data exp. 6 ijatto

8/3/2019 Data Exp. 6 Ijatto

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RESULT

DATA SHEET 1

Gamma Scintillation Detector

Counter:Source: 137Cs (1Ci)

Voltage: 1000V

Channel (LL) Counts Channel Counts0.2 1954 3.4 230.3 2354 3.5 130.4 19520.5 16030.6 1681

0.7 17430.8 22700.9 26141.0 23231.1 19371.2 15791.3 14591.4 13631.5 13051.6 12561.7 1270

1.8 13321.9 14172.0 11822.1 8292.2 4482.3 2702.4 2202.5 2072.6 2712.7 8082.8 39602.9 80973.0 58373.1 14083.2 1633.3 31


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Graph 1

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Count/sample

Channel number


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DATA SHEET 2


Counter:

Source: 60Co

Voltage: 1000V

Channel (LL) Counts Channel Counts0.2 536 3.4 4210.3 803 3.5 4600.4 658 3.6 4530.5 549 3.7 4890.6 549 3.8 4750.7 568 3.9 4820.8 592 4.0 442

0.9 729 4.1 4231.0 888 4.2 4151.1 844 4.3 3541.2 676 4.4 3231.3 643 4.5 3211.4 563 4.6 3201.5 513 4.7 2901.6 473 4.8 3031.7 429 4.9 5311.8 422 5.0 9391.9 423 5.1 1159

2.0 389 5.2 7422.1 369 5.3 3082.2 407 5.4 1292.3 413 5.5 2122.4 427 5.6 4912.5 373 5.7 8072.6 372 5.8 8832.7 419 5.9 5112.8 387 6.0 2162.9 368 6.1 85

3.0 424 6.2 333.1 364 6.3 243.2 3943.3 426


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Graph 2

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DATA SHEET 3


Counter:

Source: unknown

Voltage: 1000V

Channel (LL) Counts Channel Counts0.2 4577 3.4 16580.3 4874 3.5 63230.4 4679 3.6 146390.5 4265 3.7 147130.6 4309 3.8 70620.7 3859 3.9 12970.8 4231 4.0 227

0.9 5970 4.1 911.0 5643 4.2 781.1 4799 4.3 1021.2 3995 4.4 1031.3 3507 4.5 881.4 3339 4.6 731.5 3143 4.7 681.6 30591.7 29291.8 29271.9 2871

2.0 28572.1 28812.2 30612.3 30142.4 29562.5 31892.6 32462.7 26682.8 18572.9 1138

3.0 7263.1 5813.2 4923.3 679


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Graph 3

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Count/Channel

Channel Number


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Energy Calibration Curve

Event (KeV) Disc. Setting (LL)(V)

Photopeak 661.6 2.9

Photopeak 1173.2 5.1

Photopeak 1332.5 5.8

Graph 4

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Energy(eV)

Channel Number

3.7

830


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From data sheet 3 (unknown source), we know that the energy peak is at 3.7 (channel number).

When we plot the point at energy calibration curve of the spectrometer, we got the energy of the

unknown source which is 830 eV.

Calculation of Energy Resolution of NaI(TI) Detector

Using formula (1) from graph 1 the resolution, R is

= 0.25

= 8.62%

From graph 2 there are two resolutions because of two peaks

= 0.35 = 0.30

= 6.86% = 5.17%

From graph 3 the resolution, R is

= 0.30

= 8.12%


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Discussion

In this experiment we will study the radioactive decay of a nucleus by detecting gamma rays

emitted consequent to the decay. Gamma ray detection is a slightly complicated, multi-step

process: the gamma ray enters a NaI(Tl) scintillator crystal where it produces a rapidly moving

free electron that, in turn, loses its energy by excitation of the ions in its path as it travels through

the crystal. This excitation energy is given off in various ways, one them being emission of

visible light (fluorescence). Thus a single high energy gamma ray entering the scintillator

produces a flash of low energy photons. These photons are directed to the photosensitive surface

of a photomultiplier tube, where they eject electrons via the photoelectric effect. The electrons

are collected in the photomultiplier and amplified to yield a current pulse, which is converted to

a voltage pulse whose height is proportional to the number of photoelectrons and is thus

proportional to the number of photons reaching the tube, which in turn is proportional to the

initial energy of the fast electron.

When a radioactive source is placed near the scintillator, the photomultiplier produces a series of

pulses, each corresponding to the decay of a single nucleus. The amplitude of each pulse is

related to the energy of the electron freed by the gamma ray. These pulses are studied using

either a single- or multi-channel analyzer. A single channel analyzer (SCA) counts on the

number of voltage pulses whose height falls within a given (adjustable) window of values, while

a multi-channel analyzer (MCA) sorts the pulses according to height and the counts the number

in each window to give a spectral (energy) distribution of the fast electrons. Figure 4 shows a

typical MCA spectrum. In order to relate this spectrum to the nuclear decay, we need to

understand how gamma rays interact with matter.When entering a crystal, gamma rays produce fast charged electrons by three different processes:

the photoelectric effect, the Compton effect (Compton scattering) and pair production. It is these

fast electrons, which give rise to scintillations, not the gamma ray. The observed spectral

distribution will thus depend on the detailed interaction process of the gamma rays in the crystal.

Consider a beam of mono-energetic gamma rays striking the scintillator. For our purposes the

most important energy loss mechanism is the photoelectric effect. When a gamma ray strikes an

ion in the crystal, it is absorbed and all of its energy is transferred to one of the bound electrons,

which is freed and moves rapidly through the crystal. Since the energy of the gamma ray

(typically about 0.5 MeV) is much greater than the binding energy of the electron of the ion

(typically 10 to 100 eV), the energy of the freed electron can be considered equal to that of the

incoming gamma. (Especially since the energy resolution of the detector is only about 10%.)

Thus the photoelectric effect results in a peak, called the photopeak, in the photomultiplier

spectrum at an energy equal to that of the incoming gamma ray.


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From graph 1 we got the peak value is at 2.9 (channel value) while from the graph 2 we got the

peak value at 5.1 and 5.8. When we draw a graph of vs discriminator setting using this value

we got a linear graph. This is known as the energy calibration curve of the spectrometer (see

figure 6). We use this curve to find the gamma ray energy of an unknown source by reassuring

is spectrum under the same condition. From the graph 3 which is using unknown source, we got

the peak value at 3.7 (channel number). When we plot this value at graph 4, we got the energy is

about 830 eV. According to the unknown sources peak energy is830 keV and based on the

graph 4 it is the Mn54 and its half-life is 312.2

A NaI(Tl) detector has an energy resolution of only about 10%. When a beam of mono-

energetic gamma rays strikes the scintillator, there is a fluctuation from gamma ray to gamma ray

in the height of the voltage pulse from the photomultiplier, which shows up as a broadening of

the photopeak. The pulse height variation is chiefly due to statistical fluctuations in the number

of electrons emitted at the cathode of the photomultiplier when a flash of photons arrives from

the scintillator, but is also due to the occasional escape of electrons, X-rays or gamma rays fromthe crystal, all of which depend on how large the NaI crystal is. The fractional full width of a

peak at half its maximum height (FWHM) is a convenient measure of the resolution of the

instrument. From the calculation of the energy resolution, R at each graph we got the percent is

below than 10% which means the ability to resolve two peaks that are fairly close together in

energy is high for graph 1 and 3 and low for graph 2.

Reference

AN34: Experiments in Nuclear Science, 3rd Ed., EG & G Ortec. Experiment 1, Basic

identifications in Electronic Measurement Systems, pp 1-7; Experiment 3, Gamma Spectroscopy

Using NaI, pp 15-24; Linear and Logic Signals in EG & G Ortec NIM Instruments, pp159-160

D.W. Preston & E.R. Dietz: The Art of Experimental Physics, Appendix B Counting and

Sorting Particles: The Scintillation Counter, pp376-385

general theory

Serway, Moses and Moyer: Modern Physics, pp 431-432 (scintillator/photomultiplier), 389-

392 (beta and gamma decay)

Beiser: Concepts of Modern Physics, pp 443, 471-473 (gamma decay)

Halliday: Introductory Nuclear Physics

Siegbahn: Alpha, Beta and Gamma Ray Spectroscopy

data exp. 6 ijatto

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