radium. The conversion factor is 1 Ci = 3.7 x 1010 Bq.
name is rad, which is the acronym for radiation absorbed dose: 1 rad = 0.01 J/kg.
Therefore 1 Gy = 100 rad.
by radioactive isotopes. The unit more commonly used is rem (roentgen equivalent in
man), which is the dose equivalent that has the same biological effect as 1 rad of b or
l radiation, with negligible or no mass. However, when the mass is not negligible
e.g., proton, neutron, or a -particle, the damage done on biological tissue depends not only on the number of ions absorbed but also on their densities, in which case a multiplication factor is introduced to convert rad to rem: 1 rem = 1 rad x RBE (Relative Biological Effectiveness). 1 RBE is based on the damage done by 200 KeV X-rays . The SI unit is seivert (Sv) where 1 Sv = 1 Gy x RBE. Conversion factors for different types of radiation are listed in RBE tables. These were largely compiled from studies made of the effects on victims of atomic bomb explosions.