Difference between revisions of "June, 7, 2007 Cf rate measurements using Photonics PMT"
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− | The Cf-252 source was placed 16.5 cm away from the face of the BC408 scintilator which is attached to the Photonics PMT whose base was set to | + | The Cf-252 source was placed 16.5 cm away from the face of the BC408 scintilator which is attached to the Photonics PMT whose base was set to -1200 Volts. The PMT output signal was descriminated by a Lecroy Model 821 descriminator set to pass any voltage below -0.5 mV. The descriminated output was then sent to a counter Model TC 531. |
Revision as of 19:56, 8 June 2007
The Cf-252 source was placed 16.5 cm away from the face of the BC408 scintilator which is attached to the Photonics PMT whose base was set to -1200 Volts. The PMT output signal was descriminated by a Lecroy Model 821 descriminator set to pass any voltage below -0.5 mV. The descriminated output was then sent to a counter Model TC 531.
Configuration Definitions
1: Placed the Cf source 16.5 cm from face of Scintillator. neutrons passing through scintillator would also pass through PMT
2: Inserted a 2.5 cm thick steel shielding wall between Scintillator and Cf source. Right up against Scintillator
3: Replaced steel wall with 5 cm thick lead shielding brick
4: Replaced lead brick with 6.7 cm thick paraffin block ( Paraffin N.F. , Household wax made by Parowax. 4 cackes, total net weight 16 oz (454 g))
5: Increased thickness of paraffin block to 13.4 cm
6: No source backround
Measurements
Using Photonics PMT and BC
Configuration | Rate (counts/min) | Statistical Error in Rate (Counts/min) |
---|---|---|
1 | 2.7e3 | 0.09e3 |
2 | 2.2e3 | 0.01e3 |
3 | 0.8e3 | 0.02e3 |
4 | 2.2e3 | 0.08e3 |
5 | 1.8e3 | 0.02e3 |
6 |
Analysis
Properties of Materials used
Steel : Zeff = 26 and
=8.02Atomic composition for AISI 304) in %
Element | % |
---|---|
Si | 1 |
Cr | 19 |
Mn | 2 |
Fe | 68 |
Ni | 10 |
Lead: Z =
Photon Mass Attenuation Lengh
Example Calculation
A 1 MeV photon has
The photon intensity is attenuated such that
The half length
Therefore
In the case of steel:
So if the Californium source were just 1 MeV photons then I would expect a 2.5 cm thick steel wall to drop the rate by at least a factor of 2. In reality the photons from Cf-252 are a distribution of energies so we would need to use a montecarlo to make a more accurate prediction. All that can be concluded at this time is that a drop in rate from 2700 to 2200 counts per minute corresponds to a 19% drop in rate which indicates that the particles causing light in the scintillator are not just 1 MeV photons. One can conjecture that because the photon distribution from CF-252 contains more photons at energies below 1 MeV than above that there must be other particles besides photons (probably neutrons) making a signal in the scintillator.