Staff Research Assistant for P-25, Los Alamos National Labs.
The Multiplicity Vertex Detector (MVD) for the PHENIX experiment at Brookhaven National Labs is using a rigid foam called Rohacell 71 as the support for silicon detectors [1]. Rohacell comes in many different densities. The number at the end of Rohacell represents the density of the foam in kg/m3. It is expected that the MVD will be exposed to ~20 krad of radiation over the life time of the detector. It has been investigated as to whether or not Rohacell 71 can withstand such an exposure. I contacted Rohm Tech Inc. [2], the maker of Roahcell foam, in order to find some literature on the radiation hardness of Rohacell. They told me to call the Richmond Aircraft Products company in Norwalk, California at 310.404.2440 (fax:310.404.9011). Richmond faxed to me a table on the material behavior upon certain doses of radioactive radiation.
Richmond Aircraft Products states that the "compressive test specimens 50x50x30mm3 and the flexural test specimens 100x10x10 mm3 were irradiated with a dose of 0.15 Mrad and 10 Mrad from a cobalt 60 source." The table concludes that there was "no significant decrease in the tested properties up to a dose of 0.15 Mrad." Below is a reproduction of the table for Rohacell 71.
| Compressive Strength N/mm2 (psi) |
Flexural strength N/mm2 (psi) |
Deflection at fracture mm (in.) |
Force at 2 mm deflection N (p) |
Exposure |
|---|---|---|---|---|
| 1.5 (213) | 2.4 (341) | 6.0 (0.24) | 11.0 (2.4) | nonirradiated |
| 1.5 (213) | 2.3 (329) | 5.8 (0.23) | 11.2 (2.4) | 0.15 Mrad |
| 1.3 (185) | 1.8 (285) | 2.5 (0.10) | 12.0 (2.5) | 10 Mrad |
More information was found on the internet. The IU Atlas project at CERN had conducted tests on Rohacell 31. They found that at exposures above 9.2 Mrad rohacell "completely gave out and could not hold any more weight." This only happened after they subjected the exposed rohacell sample to over 400 grams of weight. [Radiation test on Transition Radiation Materials, H. Orgen, et. al., February 29, 1996, needmore.physics.indiana.edu/~fred/iu_atlas/iu_atlas.html]
The Atlas test does not necessarily effect us. First, we are not using Rohacell 31 as our support structure material, but the more dense Rohacell 71. I mentioned this study because it shows that it will take much more radiation to effect our Rohacell than what we expect the detector will absorb over its life time. We expect that only about 20 krad, or 0.02 Mrad of radiation will be absorbed by the silicon barrels over the life time of the detector. Looking at the table above, one can see that even at 0.15 Mrad of radiation, there is little decrease in the strength of Rohacell 71. This exposure is about 8 times more than the exposure we expect the MVD to receive.
To conclude, we do not see anything that would cause us to be concerned about
the Rohacell due to radiation when the MVD is assembled and functioning.
The strength of the Rohacell should be about the same from when the
detector is first assembled to the end of the life time of the detector.
References
[1] Rohacell as a Support Structure Material for the PHENIX Multiplicity Vertex Detector, Submitted to NIM Section A, December 1996.
[2] Rohm Tech Inc., 195 Canal Street, Malden, MA 02148. Tel:617.321.6984, Fax:617.322.0358.