A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy

We find that the jump-into-contact of the cantilever in the atomic force microscope (AFM) is caused by an inherent instability in the motion of the AFM cantilever. The analysis is based on a simple model of the cantilever moving in a nonlinear force field. We show that the jump-into-contact distance can be used to find the interaction of the cantilever tip with the surface. In the specific context of the attractive van der Waals interaction, this method can be realized as a new method of measuring the Hamaker constant for materials. The Hamaker constant is determined from the deflection of the cantilever at the jump-into-contact using the force constant of the cantilever and the tip radius of curvature, all of which can be obtained by measurements. The results have been verified experimentally on a sample of cleaved mica, a sample of Si wafer with natural oxide and a silver film, using a number of cantilevers with different spring constants. We emphasize that the method described here is applicable only to surfaces that have van der Waals interaction as the tip-sample interaction. We also find that the tip to sample separation at the jump-into-contact is simply related to the cantilever deflection at this point, and this provides a method to exactly locate the surface.Comment: 11 pages, 4 figures, 1 tabl

Investigations on the Peach 4 Debrite, a Late Pleistocene Mass Movement on the Northwest British Continental Margin

The Peach 4 debrite is the most recent in a series of large scale Pleistocene MTDs within the Barra fan on the northwest British continental margin. Geophysical data indicate that Peach 4 was formed through a combination of blocky and muddy debris flows and affects an area of ~ 700 km2. BGS core sample 56 -10 36, located directly over the Peach 4 debrite, provides a minimum age of 14.68 ka cal BP for the last major failure. An upwards fining turbidite sequence in BGS core sample 56 -10 239 is associ-ated with increased As and S concentrations, indicators of diagenetic pyrite which forms under anoxic conditions. It is proposed that As and S concentrations may pro-vide a method of distinguishing between contourite and turbidite sedimentation, though further research is required

Understanding key barriers to fathers' case planning involvement

Fathers who are uninvolved or play minimal roles in their children's lives may unwittingly have adverse effects on their psychosocial development. In 2003, only 54% of nearly a half million children in foster care in the United States had contact with their fathers, compared to 72% of children from the general population. There are multiple, complex personal, familial, societal, and agency barriers that limit fathers’ involvement with their children. We provide recommendations for child welfare agencies to modify their policies to be equitable and financially helpful to fathers, and engage fathers in case planning about their children's safety, well-being and permanency

African-American Fathers' Perspectives on Facilitators and Barriers to Father–Son Sexual Health Communication

African-American males ages 13 through 24 are disproportionately affected by sexually transmitted infections (STIs) and human immunodeficiency virus (HIV), accounting for over half of all HIV infections in this age group in the United States. Clear communication between African-American parents and their youth about sexual health is associated with higher rates of sexual abstinence, condom use, and intent to delay initiation of sexual intercourse. However, little is known about African-American fathers' perceptions of what facilitates and inhibits sexual health communication with their preadolescent and adolescent sons. We conducted focus groups with 29 African-American fathers of sons ages 10–15 to explore perceived facilitators and barriers for father–son communication about sexual health. Participants were recruited from barbershops in metropolitan and rural North Carolina communities highly affected by STIs and HIV, and data were analyzed using content analysis. Three factors facilitated father–son communication: (a) fathers' acceptance of their roles and responsibilities; (b) a positive father–son relationship; and (c) fathers' ability to speak directly to their sons about sex. We also identified three barriers: (a) fathers' difficulty in initiating sexual health discussions with their sons; (b) sons' developmental readiness for sexual health information; and (c) fathers' lack of experience in talking with their own fathers about sex. These findings have implications for father-focused prevention interventions aimed at reducing risky sexual behaviors in adolescent African-American males

Values that Fathers Communicate to Sons about Sex, Sexuality, Relationships, and Marriage

African American males between ages 13 and 24 are disproportionately affected by unintended pregnancy and sexually transmitted infections (STIs) such as chlamydia, gonorrhea, syphilis, and HIV/AIDS; indeed, they account for 50% of HIV infections among all youth. Clear communication between parents and their youth about sex is associated with higher rates of sexual abstinence, condom use, and intent to delay initiation of sexual intercourse, which can prevent STIs and unintended pregnancy. However, barriers exist for parents to educate their youth about sexual health. The purpose of this article is to explore the values fathers communicate to their sons to prevent their risky sexual behaviors that lead to STIs, HIV, and adolescent parenthood. This was a qualitative study conducted from May and June 2015. African American fathers (N = 29) who had sons, ages 10 to 15 years, participated in five focus groups across metropolitan and rural North Carolina communities in barbershops. A qualitative content analysis revealed four themes regarding areas that fathers imparted their values onto their sons to protect them from sexual health risks: (a) sex, (b) sexuality, (c) relationships, and (d) marriage. The findings have implications for social work and public health practice

Parent–youth communication to reduce at-risk sexual behavior: A systematic literature review

There are nearly 110 million cases of sexually transmitted infections (STIs) in the United States. The Centers for Disease Control and Prevention estimate that annually there are more than 19.7 million new STI cases. Of those, more than half are accounted for by youth aged 15–24 years. Although some STIs are not considered to be life threatening, they can lead to severe health problems, risk of HIV infection, or infertility if they are not properly treated. Some research has shown that parent–youth communication can reduce youth’s at-risk sexual behaviors. The following is a systematic review of the literature on parent–youth sexual communication and family-level interventions designed to reduce risky sexual behavior in youth

Precision Measurement of the Radiative B\Beta Decay of the Free Neutron

The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the $\beta$ decay of the free neutron. We report on the RDK II experiment which measured the photon spectrum using two different detector arrays. An annular array of bismuth germanium oxide scintillators detected photons from 14 to 782~keV. The spectral shape was consistent with theory, and we determined a branching ratio of 0.00335 $\pm$ 0.00005 [stat] $\pm$ 0.00015 [syst]. A second detector array of large area avalanche photodiodes directly detected photons from 0.4 to 14~keV. For this array, the spectral shape was consistent with theory, and the branching ratio was determined to be 0.00582 $\pm$ 0.00023 [stat] $\pm$ 0.00062 [syst]. We report the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies